Executive Summary 

iii 

Executive Summary 

Clomazone is a broad spectrum flowable (emulsifiable concentrate, wettable powder, or 

micro-encapsulated) herbicide used to control annual grasses and broadleaf weeds in a wide 

variety of crops and locations. Clomazone, as an emulsifiable concentrate, was first 

registered for use in 1986; the micro-encapsulated formulation was first registered for use in 

1995. Clomazone is an isoxazolidinone herbicide and there are no other registered 

compounds currently in this class. Clomazone is absorbed by the roots and shoots and 

translocated upwards in the xylem throughout the plant. It inhibits carotenoid biosynthesis; 

the specific target enzyme is not known. Clomazone toxicity is believed to be in part caused 

by a plant metabolite, 5-ketoclomazone, and is dependent upon a plant’s ability to oxidize the 

parent compound to this active metabolite. Cases have been identified where this is not the 

sole mechanism of toxicity, not all mechanisms are known. With foliar contact clomazone 

is not translocated throughout the plant and effects are more localized at the point of contact. 

Clomazone is registered for use on several crops, including soybeans, cotton, sweet potato, 

sugarcane, peppers (bell, hot, pimento, and sweet), pumpkin, tobacco, winter and summer 

squash, cabbage, mint, peas (succulent), beans (succulent), melons, watermelons, rice, and 

the root crops arracacia, cassava, yam, and yautia. It is generally applied as a pre-plant, preemergent, although there are a few uses where it is applied as a post-plant, pre-emergent or 

early emergence (1- to 2 leaf stage). 

Environmental Fate

Clomazone is a semi-volatile compound of low to high persistence in various environmental 

compartments, and of moderate mobility in soil (see Chapter 2for details). The major 

exposure route of concern is displacement of sorbed clomazone from soil by water vapor 

after rain or during episodes of high humidity, volatilization from the soil, and downwind 

transport of clomazone vapors. Once displaced from soil clomazone is not expected to be 

persistent in air for more than a couple of days. 

Risks to Nonlisted Species from Uses Other Than Rice

Considering all lines of evidence risks to nonlisted mammals, birds, reptiles, amphibians, 

beneficial insects, freshwater and estuarine-marine fish and invertebrates were concluded to 

be within acceptable acute and chronic levels for all clomazone uses with the exception of 

wet seeded rice. Direct effects on non-target vegetation off the field are expected for all uses. 

Incident and field data were used along with existing spray drift exposure models to help 

define to what extent clomazone is expected to move off-site, and when it does move off-site 

to provide some insight on the types and severity of effects to exposed nontarget plants. This 
Executive Summary 

iv 

approach was taken due to the lack of a viable air dispersion model, and relationships 

quantifying environmental and/or application factors that control clomazone displacement 

from the soil, especially in regards to differences between the EC, WP and ME formulations. 

Effects to off-site diverse plant communities with emergent vegetation are expected to be 

reduction of growth and whitened foliage, with mortality potentially amongst highly sensitive 

species based on laboratory results. Growth effects, when severe enough, have the potential 

to cause population dynamic changes in which competition among plant communities can 

change. Effects to nontarget plants are expected to be restricted to relatively localized areas 

from a treated site, with the greatest occurrence likely being associated with areas 

immediately adjacent to the field margin out about 1,500 feet due to transport of clomazone 

in runoff, spray drift and/or vapor. Effects on non-target plants from use of clomazone, 

associated with displacement from the soil have been observed out a couple of miles from a 

use site. Most of these events occurred when clomazone was first registered for use on 

soybeans with the EC formulation before spray drift and soil displacement reduction 

measures were incorporated on the label, especially for areas with temperature and weather 

conditions vulnerable to soil displacement. The furthest distance non-target plant effects 

have been observed for the lower volatilization prone ME formulation is about a mile. 

Events outside 1,500 feet for the EC, WP, and ME formulations are expected to be infrequent 

as indicated by the incident information. 

There is buffer language on current Section 3 labels which either explicitly prohibits 

application to non-field areas including fence rows, waterways, ditches, and road sites or 

implicitly prohibits application to non-field areas by limiting application to only those 

surfaces that will be soil incorporated. Section 3 labels for EC and WP registrations prohibit 

preemergent soil incorporated applications within 1,000 ft of towns and subdivisions, 

commercial vegetable production1

, commercial fruit production, commercial nurseries, and 

commercial greenhouses and 100 ft of desirable plants2

 whereas soil unincorporated 

applications are prohibited within 1,500 ft of towns and subdivisions, commercial vegetable 

production, commercial fruit production, commercial nurseries, commercial greenhouses and 

desirable plants. Section 3 labels for ME registrations prohibit applications within1,200 ft of 

towns and subdivisions, commercial vegetable production, commercial fruit production, 

 

1 Most labels have at least an exception for sweet corn some labels also include exceptions which include one or 

more of the following: peppers, pumpkins, succulent peas, sweet potato, and winter squash.

2 Spruce trees and deciduous trees (apple (fruit and ornamental), ash (green, white, mountain), basswood, 

boxelder, catalpa, cherry (fruit and ornamental), cottonwood, elm, ginkgo, hackberry, mulberry, peach, pear 

(fruit and ornamental), pecan, poplar, Russian olive, tree of heaven, tulip tree, walnut trees, willow species); 

shrubs and vines (azalea, burningbush, grape, honeysuckle, roses, yew); agronomic crops (alfalfa, oats); ferns; 

herbs; strawberry; raspberry; and blackberry
Executive Summary 

v 

commercial nurseries, commercial greenhouses and some labels have 200 ft and others 300 ft 

of desirable plants. Effects to off-site diverse plant communities with emergent vegetation 

are expected to be reduction of growth and whitening of foliage, with mortality potentially 

amongst highly sensitive species. Growth effects, when severe enough, have the potential to 

cause population dynamic changes in which competition among plant communities can 

change. 

Risks to Nonlisted Species from Use on Rice

Dry Seeded Rice. Based on measured field residue levels in flood water, effects to aquatic 

plants, freshwater fish, amphibians, and invertebrates and marine-estuarine fish and 

invertebrates are considered within acceptable risk levels for the dry seeded rice use. 

Residue levels on dietary items from spray drift are also within acceptable risk levels for 

birds, reptiles, amphibians (land phase), and mammals. There are direct effects expected to 

non-target vegetation off the field. At the rice use label rate of 0.6 lbs ai/A direct effects 

above the level of concern (LOC) potentially exist for dicots and nonflowering plants (e.g., 

lichens, ferns and allies), but monocot exposure levels are below the LOC. For those labels 

which do not state use of a coarse droplet size, especially for aerial applications to rice this 

effect may potentially reach several thousand feet. For other labels with such restrictions in 

place, this effect is expected to be restricted to relatively localized areas off a treated site, the 

highest occurrence and likelihood being adjacent to the site from spray drift and/or vapor 

drift. Effects to off-site diverse plant communities with emergent vegetation are expected to 

be reduction of growth and whitened foliage, with mortality potentially amongst highly 

sensitive species. Growth effects, when severe enough, have the potential to cause 

population dynamic changes in which competition among plant communities can change. 

Wet Seeded Rice. Effects to nonlisted vascular aquatic plants, and freshwater fish, 

amphibians, and invertebrates and estuarine-marine fish are within acceptable risk levels. 

Acute RQ values for freshwater fish and amphibians, and invertebrates exceeded endangered 

species LOC values but the individual risk of mortality based on mean slope responses are 1 

in 89 for fish and amphibians and <1 in 1011 for invertebrates on the day of application which 

reduces to 1 in 586 for fish and amphibians and 1 in <1 in 1027 for invertebrates 14 days postapplication, the holding time required on one Section 3 label. Acute risks to estuarine-marine 

invertebrates exceed acute LOC for unto 97 days post-application, however, the odds of 

mortality reduce to <1 in 6,000 by 28 days post-application. Nonvascular plant RQ values 

exceed the nonlisted species LOC at up to 42 days post-application. However, an evaluation 

of the range of nonvascular plants likely to be affected indicated only one of the four test 

species, a freshwater diatom, exceeded the nonlisted species LOC. The other three 
Executive Summary 

vi 

nonvascular plants (a cyanobacterium, a green alga, and a saltwater diatom) would not. 

Therefore risks to the nonvascular, algal biomass and primary productivity are likely within 

acceptable levels but the diversity of species is likely affected. Residue levels on dietary 

items from spray drift are also within acceptable risk levels for birds, reptiles, amphibians 

(land phase), and mammals. There are direct effects to non-target vegetation off the field. 

For those labels which do not state use of a coarse droplet size, especially for aerial 

applications to rice this effect may potentially reach several thousand feet. For other labels 

with such restrictions in place, this effect is expected to be restricted to relatively localized 

areas off a treated site, the highest occurrence and likelihood being adjacent to the site from 

spray drift and/or vapor drift. Effects to off-site diverse plant communities with emergent 

vegetation are expected to be mainly reduction of growth and whitened foliage, with 

mortality potentially amongst highly sensitive species. 

Effects Determination for Listed Species by Formulation and Use

Clomazone was formulated as an ME to reduce volatilization of clomazone from soil during 

events (e.g., rain, high humidity) that would otherwise displace clomazone applied as the EC 

or WP. These formulation differences have resulted in notable differences observed in at 

least the off-field distal extent of clomazone effects reported in incident reports. Because the 

action area for the endangered species effects determination is based on the furthest distal 

extent for any effect, the difference in distal extent between these formulations result in 

different conclusions about the size of the affected area. Because formulation type in the 

case of clomazone plays a role in the size of the area affected, and may play a role for 

consideration for potential options for mitigation, if any are needed, a separate analysis for 

the extent of effects for the ME from EC and WP formulations was performed. Additionally 

because in the case of clomazone, rice versus nonrice uses have a significant impact on the 

downstream distal extent of aquatic animal and plant effects, a separate analysis was 

performed for the downstream extent of effects for rice versus nonrice uses. 

The furthest distal extent for expected direct or indirect effects to terrestrial plants and 

animals from spray drift and volatilization was determined based on an assessment of 

reported distances from treated fields in the incident report data base. The furthest distance 

for which effects to plants have been observed is 2 miles for the EC formulation (the WP 

formulation is assumed to have the same potential), and 1 mile for the ME formulation. Rice 

uses are only registered with the ME formulation. For nonrice uses and dry seeded rice use 

aquatic organisms are not affected or effects do not extend beyond the field or edge-of-field. 

For wet seeded rice the extent of downstream effects is assumed to extend to the ocean. This 

extent is highly conservative given that for wet seeded rice the estimated concentration will 
Executive Summary 

vii 

only occur on the day of release but the tools to evaluate downstream dilution from releases 

off an agricultural field has not been developed for the contiguous United States, Hawaii, 

Alaska, or its territories. 

For convenience these areas of effect are referred to as action areas within the assessment but 

it is recognized that for the overall review the action area is the furthest extent considering all 

uses simultaneously. Specifically the effect areas evaluated for the co-occurrence analysis 

included (1) for EC and WP formulation nonrice uses, a 2 mile radius out from clomazone 

use sites; (2) for ME formulation nonrice uses and dry seeded rice use, a 1 mile radius out 

from clomazone use sites; and (3) for ME formulation wet seeded rice uses, a 1 mile radius 

out from use sites for effects to the terrestrial environment and all downstream miles 

extending from rice sites to the ocean. The action area for the clomazone registration review 

action consists of a 2 mile radius out from any clomazone use site which then further extends 

for adjacent flowing waters downstream to the ocean for rice uses. Operationally locations 

where clomazone is used are defined as all cultivated land that has had one or more of the 

crops for which clomazone is registered cultivated on it, except California where the only 

registered clomazone use is wet seeded rice. 

The geographical extent of a 2 mile radius out from a use site is based on the farthest 

physical extent direct or indirect effects from exposure to clomazone are predicted to occur 

based on best available information. This action area should not be misconstrued as 

automatically meaning that unreasonable adverse effects to nonlisted species or the 

environment is occurring. The extent of the action area is based on any effect adverse or not, 

the effect may be to a single individual, and may not result in decreased survival, growth, or 

reproduction. The effects determination considers all available lines of evidence including 

estimates of risk, probability of mortality, type, degree, and magnitude of indirect and direct 

effects to listed and nonlisted species within the action area to make an effects determination 

for a specific listed species and potential for habitat modification to critical habitat. 

Effects Determination Associated with All Uses Except Rice 

The effect areas evaluated for the co-occurrence analysis included (1) for the EC and WP 

formulations a 2 mile radius out from clomazone use sites; and (2) for the ME formulations a 

1 mile radius out from clomazone use sites. The effects determination summarized here for 

all uses except rice is based on the action area of a 2 mile radius out from clomazone use 

sites. 

May affect but NLAA determinations were made for listed vascular aquatic plants (there are 

no nonvascular listed species), freshwater and marine listed fish and invertebrates, aquatic 
Executive Summary 

viii 

amphibians, and marine mammals. May affect but NLAA determinations were made for 

listed large land birds, mammals and reptiles not known to be reliant on one or a limited 

number of nonlisted or listed terrestrial or semi-aquatic plant species. This conclusion was 

based on a review of multiple lines of evidence for both direct and indirect effects, effects to 

critical habitat, and included estimates of risk, odds of mortality, timing of application, and 

refinement of exposure levels where appropriate. Additionally, some species were identified 

as being outside the action area of clomazone, and a no effect determination was made for 

these. Species designated as NE or May affect but NLAA, are listed in Appendix I of this 

document. 

May affect and LAA determinations were made for 8 amphibians, 1 freshwater crustacean, 1 

arachnid, 1 conifer, 26 ferns and allies, 2 lichens, 2 aquatic plants, 507 flowering plants, 51 

birds, 23 terrestrial insects, 20 mammals, 47 land and arboreal snails, and 8 land reptiles. 

This conclusion was based on a review of multiple lines of evidence for both direct and 

indirect effects, effects to critical habitat, and included estimates of risk, odds of mortality, 

and refinement of exposure levels where appropriate. This list likely includes species that 

upon further refinement fall outside the action area. For locations where there was no 

National Land Cover Data for cultivated land (e.g., U.S. territories and Hawaii) in the version 

of the IMS system used for the cooccurrence analysis, it was assumed that any listed species 

at these locations would overlap with the clomazone action area except those listed species 

known to occur only on islands that do not support agribusiness and/or the whole island is a 

nature reserve such as Nihoa Island in the Northwestern Hawaiian Islands, Mona and Monito 

Island of Puerto Rico, the California Channel Islands. Species designated as May affect and 

LAA, are listed in Appendix I of this document. 

Effects Determination for Listed Species Associated with Wet Seeded Rice 

The action area for reregistration of clomazone use on wet seeded rice consists of a 1 mile 

radius out from rice use sites and all flowing waters downstream of rice fields. Operationally 

rice field sites are defined as all cultivated land that has had rice cultivated on it. No direct 

effect to listed vascular aquatic plants, estuarine and marine fish, marine mammals and birds 

are expected. Direct effects to freshwater invertebrates are considered discountable as the 

risk of individual mortality is less than 1 in 1011. No direct effects to land mammals, birds, 

reptiles, or amphibians are expected. Direct effects to freshwater fish and amphibians, and 

estuarine and marine invertebrates are expected. There are direct effects to non-target 

vegetation off the field. For those labels which do not state use of a coarse droplet size, 

especially for aerial applications to rice this effect may potentially reach several thousand 

feet. For other labels with such restrictions in place, this effect is expected to be restricted to 
Executive Summary 

ix 

relatively localized areas off a treated site, the highest occurrence and likelihood being 

adjacent to the site from spray drift and/or vapor drift. Effects to off-site diverse plant 

communities with emergent vegetation are expected to be mainly reduction of growth, with 

mortality potentially amongst highly sensitive species. Therefore, for large mammalian and 

avian listed species not reliant on a listed plant species or not reliant on one or a limited 

number of nonlisted plant species either as prey or habitat indirect effects are expected to be 

insignificant. 

May affect but NLAA determinations were made for listed vascular aquatic plants (there are 

no nonvascular listed species), freshwater and marine listed fish and invertebrates, aquatic 

amphibians, and marine mammals. May affect but NLAA determinations were made for 

listed large land birds, mammals and reptiles not known to be reliant on one or a limited 

number of nonlisted or listed terrestrial or semi-aquatic plant species. This conclusion was 

based on a review of multiple lines of evidence for both direct and indirect effects, effects to 

critical habitat, and included, estimates of risk, odds of mortality, and refinement of exposure 

levels where appropriate. Additionally, some species were identified as being outside the 

action area of clomazone, and a no effect determination was made for these. Species 

designated as no effect and may affect but NLAA, are listed in Appendix K of this document. 

May affect and LAA determinations were made for flowering and nonflowering plants 

because of exceedance of endangered species LOC. May affect and LAA determinations 

were made for listed freshwater fish and amphibians, and estuarine and marine invertebrates. 

May affect and LAA determinations were made for listed freshwater invertebrates that are 

obligate on a specific freshwater fish species and for estuarine and marine fish that are 

obligate on a specific estuarine-marine invertebrate. May affect and LAA determinations 

were made for small mammals, reptiles, and amphibians that because of their size and home 

range may rely on a limited number of plant species for their food resource or habitat. 

May affect and LAA determinations were made for 10 amphibians, 14 birds, 1 fern and 

allies, 2 lichen, 2 conifers and cycad, 185 flowering plants, 16 land mammals, 9 land reptiles, 

34 fish, 6 freshwater insects, 2 marine invertebrates, and 22 terrestrial insects.. This 

conclusion was based on a review of multiple lines of evidence for both direct and indirect 

effects, effects to critical habitat, and included estimates of risk, odds of mortality, and 

refinement of exposure levels where appropriate. Based on the co-occurrence analysis there 

are some locations for these species which fall outside the action area. Species designated as 

May affect and LAA, are listed in Appendix K of this document. 
Executive Summary 

x 

Effects Determination for Listed Species Associated with Dry Seeded Rice 

The action area for use on dry seeded rice consists of a 1 mile radius out from rice fields, 

except California where its use is prohibited by the label. Operationally rice field sites are 

defined as all cultivated land that has had rice cultivated on it. Based on measured field 

residue levels in flood water, direct effects to freshwater fish, amphibians, and invertebrates 

and marine-estuarine fish and invertebrates are below the acute endangered species LOC, and 

effects to aquatic plants are below the listed species LOC. No direct effects to birds, reptiles, 

amphibians (land phase), and mammals are expected. There are direct effects to non-target 

vegetation off the field. For those labels which do not state use of a coarse droplet size, 

especially for aerial applications to rice this effect may potentially reach several thousand 

feet. For other labels with such restrictions in place, this effect is expected to be restricted to 

relatively localized areas off a treated site, the highest occurrence and likelihood being 

adjacent to the site from spray drift and/or vapor drift. 

May affect but NLAA determinations were made for listed vascular aquatic plants (there are 

no nonvascular listed species), freshwater and marine listed fish and invertebrates, aquatic 

amphibians, and marine mammals. May affect but NLAA determinations were made for 

listed large land birds, mammals and reptiles not known to be reliant on one or a limited 

number of nonlisted or listed terrestrial or semi-aquatic plant species. This conclusion was 

based on a review of multiple lines of evidence for both direct and indirect effects, effects to 

critical habitat, and included, estimates of risk, odds of mortality, and refinement of exposure 

levels where appropriate. Additionally, some species were identified as being outside the 

action area of clomazone, and a no effect determination was made for these. Species 

designated as no effect and may affect but NLAA, are listed in Appendix J of this document. 

May affect and LAA determinations were made for flowering and nonflowering plants 

because of exceedance of endangered species LOC. May affect and LAA determinations 

were made for small mammals, reptiles, and amphibians that because of their size and home 

range may rely on a limited number of plant species for a food resource or habitat. 

May affect and LAA determinations were made for x amphibians, 3 amphibians, 11 birds, 1 

fern and allies, 1 lichen, 45 flowering plants, and 7 land mammals. This conclusion was 

based on a review of multiple lines of evidence for both direct and indirect effects, effects to 

critical habitat, and included estimates of risk, odds of mortality, and refinement of exposure 

levels where appropriate. Based on the co-occurrence analysis there are locations for some 

of these species which fall outside the action area. Species designated as May affect and 

LAA, are listed in Appendix K of this document. 
Table of Contents 

Registration Review Ecological Risk Assessment Clomazone xi 

Table of Contents 

Chapter 1 Introduction....................................................................................... 1

1.1 Purpose of Assessment .......................................................................................................1

1.2 Clomazone Use...................................................................................................................2

1.2.1 Existing Registered Uses................................................................................................2

1.3 Problem Formulation..........................................................................................................8

1.3.1 Application Rates ...........................................................................................................8

1.3.2 Analysis Plan..................................................................................................................8

1.3.2.1 Exposure Assessment .................................................................................................8

1.3.2.2 Degradates ................................................................................................................10

1.3.2.3 Effects Assessment...................................................................................................11

1.3.2.4 Risk Characterization ...............................................................................................11

Chapter 2 Exposure Assessment ................................................................... 13

2.1 Aquatic Exposure Assessment .........................................................................................13

2.1.1 Nonrice Uses--Modeled................................................................................................13

2.1.2 Screening Analysis – Rice Uses ...................................................................................17

2.1.2.1 Wet Seeded Rice.......................................................................................................17

2.1.2.2 Dry Seeded Rice .......................................................................................................17

2.1.3 Aquatic Rice Field Studies ...........................................................................................19

2.1.4 Volatilization: Transport and Exposure........................................................................22

2.2 Terrestrial Exposure .........................................................................................................24

2.2.1 Dietary-Based and Dose-Based Clomazone Residue Levels .......................................24

2.2.1.1 On-Field....................................................................................................................24

2.2.1.2 Spray-Drift................................................................................................................27

2.2.1.3 Volatilization ............................................................................................................28

2.2.2 Off-Field Terrestrial and Wetland/Riparian Plant Clomazone Exposure Levels .........29

2.2.2.1 Runoff and Spray Drift Loading...............................................................................29

2.2.2.2 Volatilization Loading..............................................................................................30

Chapter 3 Effects Characterization................................................................. 32

3.1 Terrestrial Vertebrate Toxicology ....................................................................................32

3.1.1 Effects on Birds ............................................................................................................34

3.1.2 Effects on Mammals.....................................................................................................35

3.2 Terrestrial Invertebrate Toxicology..................................................................................36

3.3 Fish Toxicology................................................................................................................37

3.3.1 Effects to Freshwater Fish ............................................................................................38

3.3.2 Effects to Estuarine-Marine Fish..................................................................................39

3.4 Aquatic Invertebrate Toxicology......................................................................................40

3.4.1 Effects to Freshwater Invertebrates ..............................................................................41

3.4.2 Effects to Estuarine-Marine Invertebrates....................................................................41

3.5 Plant Toxicology ..............................................................................................................42

3.5.1 Effects to Terrestrial Plants ..........................................................................................43

3.5.2 Effects to Aquatic Plants ..............................................................................................48

Chapter 4 Results: Risk Characterization ...................................................... 49

4.1 Risk Estimation ................................................................................................................49

4.1.1 Direct Effects to Non-Target Terrestrial Vertebrates ...................................................49

4.1.1.1 Direct Effects to Birds, Reptiles, and (Land-Phase) Amphibians ............................49
Table of Contents 

Registration Review Ecological Risk Assessment Clomazone xii 

4.1.1.2 Direct Effects to Mammals.......................................................................................49

4.1.2 Direct Effects to Non-Target Terrestrial Invertebrates.................................................53

4.1.3 Direct Effects to Terrestrial and Wetland/Riparian Plants ...........................................53

4.1.3.1 Runoff And Spray drift Loading ..............................................................................53

4.1.3.2 Volatilization Loading..............................................................................................54

4.1.4 Direct Effects to Freshwater Fish (Amphibians) and Plants ........................................55

4.1.4.1 Label Uses Other Than Rice.....................................................................................55

4.1.4.2 Rice Use....................................................................................................................58

4.1.5 Probit Slope Response Analysis of LOC Values and Acute RQ Values......................60

4.2 Risk Description ...............................................................................................................62

4.2.1 Non-Listed Species.......................................................................................................63

4.2.1.1 Direct Effects to Terrestrial Plants and Animals ......................................................63

4.2.1.2 Direct Effects to Aquatic Plants and Animals ..........................................................81

4.2.2 Listed Species Effects Analysis....................................................................................86

4.2.2.1 Action Area ..............................................................................................................86

4.2.2.2 Direct Effects To Listed Species For Uses Other Than Rice .................................103

4.2.2.3 Direct Effects to Listed Species For Use on Wet Seeded Rice ..............................109

4.2.2.4 Direct Effects to Listed Species For Use on Dry Seeded Rice...............................114

4.2.2.5 Indirect Effects to Listed Species For Uses Other Than On Rice ..........................116

4.2.2.6 Indirect Effects to Listed Species For Rice Uses....................................................117

4.2.2.7 Effects to Critical Habitat.......................................................................................119

4.2.2.8 Co-Occurrence Analysis.........................................................................................120

4.2.2.9 Effects Determination.............................................................................................122

Chapter 5 Uncertainties ................................................................................. 125

5.1 Exposure Assessment Uncertainties...............................................................................125

5.1.1 Volatilized Clomazone ...............................................................................................125

5.1.1.1 Maximum Exposure analysis: Edge-of-field PRZM Analysis ...............................126

5.1.1.2 Maximum Exposure Analysis: Agriculture............................................................127

5.2 Effects and Risk Assessment Uncertainties....................................................................128

5.2.1 Lack of definitive avian acute oral and subacute dietary clomazone test results .......128

5.2.2 Lack of ELS or life cycle test results for the most acutely sensitive freshwater fish 

species 129

5.2.3 Lack of ELS test results for the most acutely sensitive estuarine/marine fish ...........130

5.2.4 Lack of life cycle test results for the most acutely sensitive estuarine/marine 

invertebrate.................................................................................................................................130

5.2.5 Use of most sensitive species toxicity endpoint .........................................................131

5.2.6 Single and multiple active ingredient formulations relative to TGAI effects.............131

5.2.7 Lack of NLCD 82 (cultivated land) data for U.S. Territories, Hawaii, and Alaska ...132

Chapter 6 Bibliography.................................................................................. 133

Appendix A Clomazone Registrations.......................................................... 135

Appendix B Clomazone Toxicity Data .......................................................... 137

Appendix C Back Calculation of Acute LD50 and LC50 Values for No 

Mortality Test Results.......................................................................................... 156

Appendix D Formulation Effects Evaluation – Single Active and Multiple 

Active Ingredients................................................................................................ 159

Appendix E ECOTOX Open Literature Which Passed Initial Screens ....... 165

Appendix F Bibliography of ECOTOX Open Literature Excluded From The 

Effects Assessment............................................................................................. 172
Table of Contents 

Registration Review Ecological Risk Assessment Clomazone xiii 

Appendix G Non-Target Plant Incidents ....................................................... 245

Appendix H Field Studies – Plant Effects..................................................... 304

Appendix I Effect Determinations and Habitat Modification Determinations 

for Nonrice Uses .................................................................................................. 310

Appendix J Effect Determinations and Critical Habitat Modification 

Determinations for Dry Seeded Rice Uses ........................................................ 399

Appendix K Effects Determinations and Critical Habitat Determinations for 

Wet Seeded Rice Uses ........................................................................................ 430
List of Figures 

Registration Review Ecological Risk Assessment Clomazone xiv 

List of Figures 

Figure 1 Clomazone Concentration (ppb) at Three Sites on Cache River Arkansas, 2005.20

Figure 2 Clomazone Concentration (ppb) at Four Sites on L’Anguille River, Arkansas, 2005

.......................................................................................................................................21

Figure 3 Seedling Emergence IC25 Species Sensitivity Distribution for Clomazone ............45

Figure 4 Seedling Emergence IC50 Species Sensitivity Distribution for Clomazone ............46

Figure 5 Distribution of Distances from a Treated Field to Clomazone Effects....................66

Figure 6 Reported Number of Non-Target Plant Incidents Associated with Clomazone 

Formulations and Key Regulatory Decisions.................................................................67

Figure 7 Terrestrial Plant EEC Values Compared to Seedling Emergence IC25 Species 

Sensitivity Distribution for Clomazone ...........................................................................69

Figure 8 Wetland or Semi-Aquatic Plant EEC Values Compared to Seedling Emergence 

IC25 Species Sensitivity Distribution for Clomazone ......................................................71

Figure 9 Terrestrial, Wetland, and Semi-Aquatic Plant EEC Values Due to Volatilization 

Compared to Seedling Emergence IC25 Species Sensitivity Distribution.......................73
List of Tables 

Registration Review Ecological Risk Assessment Clomazone xv 

List of Tables 

Table 1 Summary of Current Clomazone Labeled Uses........................................................6

Table 2 PRZM Scenarios, Application Dates, and Application Rates Used to Model 

Clomazone EECs...........................................................................................................13

Table 3 Physical-Chemical Properties of Clomazone..........................................................14

Table 4 Environmental Fate Properties of Clomazone ........................................................14

Table 5 PRZM/EXAMS Modeling Input Parameters ............................................................15

Table 6 Screening Exposure Analysis: PRZM/EXAMS Standing Water Body.....................16

Table 7 Summary of Marchesan et al. (2007) Clomazone Monitoring Results in Brazil......22

Table 8 Dietary-Based Clomazone EEC Values for the Highest Application Rate Crop, 

Sweet Potatoes, and for Both a Broadcast and a Banded Treatment ...........................25

Table 9 Avian Dose-Based Clomazone EEC Values for the Highest Application Rate Crop, 

Sweet Potatoes, as a Banded Application Treatment....................................................25

Table 10 Mammalian Dose-Based Clomazone EEC Values for the Highest Application Rate 

Crop, Sweet Potatoes, as a Banded Application Treatment..........................................25

Table 11 Mammalian Dose-Based Clomazone EEC Values for the Highest Application Rate 

Crop, Sweet Potatoes, as a Broadcast Treatment.........................................................26

Table 12 Mammalian Dose-Based Commence EC Formulation EEC Values for Cotton and 

Soybeans t Potatoes, as a Banded Treatment ..............................................................27

Table 13 Dietary-Based Clomazone EEC Values Due to Volatilization for the Highest 

Application Rate Crop, Sweet Potato at 1.5 lbs ai/A......................................................28

Table 14 Avian Dose-Based Clomazone EEC Values Due to Volatilization for the Highest 

Application Rate Crop, Sweet Potatoes at 1.5 lbs ai/A..................................................28

Table 15 Mammalian Dose-Based Clomazone EEC Values Due to Volatilization for the 

Highest Application Rate Crop, Sweet Potatoes at 1.5 lbs ai/A.....................................29

Table 16 Estimated Clomazone Loading to Off-Site Terrestrial and Semi-Aquatic Areas 

Adjacent to a Treated Field for Ground Spray Applications at the Upper and Lower 

Maximum Application Rate Range.................................................................................30

Table 17 Input Parameters for TerrPlant v1.2.2 Used for Modeling Runoff and Spray Drift 

Loading to Off-Field Terrestrial And Semi-Aquatic Areas Adjacent to a Treated Field..30

Table 18 Off-Site Loading Due to Volatilization Based on Relative Humidity ......................31

Table 19 Off-Site Loading Due to Volatilization Assuming 50% of Rate Based on Relative 

Humidity.........................................................................................................................31

Table 20 Summary of Specific Measurement Endpoint Values Selected to Calculate RQ 

Values to Evaluate Risk for Birds and Mammal Assessment Endpoints .......................33

Table 21 Body Weight Adjusted Acute Oral LD50 Values Based on Extrapolated Mallard 

Duck LD50 Values from Limit Test Results .....................................................................35

Table 22 Summary of Specific Measurement Endpoint Values Selected To Calculate RQ 

Values to Evaluate Risk for Terrestrial Invertebrate Assessment Endpoints.................37

Table 23 Summary of Specific Measurement Endpoint Values Selected To Calculate RQ 

Values to Evaluate Risk for Fish and Amphibian Assessment Endpoints .....................38

Table 24 Summary of Specific Measurement Endpoint Values Selected to Calculate RQ 

Values to Evaluate Risk for Aquatic Invertebrate Assessment Endpoints .....................40

Table 25 Summary of Specific Measurement Endpoint Values Selected to Calculate RQ 

Values to Evaluate Risk for Terrestrial and Aquatic Plant Assessment Endpoints........42

Table 26 Vegetative Vigor Study Morphological Observation Results – Clomazone Dose at 

Which Discoloration and Mortalities were First Observed .............................................47

Table 27 Avian chronic dietary-based RQ values for a banded application on sweet 

potatoes .........................................................................................................................49
List of Tables 

Registration Review Ecological Risk Assessment Clomazone xvi 

Table 28 Upper bound acute and chronic clomazone dose-based RQ values for mammals 

for a banded application on sweet potatoes ..................................................................51

Table 29 Upper bound acute and chronic clomazone dose-based RQ values for mammals 

for a broadcast application on sweet potatoes...............................................................51

Table 30 Upper bound acute and chronic clomazone dose-based RQ values for mammals 

ingesting short grass at application rates from 0.75 lbs ai/A to 2.5 lbs ai/A...................52

Table 31 Mammalian chronic dietary-based clomazone RQ values for a banded application 

on sweet potatoes..........................................................................................................52

Table 32 Upper bound and average acute Commence EC dose-based RQ values for 

mammals for a banded application on cotton or soybeans............................................52

Table 33 RQ values for plants in dry and semi-aquatic areas exposed to clomazone through 

runoff and/or spray drift..................................................................................................53

Table 34 RQ values for near field plants exposed to clomazone from volatilization............54

Table 35 RQ values for near field plants exposed to clomazone from volatilization............55

Table 36 RQ values for freshwater fish, invertebrates, and plants based on 1-in-10 year 

acute and chronic exposure concentrations in a vulnerable receiving water.................55

Table 37 RQ values for estuarine-marine fish, invertebrates, and aquatic plants based on 1-

in-10 year acute and chronic exposure concentrations in a vulnerable receiving water, 

With and Without PRZM Volatilization ...........................................................................56

Table 38 Wet Seeded Rice Non-Listed and Listed Aquatic Plant RQs and Acute Fish and 

Aquatic Invertebrate RQ Values for Clomazone Residues in Rice Paddy Water at 

Various Holding Times...................................................................................................58

Table 39 Dry Seeded Non-Listed and Listed Aquatic Plant RQs and Acute Fish and Aquatic 

Invertebrate RQ Values for Clomazone Residues in Rice Paddy Water at Permanent 

Flood..............................................................................................................................60

Table 40 Probit Slope and Odds of Mortality for an Individual at the LOC...........................61

Table 41 Odds of Mortality for an Individual Bird or Mammal at a Given Acute RQ Value for 

the TGAI and Commence EC ........................................................................................61

Table 42 Odds of Mortality for an Individual Fish or Aquatic Invertebrate at a Given Acute 

RQ Value .......................................................................................................................62

Table 43 Distance from Treated Field Spray Drift Loading Exceeds LOC...........................64

Table 44 Upper bound avian dietary levels for a banded application on sweet potatoes 

compared to extrapolated dietary acute toxicity values .................................................76

Table 45 Upper bound mammalian dietary levels for a banded application on sweet 

potatoes compared to extrapolated dietary acute toxicity values ..................................78

Table 46 Mean acute and chronic dose-based RQ values for mammals for a banded 

application on sweet potatoes........................................................................................80

Table 47 Summary of Indirect and Direct Effects to Listed Species and Preliminary Effects 

Determination for Clomazone Uses Other Than Rice....................................................90

Table 48 Summary of Indirect and Direct Effects to Listed Species and Preliminary Effects 

Determination for Clomazone Uses On Wet Seeded Rice ............................................95

Table 49 Summary of Indirect and Direct Effects to Listed Species and Preliminary Effects 

Determination for Clomazone Uses On Dry Seeded Rice...........................................100

Table 50 Clomazone uses designated in problem formulation and corresponding IMS sites

.....................................................................................................................................121

Table 51 PRZM Edge-of-Field Exposure ...........................................................................127

Table 52 Maximum Surface Water EEC Values from PRZM/EXAMS Model .....................128
List of Abbreviated Terms 

Registration Review Ecological Risk Assessment Clomazone xvii 

List of Abbreviated Terms and Symbols 

@ symbol for “at” 

# symbol for “number” 

% symbol for “percent” 

> symbol for “greater than” 

> symbol for “greater than or equal to” 

< symbol for “less than” 

< symbol for “less than or equal to” 

μg/kg symbol for “micrograms per kilogram” 

μg/L symbol for “micrograms per liter” 

oC symbol for “degrees Celsius” 

2-CBA 2-chlorobenzoic acid 

Ai active ingredient 

AAPCO Association of American Pesticide Control Officials 

Acc# accession number 

amu atomic mass unit 

Apr April 

Aug August 

bw body weight 

CA California 

CARB California Air Resources Board 

CDPR California Department of Pesticide Regulation 

CI confidence interval 

CL confidence limit 

CT Connecticut 

DE Delaware 

DP# 

EC emulsifiable concentrate 

EC05 5% effect concentration 

EC25 25% effect concentration 

EC50 50% (or median) effect concentration 

ECOTOX EPA managed database 

EEC estimated environmental concentration 

EFED Environmental Fate and Effects Division 

e.g. Latin exempli gratia (“for example”) 
List of Abbreviated Terms 

Registration Review Ecological Risk Assessment Clomazone xviii 

E 

EIM Environmental Information Management System 

EPI Estimation Programs Interface 

ESU Evolutionarily significant unit 

et al. Latin et alii (“and others”) 

etc. Latin et cetera (“and the rest” or “and so forth”) 

EXAMS 

FESTF FIFRA Endangered Species Task Force 

FIFRA Federal Insecticide Fungicide and Rodenticide Act 

FMC65317 N-[(2-chlorophenyl) methyl]-3-hydroxy-2,2-

dimenthylpropanamide 

FQPA Food Quality Protection Act 

ft Feet 

GENEEC Generic Estimated Exposure Concentration model 

HMONC habitat modification of no concern 

HMOPC habitat modification of potential concern 

HPLC high pressure liquid chromatography 

IA Iowa 

IC05 5% inhibition concentration 

IC50 50% (or median) inhibition concentration 

i.e. Latin for id est (“that is”) 

IL Illinois 

IMS Information Management System 

IN Indiana 

Jan January 

Jun June 

kg Kilogram(s) 

kJ/mole kilojoules per mole 

km kilometer(s) 

Koc symbol for the organic carbon partitioning coefficient 

KY Kentucky 

LAA likely to adversely affect 

lb ai/acre pound(s) of active ingredient per acre 

LC50 50% (or median) lethal concentration 

LD50 50% (or median) lethal dose 

lg large 
List of Abbreviated Terms 

Registration Review Ecological Risk Assessment Clomazone xix 

LOAEC lowest observable adverse effect concentration 

LOAEL lowest observable adverse effect level 

LOC level of concern 

LOD level of detection 

LOEC lowest observable effect concentration 

LOQ level of quantitation 

m meter(s) 

MA may affect 

MATC maximum acceptable toxicant concentration 

MD Maryland 

m2

/day square meters per days 

ME Microencapsulated 

mg milligram(s) 

mg/kg milligrams per kilogram (equivalent to ppm) 

mg/L milligrams per liter (equivalent to ppm) 

mi mile(s) 

MI Michigan 

MJD Multi-Jurisdictional Database 

MLRA major land resource area 

mmHg millimeter of mercury 

MN Minnesota 

MRID master record identification number 

MW molecular weight 

NASS National Agricultural Statistics Service 

NAWQA National Water Quality Assessment 

NCOD National Contaminant Occurrence Database 

NE no effect 

NLAA not likely to adversely affect 

NLCD National Land Cover Dataset 

NH New Hampshire 

NJ New Jersey 

NM New Mexico 

NMC N-methyl carbamates 

NMFS National Marine Fisheries Service 

NOAA National Oceanic and Atmospheric Administration 

NOAEC no observable adverse effect concentration 
List of Abbreviated Terms 

Registration Review Ecological Risk Assessment Clomazone xx 

NOAEL no observable adverse effect level 

NOEC no observable effect concentration 

NRCS Natural Resources Conservation Service 

NTU nephelometric turbidity unit 

NY New York 

Oct October 

OH Ohio 

OPP Office of Pesticide Programs 

OPPTS Office of Prevention, Pesticides and Toxic Substances 

PA Pennsylvania 

pH symbol for the negative logarithm of the hydrogen ion 

activity in an aqueous solution, dimensionless 

pKa symbol for the negative logarithm of the acid dissociation 

constant, dimensionless 

ppb parts per billion (equivalent to μg/L or μg/kg) 

ppm parts per million (equivalent to mg/L or mg/kg) 

PRZM 

RI Rhode Island 

SLN Special Local Need 

sm small 

UCL upper confidence limit 

USDA United States Department of Agriculture 

USEPA United States Environmental Protection Agency 

USFWS United States Fish and Wildlife Service 

VA Virginia 

VT Vermont 

WI Wisconsin 

WP wettable powder 

wt weight 

WV West Virginia 
Chapter 1 Introduction 

Registration Review Ecological Risk Assessment Clomazone 1 

Chapter 1 Introduction 

1.1 Purpose of Assessment 

The purpose of this assessment is to evaluate the potential risk to populations of non-listed 

(i.e., not federally endangered or threatened) species and the potential risk to endangered and 

threatened (listed) species from exposure to clomazone from its registered uses. This risk 

assessment incorporates the latest effects data, modeling, and risk methodologies that may 

not have been available at the time clomazone registration actions occurred. Risks from 

direct and indirect effects are derived and evaluated in accordance with the risk assessment 

methodology described in the Agency’s Overview Document (USEPA, 2004). These risk 

findings are then used as part of an “effects determination” for listed species. The Agency 

will reach one of the following three conclusions regarding the potential for each registered 

clomazone use to affect federally listed species: 

• “No effect”; 

• “May affect, but not likely to adversely affect”; or 

• “May affect, and likely to adversely affect”. 

If the results of the risk assessment show no indirect effects and the levels of concern (LOC) 

are not exceeded for direct effects for a given listed species taxonomic grouping (e.g., 

freshwater fish, small herbivorous mammal), a “no effect” (NE) determination is made, 

based on use of clomazone within the action area for a “species” with the given taxonomic 

group. If, however, there is a potential for indirect effects and/or exposure exceeds the listed 

species LOC values for direct effects for a given group, the Agency concludes a preliminary 

“may affect” (MA) “species” within the taxonomic group. The Agency then considers 

additional lines of evidence such as the geographical nature of the exposure, as well as more 

in-depth evaluations of the toxicological and ecological requirements to determine a rationale 

for a “not likely to adversely affect” (NLAA) or “likely to adversely affect” (LAA) 

determination. 

Similarly the Agency will reach one of the following conclusions regarding the potential for 

clomazone uses to result in destruction or adverse modification of critical habitat: 

• “No effect”; 
Chapter 1 Introduction 

Registration Review Ecological Risk Assessment Clomazone 2 

• “Habitat modification(s) of no concern”; 

• “Habitat modification(s) of potential concern”. 

The Agency uses the risk assessment analysis for direct effects to categories of biological 

resource requirements to draw conclusions about effects to principle constituent elements of 

critical habitat. The Agency is limited in a practical sense, to those principle constituent 

elements of critical habitat that are of a biological nature. If the results of the risk assessment 

show that no LOC is exceeded for all taxonomic groups a “no effect” (NE) determination for 

habitat modification is made. If a LOC is exceeded for one or more taxonomic groupings the 

Agency then considers additional lines of evidence such as the type and degree of effect on 

the taxonomic groupings, expected resultant effects on biologically mediated environmental 

processes (e.g., increased sedimentation from loss of vegetation) as compared to baseline 

environmental conditions, cooccurrence of the action area with critical habitat, the type of 

principle constituent elements associated with critical habitat for listed species in a 

taxonomic grouping to determine a rationale for a “habitat modification of no concern” 

(HMONC) or “habitat modification of potential concern” (HMOPC). 

1.2 Clomazone Use 

Clomazone is a broad spectrum herbicide used to control annual grasses and broadleaf 

weeds. It is an isoxazolidinone herbicide and there are no other registered compounds 

currently in this class. Existing registered uses are described in Section 1.2.1. Clomazone 

end-use products may be applied alone mixed with water or also with liquid fertilizer. 

Emulsifiable concentrate formulations may be applied with dry fertilizer. Clomazone may be 

tank mixed with other crop registered herbicides3, insecticides, fungicides and/or 

nematicides. 

1.2.1 Existing Registered Uses 

The existing FIFRA Section 3 and 24c registered label uses of clomazone (trade names 

Command, Commence, CERANO, Strategy, and RiceMax) constitute the action under 

review (Appendix A). The following provides information on these registered uses. 

 

3 Canopy®, Dual®, Lasso®, Lasso Micro-Tech®, Lexone®, Lorox Plus®, Prowl®, Pursuit®, Scepter®, 

Sencor®, Sonalan®, Squadron®
Chapter 1 Introduction 

Registration Review Ecological Risk Assessment Clomazone 3 

Summary of Uses: 

Food: Existing registered food crop uses include arracacia (Peruvian carrot), 

succulent beans (lima, snap), cabbage, cucumber, manioc (cassava), 

melons, musk melons, watermelons, mint, succulent peas, peppers, 

pimento, pumpkin, rice, soybeans, summer squash, winter squash, 

sugarcane (newly planted or ratoon), sweet potato, yam, yautia, and 

agricultural fallow land (to be planted for winter wheat). Tanier, 

cassava, yams, and arracacia are members of Crop Subgroup 1-D 

(Tuberous and corm vegetables), for which sweet potato is the 

representative commodity. 

Non-Food: Existing registered non-food crop uses of clomazone include cotton 

and tobacco, except not for use on tobacco seedling beds. 

Residential: There are no existing residential uses of clomazone. 

Target Pests: Used to control annual grass weeds in dry-seeded rice and partial to 

full control of annual grasses4 and broadleaf weeds5 for the other 

agricultural crops. At lower rates of around 0.5 lbs ai/A the label 

indicates that clomazone only suppresses some annual grasses and 

controls velvetleaf and spurred anoda. Rates of 1.0 lb ai/acre or higher 

are recommended to control weeds in fields that have previously been 

in conservation programs, or where heavy weed pressure is 

anticipated. 

Formulation Types: Three formulations of clomazone are addressed in this document: 

wettable powder (WP), emulsifiable concentrate (EC), and microencapsulated (ME). There are eleven FIFRA Section 3 registered endproducts, three emulsifiable concentrate end-products, one wettable 

powder, and seven micro-encapsulated (ME) end-products. 

Additionally, there are several FIFRA Special Local Need (SLN) 

Section 24(c) registrations (Appendix A). Command 4EC, Command 

 

4 Barbarnyardgrass; broadleaf signalgrass; large and smooth crabgrass; giant, green, purple, robust and yellow 

foxtail; goosegrass; common, fall and Texas panicum; seedling Johnsongrass; field sandbur; South-western and 

wooly cupgrass; Bermudagrass; red rice; itchgrass; junglerice; shattercane; and wild proso millet 5 Velvetleaf; spurred anoda; black seeded plantain; balloonvine; carpet weed; cocklebur; common ragweed; 

curly dock; dayflower; Jimsonweed; lambsquarter; Pennsylvania smartweed; pricly sida; purslane; redweed; 

Venice mallow; Florida beggarweed; Florida pusley; kochia; redvine; tropic croton; wild poinsettia; black 

nighshade; joint vetch; pitted morningglory; and prostrate, redroot, smooth and spiny pigweed
Chapter 1 Introduction 

Registration Review Ecological Risk Assessment Clomazone 4 

4E, Command 3ME, Command 2ME, Cerano 5 MEG, and Command 

50 WP are single active ingredient formulations of clomazone. 

Strategy is a mixture of clomazone and ethalfluralin6. RiceMax is a 

mixture of clomazone and propanil7

. Commence EC is a mixture of 

trifluralin8

 and clomazone. 

Methods and Rates of Application: 

Equipment: There are FIFRA Section 24(c) labels that allow an ME formulation to 

be applied to rice using aerial equipment. Otherwise all other labels 

do not allow clomazone to be applied either by air or in any type of 

irrigation system. Clomazone is applied as a broadcast treatment or a 

band treatment using ground equipment, and is restricted to application 

at wind speeds at or below 10 miles per hour. Some labels specifically 

state to use equipment that produces a large droplet size of 450 

microns or larger, while other labels just indicate to use a large or 

coarse droplet size. Applications of the WP and EC formulations are 

required to be soil incorporated at 1 to 3 inches, depending on the 

crop, in given geographic locations (see Table 1) to reduce off-site 

movement of clomazone. 

 The RiceMax label indicates that rice fields to which the product is 

applied should have a levee system that can hold a 3-6 inch flood. 

Buffer Restrictions: There is language on current Section 3 labels which either explicitly 

prohibits application to non-field areas including fence rows, 

waterways, ditches, and road sites or implicitly prohibits application to 

non-field areas by limiting application to only those surfaces that will 

be soil incorporated. Section 3 labels for EC and WP registrations 

prohibit preemergent soil incorporated applications within 1,000 ft of 

towns and subdivisions, commercial vegetable production9

, 

commercial fruit production, commercial nurseries, and commercial 

greenhouses and 100 ft of desirable plants10 whereas soil 

 

6

 N-ethyl-N-(2-methyl-2-propenyl)-2,6-dinitro-4-(trifluoromethyl)benzenamine

7

 3’,4’-dichloropropionanilde)

8 α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine 9 Most labels have at least an exception for sweet corn some labels also include exceptions which include one or 

more of the following: peppers, pumpkins, succulent peas, sweet potato, and winter squash.

10 Spruce trees and deciduous trees (apple (fruit and ornamental), ash (green, white, mountain), basswood, 

boxelder, catalpa, cherry (fruit and ornamental), cottonwood, elm, ginkgo, hackberry, mulberry, peach, pear 
Chapter 1 Introduction 

Registration Review Ecological Risk Assessment Clomazone 5 

unincorporated applications are prohibited within 1,500 ft of towns 

and subdivisions, commercial vegetable production, commercial fruit 

production, commercial nurseries, commercial greenhouses and 

desirable plants. Section 3 labels for ME registrations prohibit 

applications within 1,200 ft of towns and subdivisions, commercial 

vegetable production, commercial fruit production, commercial 

nurseries, commercial greenhouses and some labels have 200 ft and 

others 300 ft of desirable plants. These buffer restrictions were not 

added to address a specific endangered species concern. 

Application Rates: Clomazone is limited to one application per year. Maximum label 

application rates for uses are contained in Table 1. 

Timing: For essentially all crops, clomazone may be applied as a pre-emergent 

soil treatment prior to seeding, planting, or transplanting. For crops 

such as dry-seeded rice, tobacco, and vegetables, it may alternatively 

be applied as a pre-emergent or early pre-emergence post-planting or 

post-transplanting but prior to crop emergence. For dry-seeded rice it 

also may be applied as an early post-emergent. For vegetables it is 

recommended that the seed bed be free of crop and weed residue and 

clod free whereas cotton applications may be made under conventional 

tillage or conservation tillage systems. Application to soybeans may 

also be made under minimum-tillage or no-tillage systems. 

Use Classification: General use. 

 

(fruit and ornamental), pecan, poplar, Russian olive, tree of heaven, tulip tree, walnut trees, willow species); 

shrubs and vines (azalea, burningbush, grape, honeysuckle, roses, yew); agronomic crops (alfalfa, oats); ferns; 

herbs; strawberry; raspberry; and blackberry.
Chapter 1 Introduction 

Registration Review Ecological Risk Assessment Clomazone 6 

Table 1 Summary of Current Clomazone Labeled Uses 

Crop Formulation Maximum

lb ai/acre

Application method* Geographic Restrictions 

sweet potato ME 1.5 broadcast or band (no soil 

incorporation) 

Disallowed in CA 

cotton, soybeans ME, EC 1.25 EC: broadcast or band (must 

be soil incorporated in some 

states) 

ME: broadcast or band (no 

soil incorporation) 

Disallowed in CA 

EC: one EC label requires soil 

incorporation, one EC label requires 

soil incorporation in CT, DE, IL, IN, 

IA, Maine, MD, Massachusetts, MI, 

MN, NH, NJ, NY, OH, PA, RI, VT, 

VA, WV, WI, another EC label 

requires soil incorporation in these 

same states plus KY 

arracacia (Peruvian 

carrot), cassava 

(manioc), yam, 

yautia, sugarcane 

ME 1.24 broadcast or band (no soil 

incorporation) 

Disallowed in CA 

Agricultural fallow 

land 

EC 1 broadcast (no soil 

incorporation) 

Disallowed in CA 

peppers (bell, hot, 

pimento and sweet) 

ME, EC 1 ME: broadcast or band (no 

soil incorporation) 

EC: broadcast or band (must 

be soil incorporated in some 

states) 

Disallowed in CA 

EC: requires soil incorporate in CT, 

DE, IL, IN, IA, Maine, MD, 

Massachusetts, MI, MN, NH, NJ, 

NY, OH, PA, RI, VT, VA, WV, WI, 

another EC label requires soil 

incorporation in these same states 

plus KY 

pumpkin EC, WP 1 broadcast or band (must be 

soil incorporated in some 

states) 

Disallowed in CA 

EC: one EC label has no soil 

incorporation requirement; one EC 

label requires soil incorporation in 

CT, DE, IL, IN, IA, Maine, MD, 

Massachusetts, MI, MN, NH, NJ, 

NY, OH, PA, RI, VT, VA, WV, WI; 

and two other EC labels require soil 

incorporation in these same states 

plus KY 

tobacco ME, EC 1 ME: broadcast or band (no 

soil incorporation) 

EC: broadcast or band (must 

be soil incorporated in some 

states) 

Disallowed in CA 

EC: must be soil incorporated in 

CT, DE, IL, IN, IA, Maine, MD, 

Massachusetts, MI, MN, NH, NJ, 

NY, OH, PA, RI, VT, VA, WV, WI

pumpkin and squash 

(winter) 

ME 0.75 broadcast or band (no soil 

incorporation) 

Disallowed in CA 
Chapter 1 Introduction 

Registration Review Ecological Risk Assessment Clomazone 7 

Crop Formulation Maximum

lb ai/acre

Application method* Geographic Restrictions 

rice ME 0.6 dry-seeded: broadcast by 

ground equipment (no soil 

incorporation) 

water-seeded, flooded: 

broadcast by aircraft or 

ground equipment (one 

Section 3 label requires no 

release of rice paddy water 

for a minimum of 14 days 

post-application unless there 

is water treatment) 

dry-seeded: Disallowed in CA 

cabbage, mint, peas 

(succulent), squash 

(summer) 

ME 0.5 broadcast or band (no soil 

incorporation) 

Disallowed in CA 

Peas (succulent) EC,WP 0.5 broadcast or band (must be 

soil incorporated in some 

states) 

Disallowed in CA 

EC: one EC label requires soil 

incorporation in CT, DE, IL, IN, IA, 

ME, MD, Massachusetts, MI, MN, 

NH, NJ, NY, OH, PA, RI, VT, VA, 

WV, WI, another EC label requires 

soil incorporation in these same 

states plus KY 

cucumber, melons, 

squash (summer), 

squash (winter), 

watermelons 

ME 0.375 broadcast or band (no soil 

incorporation) 

-- 

cucumber ME 0.375 broadcast or band (no soil 

incorporation) 

Disallowed in CA 

beans (succulent), 

melons (musk), and 

watermelons 

ME 0.251 broadcast or band (no soil 

incorporation) 

Disallowed in CA 

*Applied using ground equipment except where noted. All labels state that the product may not be applied through any type 

of irrigation system. 

For ME formulations where soil incorporation is not recommended on the label, the tank mix 

instructions limit other pesticides in the tank mix to those that do not require soil 

incorporation. Use of a surfactant is recommended. For cotton, diuron is not recommended 

at planting when clomazone is used as plant injury may result and either a disulfoton or 

phorate organophosphate insecticide must be applied in-furrow with the cotton seed at 

planting time at a rate of 0.75 lb ai/acre otherwise crop phytotoxicity and/or stand reduction 

can occur. In addition, labels for use on rice warn to not tank mix or apply carbamate or 

organophosphate insecticides within 14 days of use of clomazone or to apply to fields with 

rice treated with bird repellents containing methiocarb. 
Chapter 1 Introduction 

Registration Review Ecological Risk Assessment Clomazone 8 

1.3 Problem Formulation 

The problem formulation for this assessment and effects determination was provided in the 

USEPA (2007a) Clomazone Summary Document Registration Review: Initial Docket 

January 200711 and USEPA (2007b) Revised Clomazone Final Work Plan Registration 

Review July 200712 documents. Additional modifications to the problem formulation are 

discussed in this section. 

1.3.1 Application Rates 

Maximum application rates were modified for a couple of uses, such as cotton, due to either 

label modifications which were made about the time of the problem formulation, and were 

therefore not reflected in the problem formulation or typographical errors. Application rates 

in Table 1 reflect the maximum application rates considered in this assessment. 

1.3.2 Analysis Plan 

1.3.2.1 EXPOSURE ASSESSMENT

As indicated in the problem formulation one of the major routes of fate and transport off-site 

is volatilization. (Appendix G documents the well-demonstrated volatilization and 

downwind transport of clomazone). Clomazone is a semi-volatile compound, but its 

vaporization is promoted by sorption of water to soil. Many incidents of bleaching in nontarget plants have been reported near clomazone use sites, especially after rain or during high 

humidity. Incidents have occurred repeatedly days after clomazone use, and so are not 

attributable to spray drift at the time of application. The severity of non-target plant 

bleaching, which led in some cases to plant death, and recorded distances of one mile and 

more, indicates that a substantial portion of the applied clomazone may be volatilized and 

transported away from the use site. 

The association of non-target plant bleaching incidents with rain and high humidity is 

consistent with the mechanism of displacement of clomazone from its soil sorption sites by 

water vapor. The desorbed clomazone partitions to the gas phase and is then free to be 

transported by the wind. 

As proposed in the problem formulation, the California Air Resources Board (CARB) Air 

model, CALPUFF, was evaluated for use in estimating volatilization exposures for 

 

11 http://www.regulations.gov/fdmspublic/component/main?main=DocumentDetail&d=EPA-HQ-OPP-2006-0113-0017 12 http://www.regulations.gov/fdmspublic/component/main?main=DocumentDetail&d=EPA-HQ-OPP-2006-0113-0022
Chapter 1 Introduction 

Registration Review Ecological Risk Assessment Clomazone 9 

clomazone. Scenario development and parameterization of CALPUFF for use in OPP 

exposure assessments was not sufficiently advanced for it to be used in the clomazone 

assessment. 

A recent paper by Yang and Holmén (in press) measured the effect of surfactant content, 

Henry’s Law constant, and relative humidity on the volatilization of seven herbicides 

(alachlor, butachlor, metolachlor, propachlor, pendimethalin, prodiamine, and trifluralin) 

applied as emulsifiable concentrates from two soils (a CT sandy clay loam with 1.68% 

organic carbon, and a NM clay loam with 0.73% organic carbon). The authors developed an 

empirical model for volatilization based on these parameters. The amount of clomazone 

volatilized from agricultural fields and available for deposition off-site under different 

relative humidity is estimated by this method of Yang and Holmén (in press). This is a 

provisional method for use in this assessment, and has not been vetted for use by the 

appropriate EFED technical team. 

To account for the effect of volatilization on runoff load, aquatic exposure estimates were 

done in two ways. First, with the PRZM volatilization routine turned off, and second with 

the volatilization routine turned on. The volatilization routine is invoked by entering values 

for the diffusion coefficient in air (estimated to be 3,802 m2

/day, Table 5) and the enthalpy of 

vaporization (the PRZM default value of 20 kJ/mole was used). 

Maximum exposures from PRZM-EXAMS (as opposed to 1-in-10 year peak daily 

exposures), and “edge-of-field” runoff water exposures from PRZM are given to assist in the 

demarcation of the “action area” for the listed species effects determination. All of these 

estimates are done both with and without volatilization considered. 

The Tier 1 Rice model is used for wet-seeded rice use, and is modified by considering the 

decay rate observed in an aquatic field dissipation study done in a California rice paddy. The 

modification to include a decay rate from field studies is a provisional modification of the 

Tier 1 Rice model. Ambient water exposures from rice uses, taken from the scientific 

literature, are also given. 

If effects data indicate that a formulated product, whether a single or multiple active 

ingredient one, is more toxic than the active ingredient alone, the formulation effects data 

may be used qualitatively or quantitatively in accordance with the Agency’s Overview 

Document and the Services’ Evaluation Memorandum (EPA 2004; USFWS/NMFS 2004). 

Except for mammals and freshwater fish no formulation testing was available. The results of 

an analysis of formulation effects on acute oral toxicity to mammals as compared to the 

TGAI show that an assessment based on the acute oral toxicity of the TGAI appears 
Chapter 1 Introduction 

Registration Review Ecological Risk Assessment Clomazone 10 

appropriate, except potentially for Commence EC. The Commence EC formulation based on 

mammalian effects data was found to potentially be more toxic to terrestrial wildlife than 

indicated by active ingredient effects testing. In this case a quantitative assessment of risk 

from exposure to this formulation was considered for mammals in addition to the analysis 

performed on the active ingredient. Exposure modeling in this instance was limited to 

dietary exposure to residues for a time period immediately following pesticide product 

application based on the expectation that the varying physical-chemical properties of the 

individual components of a formulation will result in progressively different formulation. 

Commence EC is registered for only two uses, cotton and soybean. Because there are no 

formulation effects testing with other terrestrial species (e.g., birds) so no quantitative 

exposure analysis for this formulation was performed for other terrestrial taxa. 

1.3.2.2 DEGRADATES

A major degradate in the anaerobic aquatic metabolism study (MRID 44348403) was 

FMC65317 (N-[(2-chlorophenyl)methyl]-3-hydroxy-2,2-dimenthylpropanamide), a ringopening product. It reached 49-55% of parent at 120 to 365 days of the study. In the aerobic 

aquatic study (MRID 44348404), it reached only 11% of parent at one sampling interval. 

Thus, it would not be expected to form to the same degree under aerobic conditions, and 

would be expected to degrade once exposed to oxygen. Therefore, FMC65317 was not 

included in the exposure assessment for water. 

Estimation Programs Interface (EPI) Suite v3.12, a Windows®-based suite of 

physical/chemical property and environmental fate estimation programs, was used to 

compare the estimated physical properties of FMC65317 to parent clomazone. The 

degradate was predicted to have a lower log Kow (1.55 versus 2.55 measured for parent), a 

higher boiling point (403oC versus 275oC for parent), a higher water solubility (1,239 to 

20,100 mg/L versus 1,100 for parent), and a lower vapor pressure (6.8E-09 mmHg versus 

1.44E-04 mmHg for parent) than the parent. All of these properties indicate a greater 

propensity to remain in water or soil than parent clomazone. Thus, FMC65317 is much less 

likely to cause exposure via the volatilization route. 

Another major degradate in the anaerobic aquatic metabolism study was 2-chlorobenzoic 

acid (2-CBA). This compound reached a maximum of 14% of parent at 189 days, and is an 

expected product of degradation of FMC65317. Because 2-CBA has a measured pKa of 

2.89, it is expected to be ionized at environmental pH, and therefore to remain in the water 

column rather than volatilize. 2-CBA is expected to be readily biodegradable, and thus to 

contribute little to exposure. 
Chapter 1 Introduction 

Registration Review Ecological Risk Assessment Clomazone 11 

The uncertainty in the water exposure assessment due to volatilization (50% or more at peak 

exposure) is expected to overwhelm the uncertainty due to non-inclusion of degradates at 

later times. There is no major degradate resulting in terrestrial exposure concern. 

1.3.2.3 EFFECTS ASSESSMENT

Modifications to the toxicity data presented in the problem formulation included correction 

from test material to an active ingredient basis for several endpoints, inclusion of public 

literature results which lowered acute and chronic toxicity endpoint values for the freshwater 

fish, corrections and updates in the statistical analysis approach for plant toxicity data (e.g., a 

continuous response model was used for point estimates, if all plants died in a replicate, a 

missing value rather than a zero value was used in height and weight analyses, where a 

NOAEC could not be determined an IC05 was calculated). All of these modifications 

resulted in lower toxicity endpoint values than used in previous assessments. Additionally, 

an evaluation of available formulation effects data indicated that the Commence EC 

formulation may be more toxic to mammalian wildlife than that that indicated by active 

ingredient effects testing. In this case exposure to the formulation was also evaluated for 

mammals. The clomazone effects data and the values used in the risk assessment and effects 

determination are summarized in Chapter 3. 

1.3.2.4 RISK CHARACTERIZATION

Because exposure models and/or effects data differ from previous risk assessments 

performed for registration of these uses, risk estimates based on maximum exposure levels to 

aid in the demarcation of the action area for listed species, and screening level risk estimates 

for these clomazone registered uses were performed. As indicated in the problem 

formulation more refined risk characterizations were also performed as needed to support the 

effects determination for listed species. 

The Information Management System (IMS), a work product of the industry-based FIFRA 

Endangered Species Task Force (FESTF) was used as a pilot to evaluate the co-occurrence of 

listed species and critical habitat within the clomazone action area. FESTF was formed to 

fulfill data requirements relative to species/pesticide use proximity. The registrant of 

clomazone, FMC Corporation, is a full member of FESTF. 

Species Location Data. The species location data in the IMS were provided by EFED in 

June, 2003. In order to provide the most current information, location data from the 

NatureServe Multi-Jurisdictional Database (MJD) licensed by FESTF (the FESTF MJD; 

McGaughey et al. 2007) was added to the IMS. In the biological data for each Element 

Occurrence, representing an individual record of species presence in a defined area, the 
Chapter 1 Introduction 

Registration Review Ecological Risk Assessment Clomazone 12 

FESTF MJD lists the counties in which that Element Occurrence occurs. To ensure that all 

counties were included, the accompanying spatial data (shape file) in the FESTF MJD was 

overlaid with county boundaries and all counties that overlap with each shape file were 

determined. The lists derived from the biological data and spatial data were combined and 

incorporated into the IMS. Because some taxonomic and geographic gaps exist in the 

FESTF MJD dataset, the MJD-based information was supplemented using county occurrence 

data lists issued by the USFWS for each state13 or by contacting the responsible USFWS 

regional office. The source of each species location (i.e., the FESTF MJD and/or a USFWS 

county list) was indicated in the IMS. 

In the investigation of individual co-occurrences, the spatial data in the FESTF MJD were 

used to determine the proximity of Element Occurrences to potential clomazone use sites. 

Species located within one or two miles of potential use sites were prioritized for further 

analysis; those outside two miles were considered to be not exposed to clomazone. 

Critical Habitat Location Data. Spatial data from the USFWS Critical Habitat Portal14

 was 

overlaid on county boundaries to generate a list of counties containing critical habitat. For 

species that are not included in the Critical Habitat Portal, attempts were made to obtain 

critical habitat spatial data by contacting the lead USFWS region for that species. The list of 

counties containing critical habitat for each species was incorporated into the IMS, separate 

from the list of counties containing species locations. 

In the investigation of individual critical habitat co-occurrences, subcounty critical habitat 

spatial data from the USFWS Critical Habitat Portal may be used to determine the proximity 

of critical habitat to potential clomazone use sites. 

Crop Use Location Data. Counties containing specific crop use sites were determined from 

the 2002 Census of Agriculture (USDA 2002). The Census of Agriculture is described by 

the USDA as “the only source of uniform, comprehensive agricultural data for every state 

and county or county equivalent in the United States.” The Census is conducted every five 

years, and the 2002 Census is the most recent. 

 

13 For example, http://www.fws.gov/mountain-prairie/endspp/CountyLists/Colorado.pdf 14 http://criticalhabitat.fws.gov
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 13 

Chapter 2 Exposure Assessment 

2.1 Aquatic Exposure Assessment 

2.1.1 Nonrice Uses--Modeled 

Exposure of aquatic organisms is calculated as detailed in the Overview Document (USEPA, 

2004). Briefly, the PRZM model (10-hectare field) with appropriate crop scenarios (or 

surrogates), soil type and meteorological data is run for a 30-year simulation in a 1-hectare, 

2-meter deep static pond in the EXAMS model, and the highest exposure expected once in 

ten years is reported. The 1-in-10-year peak daily, and 21-day and 60-day averages are used 

for acute and chronic invertebrate and fish estimated environmental concentrations (EEC), 

respectively. 

The following PRZM scenarios were chosen to represent registered clomazone uses. 

Application dates were set to 2 weeks before the emergence date in the PRZM scenario 

except for sugarcane and Florida potato, where clomazone is used as a pre-plant, preemergence treatment. An application date of January 15 was used for sugarcane and Florida 

potato. The Florida potato scenario was used as a surrogate for tropical tuber crops such as 

yautia, tanier, cassava, and arracacia. 

Table 2 PRZM Scenarios, Application Dates, and Application Rates Used to Model 

Clomazone EECs 

PRZM Use Scenario 

(standard unless noted) 

Clomazone Labeled Use 

represented 

Application 

Date 

Maximum Application 

rate, lb ai/acre 

Florida sugarcane Sugarcane Jan 15 1.25 

Louisiana sugarcane Sugarcane Jan 15 1.25 

Mississippi cotton Cotton Apr 15 1.25 

North Carolina cotton Cotton May 15 1.25 

Florida peppers Peppers Aug 15 1 

Mississippi soybeans Soybeans Apr 1 1.25 

North Carolina sweet potato Sweet potatoes May 1 1.5 

North Carolina tobacco Tobacco Apr 1 1 

Florida potato (NMC)1 Tanier, cassava, arracacia, yautia Jan 15 1.275 

Michigan bean Dry peas/beans, green peas, green 

beans 

May 15 0.4875 

Oregon snap beans Dry peas/beans, green peas, green 

beans 

Jun 1 0.4875 

Florida cabbage Cabbage Oct 1 0.49 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 14 

PRZM Use Scenario 

(standard unless noted) 

Clomazone Labeled Use 

represented 

Application 

Date 

Maximum Application 

rate, lb ai/acre 

Florida cucumber Cantaloupe, cucumbers, pumpkins, 

squash, watermelons 

Oct 1 1.0 (pumpkin)—WP 

0.75 (pumpkin, winter 

squash) 

0.5 (summer squash) 

0.25 (pumpkin, melons, 

watermelons) 

Oregon mint mint Apr 1 0.5 

(1) NMC: scenario developed for N-methyl carbamate cumulative assessment. This Florida potato scenario was 

used as a surrogate for tropical tuber crops such as yautia, tanier, cassava, and arracacia. 

The following tables summarize the physical-chemical and environmental fate properties of 

clomazone. 

Table 3 Physical-Chemical Properties of Clomazone

Property Value Reference 

Chemical Name (IUPAC) 2-(2-chlorobenzyl)-4,4-dimethylisooxazolidin-3-one Ahrens, 1994 

CAS Registry Number 81777-89-1 Ahrens, 1994 

Molecular Formula C12H14ClNO2 Ahrens, 1994 

Molecular Weight 239.70 amu Ahrens, 1994 

Physical State Colorless to pale yellow, viscous liquid at room 

temperature; white, crystalline solid below melting 

point

Ahrens, 1994 

Melting Point, oC 25 Ahrens, 1994 

Boiling Point, oC 275.4 @ 760 mmHg Ahrens, 1994 

Vapor Pressure, mmHg 0.000144 @ 25 oC Ahrens, 1994 

Aqueous Solubility, mg/L 1100 @ 25 oC Ahrens, 1994 

Octanol-Water Partition 

Coefficient (Kow) 

350 

(Log KOW = 2.54)

Ahrens, 1994 

Henry’s Law constant 4.13 E-5 atm-m3

/mol Calculated from vapor 

pressure and solubility

Table 4 Environmental Fate Properties of Clomazone

Input Parameter Value Source/comment 

Molecular weight 239.70 

Vapor pressure 1.44E-4 mm Hg Ahrens, 1994 (Herbicide Handbook, 

WSSA, 7th ed.) 

Solubility in water 1100 mg/L Ahrens, 1994 

Hydrolysis half-life Stable at pH 5, 7, 9 Acc. No. 248476, ref. 29 

Aqueous Photolysis half-life 87 days Acc. No. 073672 

Aerobic Soil metabolism half-life 80 days (90% UCL on mean of 35, 

65, 74 days) 

Acc. No. 248476, ref. 30 

Anaerobic aquatic metabolism halflife 44 days for total system MRID 44348403 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 15 

Input Parameter Value Source/comment 

Aerobic Aquatic metabolism half-life 79 days (90% UCL on mean of 75, 

64, 40.3 days) 

MRID 44348404, 44348405 

Organic carbon normalized soil 

partition coefficient Koc

300 Ahrens, 1994 

Henry’s Law Constant 4.13E-5 atm-m3/mol Calculated from vapor pressure and 

solubility 

Air Diffusion Coefficient 3802 cm2/day Calculated from molecular weight, Da = 

1.55/((MW)0.65) 

(Schwarzenbach, Gschwend, Imboden, 

1993, page 195) 

Enthalpy of Vaporization 20 kJ/mol PRZM default value 

Clomazone is a semi-volatile compound of low to high persistence (half-lives 13 to 173 days 

in various compartments) and moderate mobility in soil (FAO Koc range 100 to 1000). 

Reported incidents of volatilization from soil and vapor-phase transport to off-site receptors 

indicate that this route is the primary exposure route of concern. An estimated atmospheric 

half-life of 0.487 days (EPISuite, AOP v1.91) indicates that long-range (global) transport is 

not of concern. Volatilization from water is of less concern than from soil, as indicated by 

the Henry’s law constant. 

Based on laboratory data submitted to EPA by the registrant, the following input parameters 

for the linked PRZM and EXAMS models were chosen in accordance with the February 

2002 Input Parameter Guidance15. 

Table 5 PRZM/EXAMS Modeling Input Parameters 

Input Parameter Value Source/comment 

Molecular weight 239.70 

Vapor pressure 1.44E-4 mm Hg Ahrens, 1994 (Herbicide Handbook, 

WSSA, 7th ed.) 

Solubility in water 1100 mg/L Ahrens, 1994 

Hydrolysis half-life Stable at pH 5, 7, 9 Acc. No. 248476, ref. 29 

Aqueous Photolysis half-life 87 days Acc. No. 073672 

Aerobic Soil metabolism half-life 80 days (90% UCL on mean of 35, 

65, 74 days) 

Acc. No. 248476, ref. 30 

Anaerobic aquatic metabolism halflife 44 days for total system MRID 44348403 

Aerobic Aquatic metabolism half-life 79 days (90% UCL on mean of 75, 

64, 40.3 days) 

MRID 44348404, 44348405 

Organic carbon normalized soil 

partition coefficient Koc 

300 Ahrens, 1994 

Henry’s Law Constant 4.13E-5 atm-m3/mol Calculated from vapor pressure and 

solubility 

 

15 Available for download at http://www.epa.gov/oppefed1/models/water/input_guidance2_28_02.htm 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 16 

Input Parameter Value Source/comment 

Air Diffusion Coefficient 3802 cm2/day Calculated from molecular weight, 

Da = 1.55/((MW)0.65) 

(Schwarzenbach, Gschwend, 

Imboden, 1993, p. 195) 

Enthalpy of Vaporization 20 kJ/mol PRZM default value 

Chemical Application Method 1 (soil applied, default incorporation 

depth 4 cm) 

Spray Drift 1% for ground application Guidance for ground applications 

Application Efficiency 99% Guidance for soil applications 

without incorporation 

The following exposures were returned by PRZM-EXAMS. These are 1-in-10 year peak 

exposures, and are given both with the PRZM volatilization routine “on” and “off.” This 

routine has not yet been subjected to full quality assurance, however both figures are given 

for characterization purposes, to account for the known volatility of clomazone. The 

volatilization routine in PRZM is invoked by entering values for the diffusion coefficient in 

air and the enthalpy of vaporization. These parameters describe the ability of the vaporphase molecule to diffuse in air, and the amount of energy required to transfer the molecule 

from the liquid to the gas phase. Volatilization in EXAMS is invoked by entering a value for 

the Henry’s Law constant. 

Table 6 Screening Exposure Analysis: PRZM/EXAMS Standing Water Body 

1-in-10 year peak 

daily, ppb 

Without/With 

Volatilization

1-in-10 year, peak 21-day 

average, ppb 

Without/With 

Volatilization

1-in-10 year, peak 60-

day average, ppb 

Without/With 

Volatilization

PRZM Scenario (standard 

unless noted) 

Without With Without With Without With 

Florida sugarcane 36.4 23.2 18.8 13.5 9.1 6.4 

Louisiana sugarcane 25.0 23.2 16.3 14.9 8.6 7.4 

Mississippi cotton 29.3 16.0 17.9 8.9 8.9 5.2 

North Carolina cotton 17.1 11.3 11.2 7.1 6.0 3.7 

Florida peppers 29.0 12.8 14.5 7.1 7.4 3.2 

Mississippi soybeans 19.4 12.3 12.3 8.3 6.0 4.4 

North Carolina sweet potato 13.2 8.2 8.2 4.5 4.1 2.2 

North Carolina tobacco 6.5 2.7 3.3 1.5 1.5 0.74 

Florida potato (NMC)1 28.3 12.4 14.3 6.9 6.6 3.1 

Michigan bean 5.5 2.9 3.4 1.6 1.7 0.88 

Oregon snap beans 3.2 2.1 1.8 1.4 0.98 0.66 

Florida cabbage 6.8 3.7 3.2 1.7 1.8 0.73 

Florida cucumber, 1.0 lb/A 41.1 20.2 18.9 8.8 8.1 4.0 

Florida cucumber, 0.75 lb/A 31.6 15.1 14.6 6.6 6.3 3.0 

Florida cucumber, 0.5 lb/A 20.5 10.1 9.5 4.4 4.1 2.0 

Florida cucumber, 0.25 lb/A 10.3 5.0 4.7 2.2 2.0 1.0 

Oregon mint 1.6 0.82 0.96 0.51 0.51 0.29 

 (1) NMC: scenario developed for N-methyl carbamate cumulative assessment. This Florida potato scenario 

was used as a surrogate for tropical tuber crops such as yautia, tanier, cassava, and arracacia. 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 17 

2.1.2 Screening Analysis – Rice Uses 

2.1.2.1 WET SEEDED RICE

The Tier 1 Rice model (EPA, May, 2007)16 calculates the water-column exposure expected 

on the day of application to a flooded rice paddy, accounting only for sorption to sediment as 

a dissipation process. No other degradation processes, such as hydrolysis, photolysis, or 

metabolism, or dissipation process such as volatilization, are considered. The resulting EECs 

are therefore expected to be conservative. Using inputs of Koc = 139 ml/g, and an 

application rate of 0.6 lb/acre (0.672 kg/hectare), the Tier 1 Rice model predicts an exposure 

of 546 μg/L (ppb), if paddy water were to be released on the day of application. However, 

the wet seeded rice label (USEPA 279-3269) has a requirement to hold and not release water 

from the rice field for a minimum of 14 days, unless the water is contained in a tailwater 

recovery system or other system appropriate for preventing discharge from the rice fields. 

An estimate of the clomazone concentration at 14 days post-application is obtained by 

decaying the day of application concentration at a half-life of 23.7 days (MRID 45720601, 

California wet-seeded rice field dissipation study). An Australian study had a half-life of 7 

days, which is a faster dissipation rate. The relevant fate studies (aerobic aquatic 79 days, 

anaerobic aquatic 44 days, and aqueous photolysis 87 days) combine to give a half-life of 21 

days, which support the use of a longer half-life of 23.7 as a reasonable dissipation rate to 

use. The Australian study potentially reflects different environmental conditions. The 

exposure concentration predicted is 352 ppb on day 15 post-application using a 23.7 day 

half-life.17

2.1.2.2 DRY SEEDED RICE

Clomazone used in dry-seeded rice culture is subject to dissipation by terrestrial processes 

(volatilization and aerobic metabolism) before flooding, so paddy water concentrations are 

expected to be lower than for wet-seeded rice (i.e., clomazone applied directly to water). For 

dry seeded rice, clomazone is applied around 14 days before rice is planted, and the earliest 

that permanent flood would occur is 2 weeks (21 days) after planting (Sunil Ratnayke, 

Biological Analysis Branch, 11/06/2008 Use of Clomazone in Rice Production (DP# 

336342). If we assume that only soil metabolism is happening, the half-life of clomazone 

will be 80 days (this is the upper 90th percentile of three values). Aerobic aquatic 

metabolism has a similar half-life value of 79 days, and photolysis 87 days. The anaerobic 

aquatic half-life is 44 days. So, conservatively assuming that volatilization does not happen, 

 

16 http://www.epa.gov/oppefed1/models/water/index.htm#rice 17 ln ln ln 546ppb (0.0292) * (15 ) 352 ppb Ct = C0 − kt = − days = where C0 is the initial concentration, 0.0292 is 

the first-order rate constant, and Ct is the concentration on day t 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 18 

and the only process is aerobic soil (or aquatic) metabolism, the decay rate is ln(2)/80 = 

0.0087/day for clomazone applied to the rice field. After 35 days, C = (0.6 lb ai/acre) x

e

(0.0087 x 35) = 0.44 lbs ai/acre. Feeding this remaining load into the Tier 1 Rice Model, a result 

about 4/6 (or 2/3) as large as what is estimated for the initial concentration for wet-seeded 

rice is estimated, 364 ppb. Addition of other processes (photolysis, volatilization, etc) would 

all make the result smaller. A rigorous treatment of this exposure route would in all 

likelihood give us a result somewhere in the range of results for the rice field studies 

discussed in the following paragraphs. 

An aquatic field dissipation study (MRID 44348406) was conducted in rice fields in 

Arkansas and Texas at an application rate of 0.6 lb ai per acre. This corresponds to an 

expected concentration of 1,015 μg/kg in the top 2 inches of soil or 338 μg/kg in the top 6 

inches18. 

In the Arkansas field study the initial concentration in the top 6 inches was 292 μg/kg, or 

86.4% of expected. This fell to 21 μg/kg, or 7% of the initial by day 29. On day 30 (day of 

flooding), the concentration in the top 2 inches was 166 μg/kg, or 16.4% of the expected 

1,105 μg/kg from day 1. Thus, there appeared to be more desorption of clomazone from the 

wet soil (day 30) than from the dry soil (day 29). By days 180-269, the soil concentration 

was 12 to 17.6 μg/kg. Parent clomazone was not detected in the floodwater above 5 ppb. 

A similar pattern was seen in the Texas study. On day 0, the 6-inch soil concentration was 

203 μg/kg, or 60% of expected (338 μg/kg). By day 29, this had fallen to 73 μg/kg, or 21.6% 

of the expected initial concentration. Upon flooding (day 30), the 2-inch soil concentration 

was 140 μg/kg, or 13.8% of the expected initial (1,015 μg/kg). The soil concentration fell to 

11 to 16 μg/kg at 128 to 369 days post-treatment. Parent clomazone was not detected in the 

floodwater above 7 ppb. 

Overall, it appears that aquatic exposure from paddy water released from dry-seeded rice will 

be substantially lower than from wet-seeded rice. 

 

18

applicatio n rate, D is the soil depth, and C is the concentrat ion in the upper inches of the soil column

1.015 mg/kg where AR is the

2 inches

1

1300

1

102.79 m

1 acre - inch

1 lb

453,592 mg 0.6 lb ai/acre

inches

1

1300

1

102.79 m

1 acre - inch

1 lb

453,592 mg , mg/kg , lb ai/acre

D

3

3

3

3

D

kg

m

kg D

m CD AR

× × × × =

= × × × × =
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 19 

2.1.3 Aquatic Rice Field Studies 

Based on the information discussed below, the Tier 1 Rice model for wet seeded rice on day 

of application (Section 2.1.2) represents a highest expected exposure from labeled use. 

Aquatic Field Dissipation Studies 

A supplemental study (MRID 45720601) conducted in California on wet-seeded rice at the 

maximum label rate (0.6 lb ai/acre) yielded a maximum water concentration of 237 ppb 

parent. The residues declined with a half-life of 23.7 days in the total sediment-water 

system. This study was used in the provisional modification of the Tier 1 rice model. 

An acceptable study (MRID 44348406) conducted in Arkansas and Texas on dry-seeded rice 

focused mainly on soil dissipation. Concentrations in water (5-7 ppb) were low because 

terrestrial dissipation processes (metabolism and volatilization) were active for 29 days 

before flooding. 

Literature Studies: Rice Agriculture 

There are also published studies that measured concentrations in both rice paddies and in 

receiving waters. These studies are from Arkansas (Mattice et al., 2005), Australia (Quayle, 

2005), and Brazil (Zanella, et al., 2002; Marchesan et al., 2007). These studies demonstrate 

that the Tier 1 Rice model result (546 ppb) gives a reasonable exposure estimate, compared 

to measured values, and that exposures in receiving waters (up to 30 ppb) are lower than in 

rice fields, but persist over a period of weeks. 

Mattice et al. (2005) measured rice pesticides (Clomazone, Imazethapyr, Molinate, 

Quinclorac, and Triclopyr) in Arkansas, at four sites each on the L’Anguille and St. Francis 

rivers from near Jonesboro in the north to Marianna in the south, over a period of six years 

(2000-2005). In 2002, four sites on Lagrue Bayou from just below Peckerwood Lake north 

of Stuttgart to near the bayou’s mouth southeast of DeWitt were added. In 2003, four sites 

on the Cache River from near Jonesboro in the north to just below Interstate 40 in the south 

were added. 

The authors presented all detections from the 2005 sampling campaign, from April 27 to July 

5. Clomazone and Quinclorac were the most commonly detected rice pesticide. Clomazone 

was detected in the Cache River on five sampling dates from April 27 to June 21, with a 

maximum concentration of 28 ppb. It was also detected on four sampling dates on the 

L’Anguille River, with a maximum concentration of 25 ppb. Figure 1 and Figure 2 illustrate 

the data for these two rivers, respectively. The application rates that led to these exposures 

are unknown, but are not expected to have exceeded label rates. 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 20 

Figure 1 Clomazone Concentration (ppb) at Three Sites on Cache River Arkansas, 

2005 

0

5

10

15

20

25

30

22-Apr 2-May 12-May 22-May 1-Jun 11-Jun 21-Jun 1-Jul

date

ppb clomazone
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 21 

Figure 2 Clomazone Concentration (ppb) at Four Sites on L’Anguille River, Arkansas, 

2005 

0

5

10

15

20

25

7-May 12-May 17-May 22-May 27-May 1-Jun 6-Jun 11-Jun 16-Jun 21-Jun 26-Jun

date

ppb clomazone

In an Australian study of rice pesticides in New South Wales (Quayle, 2005), the fate of 

clomazone in flooded rice fields was determined. In 2002, clomazone (MagisterTM, 450 g 

ai/liter) was applied at 0.5 liter/hectare (225 g/hectare) and the concentration in water was 

followed for 60 days. The peak concentration in water was 202 ppb, and it declined to less 

than 1 ppb over the 60 days. The exposure predicted by the Tier 1 Rice model for this 

application rate is 183 ppb. The difference is small, and indicates that the Tier 1 Rice model 

is accurate for clomazone day-1 exposure. The reported half-life in water was 7.2 days (95% 

confidence interval 5.8 to 9.7 days). The time required for clomazone to decline to a 

“nominal level” of 3 ppb was calculated as 14 days. 

Two studies from Brazil (Zanella, et al., 2002; Marchesan et al., 2007) examined clomazone 

residues in rice paddies and surrounding watersheds. Zanella et al. (2002) measured 

clomazone concentrations in four Brazilian rivers, and in two experimental rice paddies. The 

rice paddy experiments were conducted in December, 1999, and December, 2000, at a rate of 

0.50 kg/hectare “formulated material.” The rate in kg active ingredient per hectare was not 

stated, thus a Tier 1 Rice model calculation is not possible. The maximum concentrations 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 22 

observed were 505 ppb and 377 ppb in 1999 and 2000, respectively. These results are in the 

same order as the Tier 1 Rice model result (546 ppb), indicating the suitability of the model 

for use in estimating clomazone exposures Concentrations in river water were as high as 

1.72 ppb in the Ibicuí River. 

Marchesan (2007) studied the concentrations of three rice herbicides (clomazone, quinclorac, 

and propanil) in the Vacacaí and Vacacaí-Mirim river basins from 2000 to 2003. The two 

basins have a total drainage area of 11,136 square kilometers, and 85,000 hectares is used to 

cultivate irrigated rice. During the study, 50 to 60% of the rice acres were treated with a tank 

mix of clomazone and propanil in post emergence, at rates of 350 and 1800 g/hectare, 

respectively. In the Vacacaí River, clomazone was detected at up to 8.85 ppb. Detection 

rates for clomazone were high (11 – 30% of samples), again indicating its persistence in 

receiving waters. 

Table 7 Summary of Marchesan et al. (2007) Clomazone Monitoring Results in Brazil 

Years Total No. 

Samples % Positive Minimum ppb Average ppb Maximum ppb 

Vacacaí River 

2001-2 36 30 1.32 4.09 7.72 

2002-3 30 20 1.50 4.97 8.85 

Vacacaí-Mirim River 

2000-1 104 27 0.41 1.34 5.62 

2001-2 45 11 1.24 2.38 4.82 

2002-3 45 20 0.62 2.17 5.10 

2.1.4 Volatilization: Transport and Exposure 

A recent paper by Yang and Holmén (in press) measured the effect of surfactant content, 

Henry’s Law constant, and relative humidity on the volatilization of seven herbicides 

(alachlor, butachlor, metolachlor, propachlor, pendimethalin, prodiamine, and trifluralin) 

from two soils (a CT sandy clay loam with 1.68% organic carbon, and a NM clay loam with 

0.73% o.c.). The authors concluded that relative humidity had the greatest influence on 

volatilization (50% of variance), followed by Henry’s Law constant (8.4 to 28.5% of 

variance), and then surfactant content (1.4%). The equation describing the behavior of all 

seven chemicals on both soils is: 

48 1 2 3 4 F = b S + b RH + b logKH + b

where F48 is the fractional amount volatilized in the first 48 hours, S is the fractional 

surfactant amount in the formulation, RH is the fractional relative humidity, KH is the 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 23 

Henry’s Law constant (Pa•m3/mol), and the coefficients are b1 = 0.42, b2 = 0.78, b3 = 0.07, 

and b4 = 0.04. 

Provisional application of this equation to clomazone (KH = 0.0042 Pa•m3

/mol, log KH = -

2.38) with the surfactant content S set to 0.0 for the microencapsulated formulation, indicates 

that at 100% RH, 65% of the applied amount would volatilize in the first 48 hours. At 50% 

RH, 26% volatilization is predicted, and at 25% RH, 6.9% volatilization is predicted. 

While the quantitative application of the Yang and Holmén equation to clomazone is 

uncertain and applied provisionally in this assessment, the physical mechanism is 

appropriate, and its vapor pressure falls in the same range as the studied herbicides (log KH 

= -3.07 to +1.02). The predicted amount of clomazone volatilized (65% at 100% relative 

humidity) is consistent with the observed severity and distance of reported incidents, and 

their association with rainfall or high humidity. 

The mechanism of exposure described by this process is (1) displacement of clomazone 

sorbed to soil by the action of water vapor; (2) diffusion of vapor-phase clomazone to the soil 

surface; (3) transport of clomazone vapors to off-target sites by wind, and (4) redeposition of 

clomazone vapors to water or terrestrial surfaces. 

Wetland Exposure Due to Volatilization Alone 

The exposure of aquatic organisms in wetlands near use areas, due to volatilization alone, 

was estimated. This is a provisional calculation for use with clomazone, and has not been 

evaluated by the appropriate EFED tech team for routine use in OPP risk assessments. As a 

screening estimate of exposure levels, 65% of the applied amount was assumed to volatilize, 

and subsequently be deposited in the AgDrift defined wetland19. It is not likely that the full 

amount volatilized would be transported to a wetland of this size; the calculation is done only 

as a screening exercise. For purposes of this calculation, the Tier 1 AgDrift model was used 

to deliver 65% of the maximum application rate (1.5 lb/A) to the wetland; the spray drift 

features of the model were not used. (In the Aquatic Assessment window, a value for the 

application rate and a value for the Initial Average Deposition (Fraction of Applied =0.65) 

were entered to simulate deposition of the full 65% volatilized clomazone to the wetland). 

The result of this model run was an initial average concentration of 728 ppb. 

 

19 208.7 feet wide and about 6 inches deep
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 24 

2.2 Terrestrial Exposure 

2.2.1 Dietary-Based and Dose-Based Clomazone Residue Levels 

2.2.1.1 ON-FIELD

Results of the TREX v3.1 modeling of clomazone residue levels on those dietary food items 

which potentially occur on some treated fields, for mammals and birds20 are provided in 

Table 8 through Table 10. Results include both dietary-based values (i.e., milligrams of 

clomazone per kilogram of diet (mg/kg-diet)) and dose-based values (i.e., milligrams of 

clomazone per kilogram of body weight of the exposed animal (mg/kg-bw)), and both an 

upper-bound and mean estimate of these exposure levels. The upper-bound values are 

derived using the upper-bound Kenaga nomogram which used the highest of the highest 

normalized residue values from a number of studies, as a tolerance limit approach. The mean 

Kenaga value is based on the average of the highest normalized residue values from a 

number of studies, as modified by Fletcher et al. 1994. 

Only residues predicted from the clomazone use with the highest application rate, sweet 

potatoes, were modeled both as a broadcast application (1.5 lbs ai/A), and as a banded 

application (equivalent rate of 3.0 lbs ai/A in band)21 as a screening assessment. Additional 

modeling of residue levels on dietary items conducted as a refinement are presented and 

discussed in the Risk Characterization Section 4.2.1.1. For sweet potatoes a maximum 

distance between rows of 48 inches, which results in a minimum linear row area of 10,900 

feet per acre, and the assumption that the size of the band was half of the row distance (24 

inches or 2 feet) were used as conservative inputs to translate the broadcast label rate in lbs 

ai/A to an equivalent lbs ai/A within the band21 for exposure input into TREX v3.1. For the 

foliar half-life input, the default value of 35 days was used because all registered clomazone 

uses are limited to a single application in a given year resulting in the highest residue levels 

occurring at time of application, therefore foliar half-life is not considered in the EEC 

estimate. 

 

20 Birds are also used as a surrogate for reptiles and terrestrial-phase amphibians.

21

( )

( )( ) ( )

( )

( )( )( ) 43,560 ft /A 3.0 lbs ai/A

2 ft 10,900 linear ft/A

1.5 lbs ai/A

43,560 ft /A

treated row width, ft linear ft/A

Label Broadcast Applicatio n Rate , lb ai/A Equivalent lb ai/A rate on band

2

2

= =

=
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 25 

Table 8 Dietary-Based Clomazone EEC Values for the Highest Application Rate Crop, 

Sweet Potatoes, and for Both a Broadcast and a Banded Treatment 

Dietary-based EECs (ppm) 

Dietary Item Broadcast Banded 

Upper Bound Mean Upper Bound Mean 

Short Grass 360 128 720 255 

Tall Grass 165 54.0 330 108 

Broadleaf plants/sm Insects 202 67.5 405 135 

Fruits/pods/seeds/lg insects 22.5 10.5 45 21 

Table 9 Avian Dose-Based Clomazone EEC Values for the Highest Application Rate 

Crop, Sweet Potatoes, as a Banded Application Treatment 

Dose-based EECs (mg/kg-bw) 

Upper-Bound Mean 

for Birds of Body Weight: for Birds of Body Weight: 

Dietary Items 

20 g 100 g 1000 g 20 g 100 g 1000 g 

Short Grass 820 468 209 291 166 74.0 

Tall Grass 376 214 96.0 123 70.2 31.3 

Broadleaf plants/sm Insects 461 263 118 154 87.8 39.2 

Fruits/pods/seeds/lg insects 51 29.2 13.1 23.9 13.6 6.09 

Table 10 Mammalian Dose-Based Clomazone EEC Values for the Highest Application 

Rate Crop, Sweet Potatoes, as a Banded Application Treatment 

Dose-based EECs (mgt/kg-bw) 

Upper Bound Mean 

for Mammals of Body Weight: for Mammals of Body Weight: 

Functional Feeding Group 

 Dietary Item

15 g 35 g 1000 g 15 g 35 g 1000 g 

Herbivores/Insectivores

 Short Grass 686 474 110 242 168 38.3 

 Tall Grass 315 217 50.4 103 71.3 16.2 

 Broadleaf plants/sm Insects 386 267 61.9 128 89.1 20.3 

 Fruits/pods/seeds/lg insects 42.9 29.7 6.9 20.0 13.9 3.15 

 

Granivores

 Seeds 9.53 6.59 1.53 4.41 3.15 0.63 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 26 

Table 11 Mammalian Dose-Based Clomazone EEC Values for the Highest Application 

Rate Crop, Sweet Potatoes, as a Broadcast Treatment 

Dose-based EECs (mg/kg-bw) 

Upper-Bound Mean 

for Mammals of Body Weight: for Mammals of Body Weight: 

Functional Feeding Group 

 Dietary Items

15 g 35 g 1000 g 15 g 35 g 1000 g 

Herbivores/Insectivores

 Short Grass 343 237 55 121 84.2 19.1 

 Tall Grass 157 109 25.2 51.3 35.6 8.1 

 Broadleaf plants/sm Insects 193 133 30.9 64.1 44.6 10.1 

 Fruits/pods/seeds/lg insects 21.5 14.8 3.44 9.98 6.93 1.58 

Granivores

 Seeds 4.77 3.29 0.764 2.20 1.56 0.315 

An analysis of the available formulation acute oral toxicity data compared to data for the 

target grade active ingredient (TGAI) indicate that the Commence EC formulation, a multiple 

active ingredient mixture (clomazone and trifluralin), may be more toxic to terrestrial 

wildlife than indicated by the TGAI (Appendix D). Under these circumstances, a 

quantitative evaluation of exposure to the Commence EC formulation for mammals was 

considered. Exposure modeling in this instance was limited to dietary exposure to residues 

for a time period immediately following pesticide product application based on the 

expectation that the varying physical-chemical properties of the individual components of a 

formulation will result in progressively different formulation constituents in the exposure 

media over time and was limited to mammals because there is no avian formulation toxicity 

data to quantitatively assess risk to avian wildlife. The use of TGAI results appears 

appropriate for other formulations. Commence EC is registered for only two uses, cotton and 

soybeans. Modeled acute dose-based upper-bound and average EEC values for mammals for 

these crops under banded application (6.33 lbs formulation/A22) are provided in Table 12, 

where it is assumed that the banded application rate is equivalent to twice that of the 

broadcast rate. 

 

22

9.5

0.218 lbs clomazone 1 lb formulation

2.07 lbs clomazone/gal Formulation Weight (lbs/gal) = =

( ) 3.167

8 pts/gal

8/3 pts/acre Formulation Broadcast Application Rate (lbs/acre) 9.5 lbs formulation/gal =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = ; equivalent 

banded rate is 6.33 lbs formulation/acre 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 27 

Table 12 Mammalian Dose-Based Commence EC Formulation EEC Values for Cotton 

and Soybeans t Potatoes, as a Banded Treatment 

Dose-based EECs (mg/kg-bw) 

Upper-Bound Mean 

for Mammals of Body Weight: for Mammals of Body Weight: 

Functional Feeding Group 

 Dietary Items

15 g 35 g 1000 g 15 g 35 g 1000 g 

Herbivores/Insectivores

 Short Grass 1448 1001 232 511 355 80.7 

 Tall Grass 664 459 106 216 150 34.2 

 Broadleaf plants/sm insects 815 563 131 271 188 42.7 

 Fruits/pods/seeds/lg insects 90.5 62.6 14.5 42.1 29.2 6.65 

Granivores

 Seeds 20.1 13.9 3.22 9.31 6.65 1.33 

As birds are also used as a surrogate for reptiles and terrestrial-phase amphibians within this 

assessment, it is assumed that dose-based exposure levels for these animals are the same as 

birds (i.e., food ingestion rate per body weight for a bird is the same as for an amphibian and 

reptile). However, reptiles and amphibians have a lower ingestion rate for a given body 

weight than birds (USEPA 1993). Therefore, the dose-based EECs presented here represent 

an overestimate of exposure for reptiles and amphibians of a given body size. 

2.2.1.2 SPRAY-DRIFT

Not all dietary items will necessarily be present on a field at the time of treatment, for 

example, some uses of clomazone are applied after the field has been tilled in preparation for 

planting which would mean that dietary items such as short grass, tall grass, broadleaf plants, 

fruits and pods would not be present on the field. However, as clomazone is applied as a 

ground spray for all registered uses except rice, which also includes aerial applications, there 

is the potential for residues on dietary items to occur off the treated field due to spray-drift, 

and in this case, the potential exists for all these dietary items to be present. Therefore, the 

on-field estimates of exposure provided in Section 2.2.1.1 are also used to screen for residue 

levels that potentially pose a problem off a treated field due to spray drift. It is recognized 

that these estimates are an overestimate of exposure levels off the treated field. To address 

this concern, modeling of how far off a treated field residue levels would exceed LOC values 

was conducted as part of the refinement of risk in the Risk Characterization (Section 4.2.1.1). 

Where the presence/absence of a dietary item on the treatment site makes a difference in the 

conclusion about risk from clomazone, this is also discussed in the Risk Characterization. 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 28 

2.2.1.3 VOLATILIZATION

Residues on dietary items in areas near a treated field due to volatilization alone were 

estimated for the highest application rate crop, sweet potatoes. As the total mass applied on a 

per acre basis is the same for broadcast and banded applications, only the 1.5 lbs ai/A was 

modeled. A screening estimate of this exposure was obtained by assuming that 65% of 

applied clomazone per acre would volatilize under 100% humidity as discussed in Section 

2.1.4 and be deposited on one acre off site. As previously noted, this is a provisional model 

approach that has not been fully evaluated for use routinely in OPP risk assessments. Results 

of the TREX v3.1 modeling of residue levels on dietary food items for mammals and birds 

due to volatilization are provided in Table 13 through Table 15. Results include both dietarybased values (i.e., mg/kg-diet) and dose-based values (i.e., mg/kg-bw), and both an upperbound and mean estimate of these exposure levels. 

Table 13 Dietary-Based Clomazone EEC Values Due to Volatilization for the Highest 

Application Rate Crop, Sweet Potato at 1.5 lbs ai/A 

Dietary-based EECs (ppm) Dietary Item 

Upper Bound Mean 

Short Grass 234 82.9 

Tall Grass 107 35.1 

Broadleaf plants/sm Insects 132 43.9 

Fruits/pods/seeds/lg insects 14.6 6.83 

Table 14 Avian Dose-Based Clomazone EEC Values Due to Volatilization for the 

Highest Application Rate Crop, Sweet Potatoes at 1.5 lbs ai/A 

Dose-based EECs (mg/kg-bw) 

Upper-Bound Mean 

for Birds of Body Weight: for Birds of Body Weight: 

Dietary Items 

20 g 100 g 1000 g 20 g 100 g 1000 g 

Short Grass 267 152 68.0 94.5 53.9 24.0 

Tall Grass 122 69.7 31.2 40.0 22.8 10.2 

Broadleaf plants/sm Insects 150 85.5 38.3 50.0 28.5 12.7 

Fruits/pods/seeds/lg insects 16.7 9.50 4.25 7.78 4.44 1.98 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 29 

Table 15 Mammalian Dose-Based Clomazone EEC Values Due to Volatilization for the 

Highest Application Rate Crop, Sweet Potatoes at 1.5 lbs ai/A 

Dose-based EECs (mg/kg-bw) 

Upper-Bound Mean 

for Mammals of Body Weight: for Mammals of Body Weight: 

Functional Feeding Group 

 Dietary Items

15 g 35 g 1000 g 15 g 35 g 1000 g 

Herbivores/Insectivores

 Short Grass 223 154 35.8 78.7 54.7 12.4 

 Tall Grass 102 70.7 16.4 33.3 23.2 5.27 

 Broadleaf plants/sm Insects 125 86.7 20.1 41.7 29.0 6.58 

 Fruits/pods/seeds/lg insects 13.9 9.64 2.23 6.48 4.50 1.02 

Granivores

 Seeds 3.10 2.14 0.497 1.43 1.02 0.205 

2.2.2 Off-Field Terrestrial and Wetland/Riparian Plant Clomazone Exposure 

Levels 

2.2.2.1 RUNOFF AND SPRAY DRIFT LOADING

Estimated clomazone exposure levels for non-target plants in terrestrial and semi-aquatic 

areas in areas adjacent to a treated field are provided in Table 16. Exposure levels were 

estimated by calculating clomazone soil loading on a lb ai/acre basis from runoff and spray 

drift and by calculating foliar residues on a lb ai/acre basis from spray drift using TerrPlant 

v1.2.2. Input values for TerrPlant v1.2.2 modeling are listed in Table 17. Registered uses 

modeled include use on sweet potatoes at the maximum broadcast application rate of 1.5 lbs 

ai/A23 and use on succulent beans at its maximum broadcast application rate of 0.251 lbs ai/A 

by ground spray. These uses represent the upper and lower application rate range for ME 

formulations. In addition, estimated exposure due to spray drift from aerial applications for 

rice, which is the only ME use registered for application by air, was modeled. Exposure and 

associated risk for the other registered uses will fall between these EECs. 

The equations used in TerrPlant v1.2.2 to model loading EECs to non-target plants in 

terrestrial and semi-aquatic areas are shown in Table 16. Runoff is assumed to move to a one 

acre parcel from one acre of treated area for the dry upland model and from 10 acres of 

treated area for the low-lying semi-aquatic model. 

 

23 While the application rate within a band when band applied is higher than 1.5 lbs a.i./A, the total mass 

applied per acre is equivalent to the broadcast rate. 
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 30 

Table 16 Estimated Clomazone Loading to Off-Site Terrestrial and Semi-Aquatic 

Areas Adjacent to a Treated Field for Ground Spray Applications at the Upper and 

Lower Maximum Application Rate Range 

EEC (lbs ai/A) 

Ground Spray Aerial Spray 

Loading Equation Application 

Rate of 1.5 lbs 

ai/A 

Application 

Rate of 0.251 

lbs ai/A 

Application Rate 

of 0.6 lbs ai/A to 

rice 

Runoff to dry areas (A/I)*R 0.075 0.0126 N/A 

Runoff to semi-aquatic areas (A/I)*R*10 0.750 0.126 N/A 

Spray drift A*D 0.015 0.0025 0.030 

Total for dry areas ((A/I)*R)+(A*D) 0.090 0.0151 N/A 

Total for semi-aquatic areas ((A/I)*R*10)+(A*D) 0.765 0.128 N/A 

N/A = not applicable because rice fields are assumed to have berms around the field which typically prevents runoff. 

Table 17 Input Parameters for TerrPlant v1.2.2 Used for Modeling Runoff and Spray 

Drift Loading to Off-Field Terrestrial And Semi-Aquatic Areas Adjacent to a Treated 

Field 

Input Parameter Symbol Value Units Basis 

Application Rate A 1.5 for sweet potatoes 

0.251 for succulent beans 

0.6 for rice 

lbs ai/A Registered label maximum application 

range 

Incorporation depth I 1 inch Default value when there is not explicit 

incorporation on the label 

Runoff Fraction R 0.05 none Model guidance value when solubility of 

pesticide is >100 ppm (clomazone 

solubility is 1,100 ppm) 

Drift Fraction D 0.01 for ground spray 

0.05 for aerial spray 

none Model guidance values for ground spray 

and aerial spray applications 

2.2.2.2 VOLATILIZATION LOADING

An estimate of maximum exposure was obtained by assuming that 65% of applied 

clomazone per acre would volatilize under 100% humidity as discussed in Section 2.1.4 and 

be deposited on one acre off site24. Estimated loading to non-target plants in terrestrial or 

semi-aquatic areas near use areas, due to loading from volatilization alone, is contained in 

Table 18 for use on sweet potatoes and for use on succulent beans. The sweet potato and 

succulent bean uses represent the upper and lower label application range, respectively, of 

clomazone. As the total mass applied on a per acre basis is the same for broadcast and 

banded applications, only the broadcast lbs ai/A were modeled for these uses. 

 

24 Volatilization Loading, lb a.i./A = (Broadcast Loading, lb a.i./A)(0.65) = (1.5)(0.65) = 0.975 lb a.i./A for 

sweet potato and (0.251)(0.65) = 0.163 lb a.i./A for succulent beans.
Chapter 2 Exposure Assessment 

Registration Review Ecological Risk Assessment Clomazone 31 

Table 18 Off-Site Loading Due to Volatilization Based on Relative Humidity 

Off-site lbs ai/A Due to Volatilization at Relative Humidity of:

100% 1

 50% 2 25% 3

Sweet potato, 1.5 lbs ai/A 0.975 0.39 0.104

Succulent beans, 0.251 lbs ai/A 0.163 0.065 0.017

1 65% of applied volatilizes in the first 48 hours at 100% humidity from Section 2.1.4

2 26% of applied volatilizes in the first 48 hours at 50% humidity from Section 2.1.4

3 6.9% of applied volatilizes in the first 48 hours at 25% humidity from Section 2.1.4

The preceding estimate approach does not consider the impact to volatilization due to 

formulation differences (i.e., EC and WP versus ME) or effect of soil incorporation. Limited 

information on the impact of formulation to the rate of volatilization from soil has been 

submitted to the Agency, but it does not provide a sound quantitative relationship which can 

be used to model volatilization. The ME form is patented as reducing volatilization by 50% 

as compared to other formulations25. Assuming that the rate of volatilization discussed in 

Section 2.1.2 is reduced by 50% the loading due to volatilization for the ME formulation is 

provided in Table 18 for use on sweet potatoes and for use on succulent beans. 

Table 19 Off-Site Loading Due to Volatilization Assuming 50% of Rate Based on 

Relative Humidity 

Off-site lbs ai/A Due to Volatilization at Relative Humidity of: 

100% 1

 50% 2 25% 3

Sweet potato, 1.5 lbs ai/A 0.488 0.195 0.052 

Succulent beans, 0.251 lbs ai/A 0.082 0.033 0.009 

1 32.5% of applied volatilizes in the first 48 hours at 100% humidity (50% of 65% rate from Section 2.1.4).

2 13% of applied volatilizes in the first 48 hours at 50% humidity (50% of 26% from Section 2.1.4). 

3 3.45% of applied volatilizes in the first 48 hours at 25% humidity (50% of 6.9% from Section 2.1.4). 

 

25 http://www.wipo.int/pctdb/en/wo.jsp?wo=2007101019&IA=US2007062456&DISPLAY=DESC
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 32 

Chapter 3 Effects Characterization 

A detailed summary of all available toxicity data for clomazone, except mammalian data 

submitted to HED to support the human health risk assessment is provided in Appendix B 

and is based on registrant submitted data and on open literature study results for clomazone 

that have been reviewed and found to be scientifically sound. A complete characterization of 

the mammalian toxicity data set is provided in Memo, Kidwell et al., 2/13/01; D259635, and 

for clarity only a summary of the effects is provided in Appendix B. No new toxicity data 

have been submitted by the registrant since the Clomazone Summary Document Registration 

Review: Initial Docket (EPA 2007). However, additional toxicity data incorporated from the 

public literature, changes in analyses performed on seedling emergence data, and corrections 

for percent active ingredient resulted in changes to some of the previously reported values. 

Clomazone results that were found in the open literature but which were not used in the 

assessment and the reasons for their exclusion are provided in Appendix E. A summary of 

the clomazone toxicity data and the environmental levels at which effects are expected are 

described in this Chapter. 

3.1 Terrestrial Vertebrate Toxicology 

In mammals, the primary target organ for clomazone in rat, mouse, and dog studies is the 

liver. It is not classified as a carcinogen or mutagen. Clomazone is rapidly absorbed from 

the digestive tract after oral administration and is then rapidly and extensively metabolized 

by the liver. It is excreted predominately in the urine with most being excreted within 24 to 

48 hours post exposure, and almost 100% excreted by 7 days. Clomazone has a low potential 

for bioaccumulation as indicated by its high water solubility, low octanol-water partitioning 

coefficient, and rapid metabolism. 

Information is available on acute and chronic reproductive effects of clomazone in birds and 

mammals. No information was available on effects of clomazone on reptiles or to the 

terrestrial-phase of amphibians. Effects and effect levels in birds will be used as a surrogate 

for reptiles and the terrestrial-phase of amphibians in this assessment. Toxicological 

endpoints selected for terrestrial animal assessment endpoints are contained in Table 20. The 

dose-based mammalian reproduction value differs from that identified in the Clomazone 

Summary Document Registration Review: Initial Docket (EPA 2007a); this difference is 

discussed in detail in Section 3.1.2. An analysis of acute toxicity effects of the TGAI as 

compared to available formulation effects data (Appendix D), found that except for 

Commence EC use of the TGAI results to assess effects appear appropriate. Commence EC 
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 33 

appears to be more toxic to terrestrial wildlife than indicated by active ingredient effects 

testing. A quantitative analysis of acute dietary exposure to Commence EC was included in 

the assessment and the Commence EC mammalian acute oral toxicity value and body weight 

adjusted values are also included in Table 20 along with the TGAI effects data. 

Table 20 Summary of Specific Measurement Endpoint Values Selected to Calculate 

RQ Values to Evaluate Risk for Birds and Mammal Assessment Endpoints 

Selected Measurement Endpoint Value and Source 

Assessment 

Endpoint 

Measurement 

Endpoint Species Toxicological Endpoint 

and Effect(s) 

Source / 

Study 

Classification 

Survival and 

Reproduction of 

Birds, Reptiles 

and Amphibians 

Acute mortality: 

most sensitive avian 

acute oral toxicity, 

LD50 (single-dose) 

Mallard duck 

(Anas 

platyrhynchos) 

LD50 >2,230 mg ai/kg-bw 

(14-d post exposure observation) 

No mortality or behavior effects occurred at 

highest dose tested 

Acc # 248475 

Beavers et al., 

1982 / 

Acceptable 

 Mortality: most 

sensitive subacute 

avian LC50

Bobwhite quail 

(Colinus

virginianus) and 

Mallard duck (A. 

platyrhynchos) 

8-d LC50 > 4,990 mg ai/kg-diet (ppm ai) 

(8-d = 5 day exposure plus 3 day post exposure) 

No mortality, behavioral effects, or significant 

growth effects occurred at the highest dose tested 

Extrapolated values* 

8-d LC50 = 9, 731 ppm ai if probit slope =4.5 

8-d LC50 = 22,940 ppm ai if probit slope =2 

8-d LC50 = 6,905 ppm ai if probit slope =9 

Acc # 248475 

Beavers et al., 

1982 / 

Acceptable 

 Reproduction: most 

sensitive avian 

reproductive 

NOAEC 

Bobwhite quail (C. 

virginianus) 

20 week 

Offspring and paternal NOAEC >1,020 mg ai/kgdiet (or ppm ai) 

No toxicant-induced decrease observed in 

reproduction as measured by number of eggs laid, 

egg thickness, number of cracked eggs, eggs set, 

embryo viability, number hatched, number of 14-

d old survivors, and hatchling and survivor weight

MRID 45178101 

Frey et al., 1998/ 

Acceptable 

Survival and 

Reproduction of 

Terrestrial 

Mammals 

Acute mortality: 

most sensitive acute 

oral toxicity, LD50

(single-dose) 

Rat (Rattus 

norvegicus) 

LD50 = 1,369 mg ai/kg-bw (female) 

Allometric adjusted LD50 values for mammals of 

body weight: 

15 g: 3,009 mg ai/kg-bw 

35 g: 2,434 mg ai/kg-bw 

1000 g: 1,053 mg ai/kg-bw 

Acc # 00117121 

Seaman et al. 

1982 / 

Acceptable 

 Commence EC (Formulation) Value 

LD50 = 3,936 mg/kg-bw (female) 

Allometric adjusted LD50 values for mammals of 

body weight: 

15 g: 8,651 mg/kg-bw 

35 g: 6,999 mg ai/kg-bw 

1000 g: 3,027 mg ai/kg-bw 

MRID 43431302 

(Unpublished) 

Freeman, 1992/ 

Acceptable 
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 34 

Selected Measurement Endpoint Value and Source 

Assessment 

Endpoint 

Measurement 

Endpoint Species Toxicological Endpoint 

and Effect(s) 

Source / 

Study 

Classification 

 Reproduction: most 

sensitive 

reproductive 

NOAEL 

Rat, (R. 

norvegicus) 

2-generation 

Offspring and Parental NOAEL = 1000 mg ai/kgdiet 

[84.4 mg ai/kg-bw]** 

Offspring effects observed at LOAEL (2000 

mg/kg-diet) were reduced body weight in F2a and 

F2b litters 

Allometric adjusted NOAEL values for mammals 

of body weight: 

15 g: 194 mg ai/kg-bw 

35 g: 157 mg ai/kg-bw 

1000 g: 68.0 mg ai/kg-bw 

Acc # 00151108 / 

Acceptable 

*Back-calculated using default probit slope of 4.5, and lower and upper slopes of 2 and 9, number of test organisms, 95% 

upper bound on the estimate of the proportion of mortality, (see Appendix C). 

**Although a NOAEL of 50 mg/kg-bw/day was reported in the 2-generation reproduction study, the HIARC selected a 

NOAEL of 84.4 mg/kg-bw/day from the 2-year chronic rat study, since the dietary level was the same in both studies (i.e., 

1000 mg/kg-diet) but the 2-year chronic study measured actual intake rates instead of the standard conversion factor which 

was used in the 2-generation rat study. 

Summary Document Registration Review: Initial Docket (EPA 2007a); this difference is 

discussed in detail in Section 3.1.2. 

3.1.1 Effects on Birds 

Clomazone is practically non-toxic acutely to birds, as indicated by single oral dose LD50

values of >2,230 mg ai/kg-bw and dietary 8-d26 LC50 values of >4,990 mg ai/kg-diet for both 

northern bobwhite quail and mallard duck (Accession Number (Acc #) 248475). No 

mortalities were observed at this dose in any of the oral dose or dietary exposure studies. No 

effect on either growth or behavior (50 birds exposed) was detected in the northern bobwhite 

quail oral dose study or mallard duck oral dose or dietary exposure studies. There was a 

temporary reduction of food consumption observed on day 7 in the mallard duck oral dose 

study. While these studies do not directly provide a definitive LD50 (or LC50) an estimate of 

these values may be obtained using the Hill et al. (1975) dose-response model, for 

assumption of response slopes ranging from 2 to 9 (i.e., 2, 4.5, and 927), the number of 

organisms tested in the study at the exposure dose (or concentration), and the 95% upper 

bound on the proportion of mortality for the study (see Appendix C). This is a provisional 

method used in this assessment and will not be used to specifically calculate acute RQ values 

but will be used for characterization purposes only. The resulting back-calculated LD50

values range from 3,142 mg ai/kg-bw for a slope of 9 to 10,440 mg ai/kg-bw for a slope of 2 

 

26 Study duration consists of 5 days exposure plus 3 days post-exposure.
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 35 

for the mallard duck and bobwhite quail acute oral studies and the back-calculated 8-d LC50

values range from 6,905 ppm ai for a slope of 9 to 22,940 ppm ai for a slope of 2 for the 

mallard duck and bobwhite quail subacute dietary studies. Values adjusted for body weights 

of 20, 100, and 1000 g, which are used in estimation of exposure doses in Section 2.2.1 are 

provided in Table 21. 

Table 21 Body Weight Adjusted Acute Oral LD50 Values Based on Extrapolated 

Mallard Duck LD50 Values from Limit Test Results 

Body Weight Adjusted Acute Oral LD50 (mg/kg-bw) Values Using 

Extrapolated Mallard Duck Assuming Probit Response Slopes of 2, 4.5, and 9 

Body weight Slope 21

 Slope 4.51 Slope 91

20 g 5,421 2,300 1,631 

100 g 6,900 2,928 2,076 

1000 g 9,747 4,136 2,933 

1

Mallard duck extrapolated acute oral LD50 assuming response slope of 2 is 10,440 mg/kg-bw, assuming a 

response slope of 4.5 is 4.430 mg/kg-bw, and assuming a response slope of 9 is 3,142 mg/kg-bw (Appendix C). 

There is one avian reproduction study using northern bobwhite quail exposed to technical 

grade clomazone in the diet. Chronic exposure during maturation and reproduction in 

northern bobwhite quail to clomazone at 1,020 mg ai/kg-diet, the highest dietary 

concentration tested, did not result in any detectable effect on survival, food consumption, 

growth, behavior, or reproduction of adults and no detectable effect on survival or growth of 

chicks. Therefore, there is no definitive avian reproduction NOAEC value (i.e., bobwhite 

quail reproduction NOAEC >1,020 mg ai/kg-diet) however this value can still be used to 

estimate chronic LOC values. 

3.1.2 Effects on Mammals 

A complete characterization of the mammalian toxicity data submitted to support the 

registration of clomazone is presented in a Section 3 risk assessment for the use of 

clomazone on sugarcane (Memo, Kidwell et al., 2/13/01; D259635). Only a summation is 

provided herein. Clomazone is considered a low to moderate acute toxicant to mammals. 

The lowest acute oral dose LD50 is 1,369 mg ai/kg-bw for female rats and 2,077 mg ai/kg-bw 

for male rats in the same study (Acc # 00117121). Clomazone residues in the diet appear 

less toxic than that applied as a single bolus dose as the highest dietary concentration tested 

in a 90-d test, 8000 ppm (1,200 mg ai/kg-bw), exhibited no mortality (Acc # 00132586). 

Clomazone has low acute toxicity via dermal (LD50 >2000 mg ai/kg-bw in rabbits) and 

 27 Cross-section of pesticide response-slopes and typical slope (USEPA 1986, 2000)
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 36 

inhalation (LC50 = 4.23 mg ai/L-air) routes. It is non-irritating to the eyes and mildly 

irritating to the skin. 

The NOAEL for a 2-generation rat reproduction study (Acc # 00151108) was 1000 mg ai/kgdiet. Offspring effects at the LOAEL (2000 mg ai/kg-diet) were decreased body weights in 

the F2a and F2b litters and for the parental LOAEL (2000 mg ai/kg-diet) effects included 

decreased body weight and body weight gain during critical developmental times and kidney 

effects in males. The conversion of this dietary level to the body weight basis of 50 mg/kgbw was based on use of standard food conversion values to estimate consumption. The 

HIARC selected the NOAEL of 84.4 mg/kg-bw from a 2-year toxicity/carcinogenicity rat 

study as a sounder dose-basis value for observed effects on growth as compared to levels 

affecting growth in the 2-generation rat study since the 2-year rat study used actual 

consumption data (measured weekly). Developmental and subchronic effects all occur at 

dietary concentrations or doses higher than the 2-generation rat or 2-year rat study results. 

3.2 Terrestrial Invertebrate Toxicology 

Clomazone is practically non-toxic to honey bees, as indicated by an acute dermal contact 

48-h LD50 value of >100 μg ai/bee or >781 mg ai/kg-bee28 (MRID 45178102). This acute 

contact dose is converted to a residue level based upon an average fresh weight for a honey 

bee of 128 mg (Mayer and Johansen. 1990). While this study does not directly provide a 

definitive LD50 a conservative estimate of this value may be obtained using the Hill et al. 

(1975) dose-response model, the default slope of 4.5 and the upper and lower slopes of 2 and 

9, the number of organisms tested in the study at the exposure dose (or concentration), and 

the 95% upper bound on the proportion of mortality for the study (see Appendix C). This is 

a provisional approach used in this assessment and results are used for characterization 

purposes only. The resulting back-calculated dermal contact LD50 values from 135 μg ai/bee 

at a slope of 9 to 387 μg ai/bee at a slope of 2 (or 1,054 mg ai/kg-bee to 3,022 mg ai/kg-bee, 

respectively). These back-calculated LD50 values are not used to calculate acute RQ values 

but are used to evaluate the potential for risks to terrestrial invertebrates as prey items and 

direct effects to listed insects. 

 

28 Honey bee 48-h LC50, mg a.i./kg-bee = (48-h LC50, μg a.i. / bee) / (0.128 g / bee) = (100) / (0.128) = 781 μg 

a.i./g-insect = 781 ppm
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 37 

Table 22 Summary of Specific Measurement Endpoint Values Selected To Calculate 

RQ Values to Evaluate Risk for Terrestrial Invertebrate Assessment Endpoints 

Selected Measurement Endpoint Value and Source 

Assessment 

Endpoint 

Measurement 

Endpoint Species Toxicological Endpoint 

and Effect(s) 

Source / 

Study 

Classification 

Survival of 

Terrestrial 

Invertebrates and 

beneficial insects 

Acute mortality: 

most sensitive 

acute contact 48-h 

LD50 

Honey bee 

(Apis mellifera)

48-hr LD50 >100 μg ai/bee (or >781 mg ai/kginsect)] 

No mortality or behavioral effects noted at the 

highest dose tested 

Extrapolated values*: 

48-hr LD50 =182 μg ai/bee (or 1,421 mg ai/kginsect at slope = 4.5 

48-hr LD50 =387 μg ai/bee (or 3,022 mg ai/kginsect at slope = 2 

48-hr LD50 =135 μg ai/bee (or 1,054 mg ai/kginsect at slope = 9 

MRID 

45178102 

Nengel, 1998 / 

Acceptable 

*Back-calculated using default probit slope of 4.5 [and lower and upper bound slopes of 2 and 9], number of test organisms, 

95% upper bound on the estimate of the proportion of mortality (see Appendix C). 

There is also a subchronic test with earthworms exposed to clomazone residues in soil. The 

resultant 7-d and 14-d LC50 values were 215 and 156 mg ai/kg-soil. 

3.3 Fish Toxicology 

Information is available on acute effects and chronic reproductive effects of clomazone in 

fish. Chronic toxicity data were not available for the most sensitive acute freshwater species 

or for estuarine-marine species so a modeling approach using acute-to-chronic ratios (ACRs) 

was used to estimate the chronic NOAEC. No information was available on effects of 

clomazone on amphibians. Therefore, effects and effect levels in freshwater fish will be used 

as a surrogate for amphibians in this assessment. Toxicological endpoints selected for 

freshwater fish and estuarine-marine assessment endpoints are contained in Table 23. The 

resulting values differ from values used in previous risk assessments of clomazone because 

of new data found in public literature which provided lower endpoints. 

The target organ(s) and toxic mode-of-action of clomazone in fish is unknown. However, as 

with terrestrial vertebrates, based on bioconcentration studies clomazone is rapidly 

metabolized with accumulated levels being depurated within 72 hours post exposure, and 

almost 100% excreted by 14 days post-exposure. Clomazone has a low potential for 

bioaccumulation in fish as indicated by bioconcentration factors (BCFs) of 10, 75, and 40 for 

fillet, viscera, and whole fish, respectively. 
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 38 

Table 23 Summary of Specific Measurement Endpoint Values Selected To Calculate 

RQ Values to Evaluate Risk for Fish and Amphibian Assessment Endpoints 

Selected Measurement Endpoint Value and Source 

Assessment 

Endpoint 

Measurement 

Endpoint Species Toxicological Endpoint1

and Effect(s) 

Source / 

Study 

Classification 

Survival and 

reproduction of 

freshwater 

vertebrates (fish, 

amphibians) 

Acute mortality: 

most sensitive acute 

freshwater fish 96-h 

LC50

Silver catfish 

(Rhandia 

quelen) 

96-hr LC50 = 2,900 mg ai/L (or ppb ai) 

Effects on acetylcholinesterase levels were 

investigated but the reported results contained 

irreconcilable errors in the paper and are not 

appropriate for use. 

dos Santos Miron 

et al. 2004, 2005 /

Supplemental 

 Reproduction: most 

sensitive early life 

stage (ELS) 

NOAEL 

Modeled silver 

catfish value 

ELS NOAEL = 350 ppb ai See Section 3.3.1 

Survival and 

reproduction of 

marine/ estuarine 

fish 

Acute mortality: 

most sensitive acute 

freshwater fish 96-h 

LC50

Sheepshead 

minnow 

(Cyprinodon 

variegatus) 

96-hr LC50 = 40,600 ppb ai Acc # 073830 and 

073671 

Surprenant 1985 /

Supplemental 

 Reproduction: most 

sensitive early life 

stage (ELS) 

NOAEL 

Estimated 

sheepshead 

minnow value 

ELS NOAEL = 4,890 ppb ai See Section 3.3.2 

1 All concentrations provided are in the dissolved or soluble form. 

3.3.1 Effects to Freshwater Fish 

Clomazone acute toxicity ranges from slightly toxic to moderately toxic to freshwater fish. 

The acute effects of clomazone have been studied with three species of fish, two with 

technical grade clomazone submitted to support registration (Acc # 248475), and one with a 

formulation, Gamit, from the public literature (dos Santos Miron et al. 2004, 2005) but which 

is not a registered formulation in the U.S. Clomazone acute 96-h LC50 values for freshwater 

fish with technical grade clomazone range from 19,000 ppb ai for the cold water rainbow 

trout (Oncorhynchus mykiss) to 34,000 ppb ai for the warm water bluegill sunfish (Lepomis 

macrochirus) (Acc # 248475). The formulation test 96-h LC50 of 2,900 ppb ai for silver 

catfish (Rhandia quelen) is the lowest 96-h acute value. Because there are no paired 

clomazone technical and Gamit formulation species tests, it is unknown if the formulation is 

more toxic to fish than the active ingredient alone. As this is the lowest 96-h LC50 value and 

the effect of formulation is unknown, this value was selected as the more conservative of the 

available data. 

If the formulation is more toxic than the active ingredient alone, risks to freshwater fish may 

be overestimated. Also, there were a number of study conditions and other information not 
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 39 

reported in the silver catfish paper that are likely to affect the study and potentially result in 

the 96-h LC50 value being higher than that expected for a formulation under guideline 

conditions. These factors include the reported feeding of the fish during the test, aeration of 

the test chambers without measuring ai, use of plastic exposure chambers, unknown dilution 

water source, and unknown levels of total organic carbon (TOC) and particulate matter 

concentration in the dilution water. 

There is one fish early life stage study using rainbow trout exposed to technical grade 

clomazone in water (Acc # 073830). Fish were exposed from the eyed embryo stage (18-

days post-fertilization) through hatching. The NOAEC for this study was 2,290 ppb ai. 

Effects at the LOAEC included decreased survival of young and reduced growth as 

determined by both length and weight. While no effect on embryo hatchability was observed 

at concentrations at and below 8,450 ppb ai, none of the young survived to the end of the 

study at 8,450 ppb ai. 

Because silver catfish were more acutely sensitive than either rainbow trout or bluegill 

sunfish, and the only freshwater fish ELS NOAEC for rainbow trout is above the 

concentration that results in over 50% mortality in silver catfish, this rainbow trout NOAEC 

is not appropriate assess risks to the silver catfish or similar sensitive freshwater fish species. 

The silver catfish early life stage NOAEC for clomazone of 350 ppb ai29 was modeled by 

dividing the silver catfish 96-h LC50 value by a fish ACR derived from the rainbow trout 

results of 8.29730. 

Sublethal effects of clomazone on brain and muscle actylcholinesterase (AChE) were 

reported in the open literature (dos Santos et al. 2005) but a review of the paper identified 

inconsistencies which could not be resolved. The results as reported in the paper were 

considered unreliable (Appendix E). 

3.3.2 Effects to Estuarine-Marine Fish 

Effect information on clomazone to estuarine-marine fish is limited to a single acute study 

with the sheepshead minnow (Cyprinodon variegates). Clomazone is slightly toxic acutely 

to sheepshead minnow, as indicated by the 96-h LC50 value of 40,600 ppb ai (Accession 

Number (Acc # 263081). Sheepshead minnow are less sensitive than the freshwater fish 

tested. 

 

29 ppb ai ppb ai

ACR

h LC Silver catfish ELS NOAEC 350 8.29796 50 2,900 = = − =

30

8.297 2,290

96 50 19,000 = = − = ppb

ppb

ELS NOAEC

h LC Rainbow trout ACR
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 40 

No fish early life stage or life cycle data were found for estuarine-marine fish. Therefore, the 

fish ACR of 8.297 derived using the rainbow trout acute and chronic data, as discussed in 

Section 3.3.1, was used to model an early life stage NOAEC for the sheepshead minnow (C 

variegates) of 4,890 ppb ai31. 

3.4 Aquatic Invertebrate Toxicology 

Information is available on both acute effects and chronic reproductive effects of clomazone 

in aquatic invertebrates. Chronic toxicity data were not available for the most sensitive acute 

estuarine-marine species, the mysid shrimp (Americamysis bahia) so a modeling approach 

using a freshwater invertebrate ACR for clomazone was used to estimate the reproductive 

NOAEC for the mysid shrimp. Toxicological endpoints selected for freshwater and 

estuarine-marine invertebrate assessment endpoints are contained in Table 24. 

The target organ(s) and toxic mode-of-action of clomazone in invertebrates are unknown and 

no bioconcentration or metabolism studies were found for aquatic invertebrates. Because of 

clomazone’s Koc and the low BCF observed in fish, the potential for bioconcentration in 

aquatic invertebrates is also considered to be low. 

Table 24 Summary of Specific Measurement Endpoint Values Selected to Calculate 

RQ Values to Evaluate Risk for Aquatic Invertebrate Assessment Endpoints 

Selected Measurement Endpoint Value and Source 

Assessment 

Endpoint 

Measurement 

Endpoint Species Toxicological Endpoint1

and Effect(s) 

Source / 

Study 

Classification 

Survival and 

reproduction of 

freshwater 

invertebrates 

Acute mortality: 

most sensitive acute 

freshwater 

invertebrate 48-h or 

96-h EC50 or LC50

Water flea 

(Daphnia magna)

48-hr EC50 = 5,400 ppb ai Acc # 248475 

Graney, 1982 / 

Acceptable 

 

31 ppb ai ppb ai

ACR

h LC Sheepshead now ELS NOAEC 4,890 8.29796 40,600 min 50 = = − =
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 41 

Selected Measurement Endpoint Value and Source 

Assessment 

Endpoint 

Measurement 

Endpoint Species Toxicological Endpoint1

and Effect(s) 

Source / 

Study 

Classification 

 Reproduction: most 

sensitive 

reproductive 

NOAEL 

Water flea (D. 

magna) 

21-d NOAEC = 2,200 ppb ai 

Fecundity was the most sensitive endpoint 

Acc # 073997, 

073616 

Spare and Graney, 

1982 

Acceptable 

Survival and 

reproduction of 

marine/ estuarine 

invertebrates 

Acute mortality: 

most sensitive acute 

invertebrate 48-h or 

96-h EC50 or LC50

Mysid shrimp 

(Americamysis 

bahia formally 

Mysidopsis 

bahia) 

96-hr LC50 = 566 ppb ai Acc # 263080 

Williams, 1986 / 

Acceptable 

 Reproduction: most 

sensitive early life 

stage (ELS) 

NOAEL 

Modeled mysid 

shrimp 

reproduction 

Reproduction NOAEL = 68 ppb ai See Section 

1 All concentrations provided are in the dissolved or soluble form. 

3.4.1 Effects to Freshwater Invertebrates 

Scientifically sound acute and reproductive information on clomazone toxicity to freshwater 

invertebrates is limited to the waterflea Daphnia magna. Clomazone is considered 

moderately toxic acutely to D. magna, as indicated by the 48-h EC50 value of 5,400 ppb ai 

(Acc # 248475). The reproductive NOAEC for D. magna is 2,200 ppb ai and the LOAEC is 

4,380 ppb ai. Reproduction as measured by the number of young produced was the most 

sensitive measurement endpoint affected at the LOAEC, and was reduced by approximately 

40% from the control at this concentration. 

3.4.2 Effects to Estuarine-Marine Invertebrates 

There is an acute test with technical grade clomazone for a shrimp species, the mysid shrimp 

A. bahia, and a mollusk, the Eastern oyster Crassostrea virginica. The Eastern oyster study, 

examines the acute effects on shell deposition during 96 hours of exposure. Clomazone is 

considered highly toxic to aquatic invertebrates based on the acute 96-h LC50 value of 566 

ppb ai. Shell deposition in the Eastern oyster with a 96-h EC50 of 5,300 ppb ai is not as 

sensitive as mortality in the mysid shrimp. 

No reproductive effects data were found for estuarine-marine invertebrates. Therefore, the 

fish ACR of 8.297 was used to model a reproductive NOAEC for the mysid shrimp (A. 
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 42 

bahia) of 68 ppb ai32. The ACR for D. magna based on its acute and chronic data of 2.45433, 

appears smaller than that of fish but is within 10 times that of the fish value. Because there 

are no additional data by which to evaluate if there is a taxonomic effect on ACR values, and 

the values are within 10 times of each other, no taxonomic effect is assumed and the more 

conservative ACR of 8.297 was used to estimate the reproductive NOAEC for mysid shrimp. 

The general policy within EFED for pesticides is to use an ACR derived from the same taxon 

because the mode of action of some pesticides may be taxon specific. However, because in 

this case there is no known taxa specific mode-of-action between fish, aquatic insects, or 

other aquatic invertebrates and the difference in ACR values are within the standard 

acceptable range of variability for ACR values, the more sensitive of the two were used. 

3.5 Plant Toxicology 

Clomazone is an isoxazolidinone herbicide and there are no other registered compounds 

currently in this class. It inhibits carotenoid biosynthesis; the specific target enzyme is not 

known. Clomazone is absorbed by the roots and shoots and translocated upwards throughout 

the plant. Scientifically sound toxicity information on seedling emergence is available for 10 

crop species for both the technical grade active ingredient and a ME formulation. No 

scientifically sound vegetative vigor study is available. However, results from one vegetative 

study do provide insight on loading levels at which mortality and bleaching conservatively 

occurs. Aquatic plant toxicity data are available for the technical grade active ingredient for 

six species of aquatic plants and formulation data are available for one species. 

Toxicological endpoints selected for terrestrial plant and aquatic plant assessment endpoints 

are contained in Table 25. 

Table 25 Summary of Specific Measurement Endpoint Values Selected to Calculate 

RQ Values to Evaluate Risk for Terrestrial and Aquatic Plant Assessment Endpoints 

Selected Measurement Endpoint Value and Source 

Assessment 

Endpoint 

Measurement 

Endpoint Species Toxicological Endpoint 

and Effect(s) 

Source / 

Study 

Classification 

Survival and 

biomass of 

aquatic vascular 

and non-vascular 

plants 

Non-vascular species: the 

most sensitive biomass 

IC50 for non-listed and its 

associated NOAEC for 

listed species 

Freshwater diatom 

(Navicula 

pelliculosa) 

5-d IC50 = 167 ppb ai 

5-d NOAEC = 23.6 ppb ai 

MIRD 44994801 

Kranzfelder and 

Goble, 1999/ 

Acceptable 

 

32 ppb ai ppb ai

ACR

h LC Mysid shrimp reproductive NOAEC 68 8.27996 50 566 = = − =

33

2.454 2,200

48 50 5,400 = = − = ppb

ppb

Chronic NOAEC

h EC Waterflea ACR
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 43 

Selected Measurement Endpoint Value and Source 

Assessment 

Endpoint 

Measurement 

Endpoint Species Toxicological Endpoint 

and Effect(s) 

Source / 

Study 

Classification 

 Vascular species: the most 

sensitive biomass IC50 for 

non-listed and its 

associated NOAEC for 

listed species 

Duckweed 

(Lemna 

paucicostata) 

7-d IC50 = 30,200 ppb ai 

7-d NOAEC = 7,300 ppb ai 

Michel et al., 2004 

(Published) / 

Supplemental 

Survival and 

biomass of nontarget plants in 

terrestrial and 

semi-aquatic 

areas 

Monocots: the most 

sensitive seedling 

emergence or vegetative 

vigor IC25 for non-listed 

and its associated NOAEC 

for listed species 

Onion Seedling Emergence 

IC25 = 0.031 lbs ai/A 

NOAEC = 0.0062 lbs ai/A 

Most sensitive endpoint was height 

MRID 41557501 

Chetrem, 1990 / 

Acceptable 

 Dicots: the most sensitive 

seedling emergence or 

vegetative vigor IC25 for 

non-listed and its 

associated NOAEC for 

listed species 

Lettuce Seedling Emergence 

IC25 = 0.0037 lbs ai/A 

NOAEC = 0.0008 lbs ai/A 

Most sensitive endpoint was dry weight 

MRID 42165705 

Chetrem, 1991 / 

Acceptable 

3.5.1 Effects to Terrestrial Plants 

Scientifically sound toxicity information on seedling emergence is available for 10 crop 

species (monocots: corn, onion, and ryegrass; dicots: cucumber. soybean, carrot, cabbage, 

tomato, and lettuce) for both technical grade active ingredient (TGAI) clomazone (MRID 

41557501 and 42167505) and the formulation Command ME (MRID 45397701). For 

technical grade clomazone, the most sensitive IC25 value for test species ranges from >1.5 lbs 

ai/A for soybeans (emergence, survival, height, dry weight) to 0.0037 lbs ai/A for lettuce (dry 

weight). For Command 3ME, the most sensitive IC25 value for test species ranges from 

>1.25 lbs ai/A for soybeans (emergence, survival, height, dry weight) to 0.037 lbs ai/A for 

lettuce (length). For one species (soybean), the IC25 value for both TGAI and Command ME 

was greater than the exposure level tested, for five of the plant species the TGAI value is 

lower than the Command 3ME value, and for three species the TGAI value is higher than the 

Command 3ME value. In the cases where the Command 3ME IC25 values were lower they 

were on the same order and do not appear to indicate that this formulation is more toxic than 

the active ingredient alone. The comparison between active ingredient and formulation was 

made with the IC25 rather than the NOAEC (or IC05) values because it is the more accurate 

and precise endpoint measure. The most sensitive monocot and dicot results were for the 

TGAI with onion and lettuce, respectively. 

Seedling emergence IC25 results and the associated NOAEC (or IC05) values were used in the 

absence of vegetative vigor values for height and weight in determining risks to plants from 
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 44 

spray drift and volatilized clomazone off-site loading. Typically, the IC25 values from the 

vegetative vigor studies would be used to assess risk to non-target terrestrial plants from 

spray drift and volatilization since vegetative vigor studies measure the phytotoxic effect on 

plants via foliar contact or uptake of the toxicant. The seedling emergence studies are 

usually used to assess risk to non-target terrestrial plants from runoff of toxicant residues that 

would be taken up by the plant via roots or stems. Since only supplemental information on 

mortality is available from vegetative vigor studies to use for assessing risk to non-target 

terrestrial plants from spray drift exposure, it was decided to use the results of the seedling 

emergence studies to provide some measure of non-target plant risk since loading of spray 

drift and volatilized clomazone to soil can also be taken up by the roots and stems of nontarget plants. The level that results in mortality appears to be less sensitive for vegetative 

vigor than seedling emergence. 

As several plant species were tested a species sensitivity distribution of seedling emergence 

IC25 values for each measurement endpoint was determined using the median plotting 

position and these are displayed in Figure 3. The species sensitivity distribution will be used 

to evaluate the proportion of species that are above or below levels of concern in the risk 

characterization. 

As the percent emergent IC25 was greater than 1.5 lbs ai/A for all test species, this effect 

measure is not on the figure. For soybeans the IC25 was greater than 1.5 lbs ai/A for dry 

weight, height, and survival but for determination of the species sensitivity distribution these 

values were assumed to be equal to 1.5 lbs ai/A. For tomato and lettuce where there were 

two IC25 values for each measure of effect (MRID 41557501, 42165705), the geometric 

mean of these values were taken before calculating the median plotting position. Biomass of 

half of the species tested is reduced by 25% or more, as measured by height, at clomazone 

loads between 0.006 lbs ai/A to 0.1 lb ai/acre. Similarly biomass of the half of the species 

tested is reduced by 25% or more, as measured by dry weight, at clomazone loads between 

0.0037 lbs ai/A to 0.2 lbs ai/A. 
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 45 

Figure 3 Seedling Emergence IC25 Species Sensitivity Distribution for Clomazone 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.001 0.01 0.1 1 10

Concentration (lb ai/A)

Probability

Dry Weight

Height

Survival

ltc ltc ltc

tom oni oni

oni tom

rye

car

tom

crn

cab

cuc

oat

soy

rye

car

cab

oat

crn

cuc

rye

car

oat

cab

crn

cuc

IC25 Sensitivity Distribution

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.001 0.01 0.1 1 10

Concentration (lb ai/A)

Probability

Dry Weight

Height

Survival

ltc ltc ltc

tom oni oni

oni tom

rye

car

tom

crn

cab

cuc

oat

soy

rye

car

cab

oat

crn

cuc

rye

car

oat

cab

crn

cuc

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.001 0.01 0.1 1 10

Concentration (lb ai/A)

Probability

Dry Weight

Height

Survival

ltc ltc ltc

tom oni oni

oni tom

rye

car

tom

crn

cab

cuc

oat

soy

rye

car

cab

oat

crn

cuc

rye

car

oat

cab

crn

cuc

IC25 Sensitivity Distribution

cab = cabbage; car = carrot; crn = corn; cuc = cucumber; ltc = lettuce; oni = onion; rye = ryegrass; soy = soybean; tom = 

tomato 

A species sensitivity distribution of seedling emergence IC50 values for each measurement 

endpoint was determined using the median plotting position and these are displayed in Figure 

4. As the percent emergent IC50 was greater than 1.5 lbs ai/A for all test species, this effect 

measure is not on the display. Standing biomass of half of the species tested is reduced by at 

least 50% at clomazone loads between 0.013 lbs ai/A to 0.75 lbs ai/A. 
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 46 

Figure 4 Seedling Emergence IC50 Species Sensitivity Distribution for Clomazone 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.001 0.010 0.100 1.000 10.000

Concentration (lb ai/A)

Probability

Dry Weight

Height

Survival

IC50 Sensitivity Distribution

ltc ltc

tom

cab

rye

soy

car

oat

crn

cuc

oni

oni

tom

rye

car

cab

crn

cuc

cab

ltc

oni

tom

crn

rye

cuc

oat

soy

car

oat

soy

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.001 0.010 0.100 1.000 10.000

Concentration (lb ai/A)

Probability

Dry Weight

Height

Survival

IC50 Sensitivity Distribution

ltc ltc

tom

cab

rye

soy

car

oat

crn

cuc

oni

oni

tom

rye

car

cab

crn

cuc

cab

ltc

oni

tom

crn

rye

cuc

oat

soy

car

oat

soy

cab = cabbage; car = carrot; crn = corn; cuc = cucumber; ltc = lettuce; oni = onion; rye = ryegrass; soy = soybean; tom = 

tomato 

No scientifically sound vegetative vigor study is available. In vegetative vigor studies 

conducted either clomazone volatilized and cross-contaminated other treatments including 

the controls or environmental conditions were highly variable and differed between controls 

and treatments resulting in stunting and drying out of plants, which was unassociated with 

clomazone effects. Because there is no way of knowing the degree of affect on the 

magnitude and variability or spread in weight and height of controls, any estimates of growth 

(length and weight) effects of clomazone on treatments may be either underestimated or 

overestimated. However, results from one study with cross-contamination do provide an 

estimate as to dose levels at which bleaching and plant mortalities may occur when applied to 
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 47 

foliar vegetation. The clomazone dose levels at which discoloration were first observed and 

the level at which mortalities occurred are summarized in Table 26 for each test species. As 

cross-contamination occurred, as demonstrated by discoloration (bleaching) on some 

controls, the dose levels in Table 26 for discoloration are potentially lower than what would 

have been observed under no cross-contamination. 

Table 26 Vegetative Vigor Study Morphological Observation Results – Clomazone 

Dose at Which Discoloration and Mortalities were First Observed 

Discoloration First Observed Mortalities First Observed 

Plant 

Species lb 

ai/acre 

Comment Lb 

ai/acre 

Comment 

Cabbage 0.0054 2/4 reps with slight discoloration or 

stunting 

0.401 4/4 replicates had severe injury (mortalities or 

near mortalities); 

1.2 lbs ai/ A: 2/4 and 2/4 were observed to have 

sever injury and stand loss (>50% mortality), 

respectively 

Carrot 0.0164 3/4 reps slight discoloration or 

stunting 

0.401 4/4 replicates had severe injury (stand loss 

>50% mortality); 

1.20 lb ai/acre: 4/4 replicates were observed to 

have few plants remaining 

Corn 0.0054 1/4 reps slight discoloration or 

stunting 

1.20 4/4 replicates had severe injury (mortalities or 

near mortalities) 

Cucumber 0.016 2/4 reps with slight discoloration or 

stunting 

>1.20 no mortalities 

Lettuce 0.0019 3/4 reps slight discoloration or 

stunting 

1.20 4/4 replicates had severe injury (mortalities or 

near mortalities) 

Oat 0.0054 4/4 reps with slight discoloration or 

stunting 

0.414 4/4 replicates had severe injury (stand loss 

>50% mortality) 

1.20 lbs ai/A: 4/4 replicates were observed to 

have few plants remaining 

Onion 

(study 1) 

0.0019 4/4 reps with slight discoloration or 

stunting 

0.401 1/4 replicates had severe injury (mortalities or 

near mortalities); 

1.20 lbs ai/A: 3/4 and 1/4 reps had stand loss 

(>50% mortalities0 and few plants remaining, 

respectively 

Onion 

(study 2) 

0.046 2/4 reps with slight discoloration or 

stunting 

0.414 2/4 replicates had severe injury (mortalities or 

near mortalities); 

1.23 lbs ai/A: 4/4 replicates had stand loss 

(>50% mortalities) 

Ryegrass 0.016 2/4 reps with slight discoloration or 

stunting 

1.20 1/4 replicates had severe injury (mortalities or 

near mortalities) 

Soybean 1.20 2/4 reps with slight discoloration or 

stunting 

>1.23 no mortalities 

Tomato 0.0019 2/4 reps with slight discoloration or 

stunting 

>1.20 no mortalities 

The lowest test dose was 0.0019 lb ai/acre for lettuce, onion, and tomato and 0.0054 lb ai/acre for the other test species. 
Chapter 3 Effects Characterization 

Registration Review Ecological Risk Assessment Clomazone 48 

3.5.2 Effects to Aquatic Plants 

Aquatic plant toxicity data is available for the technical grade or analytic grade active 

ingredient for seven species of aquatic plants. The growth 7-d IC50 values for three 

duckweed species range from 30,200 to 48,800 ppb ai. Nonvascular plant growth was more 

sensitive to clomazone with growth 4-d IC50 and 5-d IC50 values ranging from 115 ppb ai to 

11,100 ppb ai. For vascular plants, the lowest 7-d IC50 value for L. paucicostata of 30,200 

ppb ai was selected and for nonvascular plants the lowest 4-d IC50 value of 115 ppb ai for the 

freshwater diatom, N. pelliculosa. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 49 

Chapter 4 Results: Risk Characterization 

4.1 Risk Estimation 

4.1.1 Direct Effects to Non-Target Terrestrial Vertebrates 

4.1.1.1 DIRECT EFFECTS TO BIRDS, REPTILES, AND (LAND-PHASE) AMPHIBIANS

No dietary- or dose-based acute RQ values could be calculated because no definitive acute 

values were available. At the highest dietary level tested (4,900 mg ai/kg-diet) and dose 

tested (2,230 mg ai/kg-bw) neither mallard duck or bobwhite quail had mortality or sublethal 

effects (N = 50 birds per test). Chronic RQ values were calculated for upper bound dietarybased residue levels based on the highest clomazone application rate, sweet potato banded 

rate of 3.0 lb ai/acre (Table 27). These chronic RQ values were all below the chronic LOC 

(1.0). As these are the highest residue levels of any use, all other labeled uses also have 

dietary-based chronic RQ values below the chronic LOC. Because residues from volatilized 

and redeposited clomazone are expected to be lower (Table 13) this pathway for any use will 

also result in chronic RQ values below the chronic LOC. For dietary-based results, as birds 

are used as a surrogate for reptiles and the terrestrial-phase of amphibians in this assessment 

no chronic LOC are expected to be exceeded for these groups either. 

Table 27 Avian chronic dietary-based RQ values for a banded application on sweet 

potatoes 

Dietary Item EEC (mg/kg-diet) 1 Chronic RQ 2

Short Grass 720 <0.7 

Tall Grass 330 <0.3 

Broadleaf plants/sm Insects 405 <0.4 

Fruits/pods/seeds/lg insects 45.0 <0.1 

1 Dietary-based residue levels for a banded application on sweet potatoes from Table 8.

2 Chronic RQ = (EEC, mg/kg-diet) / chronic NOAEC, mg/kg-diet; where the chronic NOAEC is > 1,020 mg/kg-diet for 

bobwhite quail from Table 20.

4.1.1.2 DIRECT EFFECTS TO MAMMALS

For banded application of clomazone on sweet potatoes which is the highest application rate, 

and assuming exposure occurs at upper bound residue estimates, dose-based acute and 

chronic RQ values do not exceed acute endangered LOC or chronic LOC for mammals 

ingesting seeds, fruits, pods, and/or large insects. However, dose-based acute and chronic 

RQ values do exceed the acute endangered LOC and the chronic LOC, respectively, for 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 50 

mammals ingesting short grass, tall grass, broadleaf plants, or small insects (Table 28). For 

broadcast applications of clomazone to sweet potatoes where exposure to upper bound 

residue levels is assumed, the acute endangered LOC and chronic LOC are exceeded only for 

residues on short grass for 15 g and 35 g mammals (Table 29). To determine at which 

application level the dose-based RQ values drop below the endangered acute and chronic 

LOC, dose-based acute and chronic RQ values were calculated for application rates ranging 

from 0.75 lbs ai/A to 2.5 lbs ai/A for short grass, the dietary group with the highest residue 

level of the five evaluated (Table 30). There are application rates below 0.75 lbs ai/A, but at 

this application rate all acute and chronic RQ values are below the endangered acute LOC 

and chronic LOC, respectively. 

For the volatilization pathway, the highest dose-based EEC values (Table 15) are lower than 

those in Table 11 for the highest pound per acre application rate (1.5 lb ai/acre, broadcast or 

3.0 lbs ai/A in bands). Therefore, in addition to those acute RQ values that are at or below 

the acute endangered species LOC for the upper bound broadcast RQ values (Table 29), all 

dose-based short grass acute RQ values are below the LOC (15 g, acute RQ = 0.07; short 

grass 35 g, acute RQ = 0.06) and except for the 15 g mammal all chronic RQ values are 

below the chronic LOC (15 g, chronic RQ = 1.1; 35 g, chronic RQ = 0.98). 

While some dose-based chronic RQ values exceeded the chronic LOC, dietary-based chronic 

RQ values for upper bound dietary EEC values for all dietary items at the highest banded 

application rate do not exceed the chronic LOC (Table 31). Because residue levels from 

redeposited clomazone at the highest application rate (1.5 lbs ai/A broadcast which is 

equivalent to 3.0 lbs ai/A) are expected to be lower (Table 13) the dietary-based chronic RQ 

for this pathway will also not exceed the chronic LOC on a dietary-basis. 

Acute dose-based RQ values based on upper bound and mean Commence EC formulation 

dietary residue levels for banded application on cotton and soybean sites are provided in 

Table 32. All acute RQ values for dietary items were below the acute endangered species 

LOC (0.1) except for short grass at upper bound residue estimates. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 51 

Table 28 Upper bound acute and chronic clomazone dose-based RQ values for 

mammals for a banded application on sweet potatoes 

Functional Feeding Group 15 g mammal 35 g mammal 1000 g mammal 

 Dietary Item Acute1

 Chronic2 Acute1 Chronic2 Acute1 Chronic2

Herbivores/Insectivores 

 Short Grass 0.23 3.5 0.19 3.0 0.10 1.6 

 Tall Grass 0.10 1.6 0.09 1.4 0.05 0.7 

 Broadleaf plants/sm insects 0.13 2.0 0.11 1.7 0.06 0.9 

 Fruits/pods/lg insects 0.01 0.2 0.01 0.2 0.01 0.1 

Granivore 

 Seeds <0.01 <0.1 <0.01 <0.1 <0.01 <0.1 

Bolded acute RQ values exceed the endangered LOC of 0.1, bolded and italicized acute RQ values exceed the acute 

restricted use LOC of 0.2, and the bolded chronic RQ values exceed the chronic LOC of 1. 

1 Acute RQ = (dose-based upper bound EEC, mg/kg-bw) / (LD50; mg/kg-bw). Upper bound dose-based EEC values for a 

given body weight are from Table 10 and LD50 values adjusted for the given body weight are from Table 20.

2 Chronic RQ = (dose-based upper bound EEC, mg/kg-bw) / (chronic NOAEC, mg/kg-bw). The upper bound dose-based 

EEC values for a given body weight and dietary item are from Table 11 and the NOAEC values adjusted for a given body 

weight are from Table 20.

Table 29 Upper bound acute and chronic clomazone dose-based RQ values for 

mammals for a broadcast application on sweet potatoes 

Functional Feeding Group 15 g mammal 35 g mammal 1000 g mammal 

 Dietary Item Acute1 Chronic2 Acute1 Chronic2 Acute1 Chronic2

Herbivores/Insectivores 

 Short Grass 0.11 1.8 0.10 1.5 0.05 0.8 

 Tall Grass 0.05 0.8 0.04 0.7 0.02 0.4 

 Broadleaf plants/sm insects 0.06 1.0 0.05 0.8 0.03 0.5 

 Fruits/pods/lg insects 0.01 0.1 0.01 0.1 <0.01 0.1 

Granivore 

 Seeds <0.01 <0.1 <0.01 <0.1 <0.01 <0.1 

Bolded acute RQ values exceed the endangered LOC of 0.1 and the bolded chronic RQ values exceed the chronic LOC of 1. 

1 Acute RQ = (upper bound dose-based EEC, mg/kg-bw) / (LD50; mg/kg-bw). The upper bound EECs for a given body 

weight are from Table 11 and LD50 values adjusted for the given body weight are in Table 20

2 Chronic RQ = (upper bound dose-based EEC, mg/kg-bw) / (chronic NOAEC, mg/kg-bw). The upper bound dose-based 

EEC values for a given body weight and dietary item are from Table 11 and the NOAEC values adjusted for a given body 

weight are in Table 20.
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 52 

Table 30 Upper bound acute and chronic clomazone dose-based RQ values for 

mammals ingesting short grass at application rates from 0.75 lbs ai/A to 2.5 lbs ai/A 

 15 g mammal 35 g mammal 1000 g mammal 

15-g 35-g 1000-g Acute Chronic Acute Chronic Acute Chronic 

 Short Grass 2.5 lb/A 572 395 92 0.19 2.9 0.16 2.5 0.09 1.3 

 Short Grass 1.5 lb/A 343 237 55.0 0.11 1.8 0.10 1.5 0.05 0.8 

Short Grass 1.25 lb/A 286 198 45.8 0.10 1.5 0.08 1.3 0.04 0.7 

Short Grass 1.0 lb/A 229 158 36.7 0.08 1.2 0.06 1.0 0.03 0.5 

Short Grass 0.75 lb/A 171 119 27.5 0.06 0.9 0.05 0.8 0.03 0.4 

Bolded acute RQ values exceed the endangered LOC of 0.1 and the bolded chronic RQ values exceed the chronic LOC of 1. 

1 Acute RQ = (dose-based upper bound EEC, mg/kg-bw) / (LD50; mg/kg-bw). The LD50 values adjusted for the given body 

weight are from Table 20.

2 Chronic RQ = (dose-based upper bound EEC, mg/kg-bw) / (chronic NOAEC, mg/kg-bw). The NOAEC values adjusted 

for a given body weight are from Table 20.

Table 31 Mammalian chronic dietary-based clomazone RQ values for a banded 

application on sweet potatoes 

Dietary Item Dietary-based EEC (mg/kg-diet) Chronic RQ 

Short grass 720 0.7 

Tall grass 330 0.3 

Broadleaf plants/sm insects 405 0.4 

Fruits/pods/lg insects 45 <0.1 

1 Dietary-based residue levels for a banded application on sweet potatoes from Table 8.

2 Chronic RQ = (EEC, mg/kg-diet) / chronic NOAEC, mg/kg-diet; the chronic NOAEC > 1,020 mg/kg-diet for bobwhite 

quail from Table 20.

Table 32 Upper bound and average acute Commence EC dose-based RQ values for 

mammals for a banded application on cotton or soybeans 

Functional Feeding Group Upper bound acute RQ values Average RQ values 

 Dietary Item 15-g 35-g 1000-g 15-g 35-g 1000-g 

Herbivores/Insectivores 

 Short Grass 0.17 0.14 0.08 0.05 0.05 0.03 

 Tall Grass 0.08 0.06 0.04 0.02 0.02 0.01 

 Broadleaf plants/sm insects 0.09 0.08 0.04 0.03 0.03 0.01 

 Fruits/pods/lg insects 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 

Granivore 

 Seeds <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 

Bolded acute RQ values exceed the endangered LOC of 0.1 

1 Acute RQ = (dose-based EEC, mg/kg-bw) / (LD50; mg/kg-bw). EEC values for a given body weight are from Table 12 and 

LD50 values adjusted for the given body weight are from Table 20.
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 53 

4.1.2 Direct Effects to Non-Target Terrestrial Invertebrates 

No RQ values were calculated for beneficial or listed insects because no definitive acute 

contact LD50 value was available. 

4.1.3 Direct Effects to Terrestrial and Wetland/Riparian Plants 

4.1.3.1 RUNOFF AND SPRAY DRIFT LOADING

Table 33 contains the RQ values for listed and non-listed plants in dry and semi-aquatic areas 

adjacent to a treated field for the highest application rate, sweet potato at 1.5 lbs ai/A 

(equivalent to 3.0 lbs ai/A applied in a band) ground applied and the lowest label rate of 

0.251 lbs ai/A, ground applied, for use on beans (succulent), melons (musk), and watermelon 

crops. Except for wet seeded rice, which is registered for aerial applications, these values 

cover the expected RQ range for label rates of registered clomazone uses. Table 33 also 

contains the RQ values for aerial spray of clomazone on rice. Because these rice fields will 

have berms to maintain flood waters, runoff to adjacent areas is not expected and the risks 

estimated are therefore applicable to spray drift alone. 

Table 33 RQ values for plants in dry and semi-aquatic areas exposed to clomazone 

through runoff and/or spray drift 

Application Type, Rate 

Plant Type Listed Status Dry Semi-Aquatic Spray Drift 

Ground Spray, 1.5 lb ai/acre 

Monocot non-listed1 2.9 25 0.5 

Monocot listed2 15 123 2.4 

Dicot non-listed 24 207 4.0 

Dicot listed 47 403 7.9 

Aerial Spray, Rice 0.6 lb ai/acre 

Monocot non-listed -- -- 1.0 

Monocot Listed -- -- 4.8 

Dicot non-listed -- -- 8.1 

Dicot listed -- -- 16 

Ground Spray, Rice 0.6 lb ai/acre 

Monocot non-listed -- -- 0.2 

Monocot Listed -- -- 1.0 

Dicot non-listed -- -- 1.6 

Dicot Listed -- -- 3.2 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 54 

Application Type, Rate 

Plant Type Listed Status Dry Semi-Aquatic Spray Drift 

Ground Spray, 0.251 lb ai/acre 

Monocot non-listed 0.5 4.1 <0.1 

Monocot Listed 2.4 21 0.4 

Dicot non-listed 4.1 35 0.7 

Dicot listed 7.9 67 1.3 

Bolded listed RQ values exceed the endangered species LOC of 1, and bolded non-listed RQ values exceed the non-listed 

species LOC of 1. 

1 Monocot (or dicot) non-listed RQ = (EEC, lb ai/acre) / (IC25, lb ai/acre), where the EEC for dry, semi-aquatic, or spray 

drift is from Table 16 and the monocot (or dicot) IC25 value is from Table 25.

2 Monocot (or dicot) listed RQ = (EEC, lb ai/acre) / (NOAEC, lb ai/acre), where the EEC for dry, semi-aquatic, or spray 

drift is from Table 16 and the monocot (or dicot) NOAEC value is from Table 25.

4.1.3.2 VOLATILIZATION LOADING

Table 34 contains RQ values for listed and non-listed plants exposed to clomazone that 

volatilizes off a treated field with subsequent near-field redeposition for the highest 

application rate crop, sweet potato, and the lowest maximum label rate, use on beans 

(succulent), melons (musk), and watermelon crops. Estimated near field loading due to 

volatilization alone is 0.975 lbs ai/A for use on sweet potatoes and 0.163 lbs ai/A for use on 

succulent beans, assuming 65% volatilization at 100% humidity conditions on day of 

application (Section 2.2.2.2). These RQ values would be applicable to terrestrial upland 

plants and wetland/riparian plants. The assumption of 100% humidity in conjunction with 

the assumption that no dilution occurs over the travel distance provides an upper bound 

estimate which in reality is expected to be lower either due to the humidity level being lower 

and/or aerial dispersion occurring over the travel distance. RQ values associated with 

volatilization at lower humidity levels of 50% and 25% with assumed subsequent loading 

with no dispersion are provided in Table 34. 

Table 34 RQ values for near field plants exposed to clomazone from volatilization 

Sweet Potato Use Potential Off-Site 

Loading from Volatilization at 

Humidity of: 

Succulent Beans Use Potential OffSite Loading from Volatilization at 

Humidity of: Plant Group 

100% 50% 25% 100% 50% 25% 

Monocot Non-listed1 31 13 3.4 5.3 2.1 0.5 

Monocot Listed2 157 63 17 26 10 2.7 

Dicot Non-listed1 264 105 28 44 18 4.6 

Dicot Listed2 513 205 55 86 34 8.9 

Bolded listed RQ values exceed the endangered species LOC of 1, and bolded non-listed RQ values exceed the non-listed 

species LOC of 1. 1 Monocot (or dicot) non-listed RQ = (EEC, lb ai/acre) / (IC25, lb ai/acre), where the EEC is the volatilization loading and 

the monocot (or dicot) IC25 value is from Table 25.

2 Monocot (or dicot) listed RQ = (EEC, lb ai/acre) / (NOAEC, lb ai/acre), where the EEC is the volatilization loading and 

the monocot (or dicot) NOAEC value is from Table 25.
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 55 

Table 35 RQ values for near field plants exposed to clomazone from volatilization 

Sweet Potato Use Potential Off-Site 

Loading from Volatilization at 

Humidity of: 

Succulent Beans Use Potential OffSite Loading from Volatilization at 

Humidity of: Plant Group 

100% 50% 25% 100% 50% 25% 

Monocot Non-listed1 16 6 1.7 2.6 1.1 0.3 

Monocot Listed2 79 31 8.0 13 5.0 1.5 

Dicot Non-listed1 132 53 14 22 9.0 2.4 

Dicot Listed2 257 103 27 43 17 4.7 

Bolded listed RQ values exceed the endangered species LOC of 1, and bolded non-listed RQ values exceed the non-listed 

species LOC of 1. 

1 Monocot (or dicot) non-listed RQ = (EEC, lb ai/acre) / (IC25, lb ai/acre), where the EEC is the volatilization loading and 

the monocot (or dicot) IC25 value is from Table 25.

2 Monocot (or dicot) listed RQ = (EEC, lb ai/acre) / (NOAEC, lb ai/acre), where the EEC is the volatilization loading and 

the monocot (or dicot) NOAEC value is from Table 25.

4.1.4 Direct Effects to Freshwater Fish (Amphibians) and Plants 

4.1.4.1 LABEL USES OTHER THAN RICE

Table 36 contains the acute and chronic RQ values for freshwater fish (amphibians)34, 

aquatic invertebrates and aquatic plants for uses representative of all clomazone uses and use 

rates. The EEC values used to calculate these RQ values were based on the conservative 

assumption for clomazone load in runoff, that none of the clomazone volatilizes from the soil 

due to displacement by water vapor (i.e., “without” volatilization). None of the animal acute 

RQ values exceed the acute endangered species LOC of 0.05 and none of the chronic RQ 

values exceed the chronic LOC of 1.0. The vascular non-listed and listed species and nonvascular non-listed RQ values do not exceed the non-listed or listed species plant LOC (1.0). 

Table 36 RQ values for freshwater fish, invertebrates, and plants based on 1-in-10 

year acute and chronic exposure concentrations in a vulnerable receiving water 

Freshwater Fish Freshwater 

Invertebrates 

Vascular Plants Nonvascular

Acute1 Chronic2 Acute1 Chronic2 NonList3 List4 NonList3

PRZM Scenario (standard 

unless noted) 

Without Without Without Without Without Without Without 

Florida sugarcane 0.01 <0.1 0.01 <0.1 <0.5 <0.5 <0.5 

Louisiana sugarcane 0.01 <0.1 <0.01 <0.1 <0.5 <0.5 <0.5 

Mississippi cotton, 1.25 lb/A 0.01 <0.1 0.01 <0.1 <0.5 <0.5 <0.5 

North Carolina cotton, 1.25 lb/A 0.01 <0.1 <0.01 <0.1 <0.5 <0.5 <0.5 

Florida peppers 0.01 <0.1 0.01 <0.1 <0.5 <0.5 <0.5 

Mississippi soybeans, 1.25 lb/A 0.01 <0.1 <0.01 <0.1 <0.5 <0.5 <0.5 

North Carolina sweet potato <0.01 <0.1 <0.01 <0.1 <0.5 <0.5 <0.5 

North Carolina tobacco <0.01 <0.1 <0.01 <0.1 <0.5 <0.5 <0.5 

Florida potato (NMC)1 0.01 <0.1 0.01 <0.1 <0.5 <0.5 <0.5 

Michigan bean <0.01 <0.1 <0.01 <0.1 <0.5 <0.5 <0.5 

Oregon snap beans <0.01 <0.1 <0.01 <0.1 <0.5 <0.5 <0.5 

 

34 Fish results are used as a surrogate for risks to aquatic-phase amphibians.
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 56 

Freshwater Fish Freshwater 

Invertebrates 

Vascular Plants Nonvascular

Acute1 Chronic2 Acute1 Chronic2 NonList3 List4 NonList3

PRZM Scenario (standard 

unless noted) 

Without Without Without Without Without Without Without 

Florida cabbage <0.01 <0.1 <0.01 <0.1 <0.5 <0.5 <0.5 

Florida cucumber, 1 lb ai/acre 0.01 <0.1 0.01 <0.1 <0.5 <0.5 <0.5 

Florida cucumber, 0.75 lb ai/acre 0.01 <0.1 0.01 <0.1 <0.5 <0.5 <0.5 

Florida cucumber, 0.5 lb ai/acre 0.01 <0.1 <0.01 <0.1 <0.5 <0.5 <0.5 

Florida cucumber, 0.25 lb ai/acre <0.01 <0.1 <0.01 <0.1 <0.5 <0.5 <0.5 

Oregon mint, 0.5 lb/A <0.01 <0.1 <0.01 <0.1 <0.5 <0.5 <0.5 

1 Acute RQ = (acute EEC, ppb) / (acute LC50 or EC50, ppb), where the EEC is the 24-h 1 in 10 year peak without 

volatilization (Table 52) and the fish 96-h LC50 is 2,900 ppb for silver catfish (Table 23) and the invertebrate 48-h EC50 is 

5.400 ppb for D. magna (Table 24).

2 Chronic RQ = (chronic EEC, ppb) / (chronic NOAEC, ppb), where the EEC is the 60-d 1 in 10 year peak for fish and the 

21-d 1-in-10 year peak for invertebrates without volatilization (Table 52) and the fish chronic NOAEC is 350 ppb and the 

invertebrate chronic NOAEC is 2,200 ppb (Table 23 and Table 24, respectively).

3 Non-listed Plant RQ = (EEC, ppb) / (IC50, ppb), where the EEC is the 24-h 1 in 10 year peak without volatilization (Table 

52) and the IC50 is 30,200 ppb for vascular plants and 167 ppb for non-vascular plants (Table 25).

4 Listed Plant RQ = (EEC, ppb) / (NOAEC, ppb), where the EEC is the 24-h 1 in 10 year peak without volatilization (Table 

52) and the NOAEC is 7,300 ppb (Table 25).

Table 37 contains the acute and chronic RQ values for estuarine-marine fish, invertebrates 

and aquatic plants at 1-in-10 year acute and chronic EEC levels for all clomazone uses and 

use rates. Conservatively assuming no volatilization occurs off the field (“without” 

volatilization), no chronic LOC for fish or invertebrates is exceeded for any use and except 

for invertebrate acute RQ values for Florida sugarcane and cucurbits (also representative of 

squash and melons) at 1.25 and 1 lbs ai/A, respectively, none of the other uses exceed acute 

endangered LOC for fish or invertebrates. No listed or non-listed LOC is exceeded for 

vascular plants, no non-listed LOC is exceeded for non-vascular plants and except for 

sugarcane, pepper, and cucurbit (also representative of squash and melon) uses at 0.75 to 

1.275 lbs ai/A, none of the other uses exceed the listed LOC for listed non-vascular plants. 

Accounting for volatilization on the field (“with” volatilization) results in no exceedence of 

the acute listed species LOC or chronic LOC for estuarine fish and invertebrates, or 

exceedence of the listed aquatic plant LOC. 

Table 37 RQ values for estuarine-marine fish, invertebrates, and aquatic plants based 

on 1-in-10 year acute and chronic exposure concentrations in a vulnerable receiving 

water, With and Without PRZM Volatilization 

Estuarine 

Fish 

EstuarineInvertebrates Vascular 

Plants 

Non-Vascular 

Plants 

Acute1 Chronic2 Acute1 Acute1 Chronic2 NonList3 Listed4 NonList3 Listed4

PRZM Scenario 

(standard unless 

noted) 

Without Without Without With Without Without Without Without Without With 

Florida sugarcane 

1.25 lb ai/acre <0.01 <0.1 0.06 <0.01 <0.1 <0.5 <0.5 <0.5 1.5 1.0 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 57 

Estuarine 

Fish 

EstuarineInvertebrates Vascular 

Plants 

Non-Vascular 

Plants 

Acute1 Chronic2 Acute1 Acute1 Chronic2 NonList3 Listed4 NonList3 Listed4

PRZM Scenario 

(standard unless 

noted) 

Without Without Without With Without Without Without Without Without With 

Louisiana 

sugarcane, 

1.25 lb ai/acre 

<0.01 <0.1 0.04 <0.01 <0.1 <0.5 <0.5 <0.5 1.1 1.0 

Mississippi 

cotton, 1.25 lb 

ai/acre 

<0.01 <0.1 0.05 <0.01 <0.1 <0.5 <0.5 <0.5 1.2 0.7 

North Carolina 

cotton, 1.25 lb/A <0.01 <0.1 0.03 <0.01 <0.1 <0.5 <0.5 <0.5 0.7 0.5 

Florida peppers 

1.0 lb ai/acre <0.01 <0.1 0.05 <0.01 <0.1 <0.5 <0.5 <0.5 1.2 0.5 

Mississippi 

soybeans, 1.25 lb 

ai/acre 

<0.01 <0.1 0.03 <0.01 <0.1 <0.5 <0.5 <0.5 0.8 0.5 

North Carolina 

sweet potato <0.01 <0.1 0.02 <0.01 <0.1 <0.5 <0.5 <0.5 0.6 <0.5 

North Carolina 

tobacco <0.01 <0.1 0.01 <0.01 <0.1 <0.5 <0.5 <0.5 <0.5 <0.5 

Florida potato 

(NMC), 1.275 lb 

ai/acre 

<0.01 <0.1 0.05 <0.01 <0.1 <0.5 <0.5 <0.5 1.2 0.5 

Michigan bean <0.01 <0.1 0.01 <0.01 <0.1 <0.5 <0.5 <0.5 <0.5 <0.5 

Oregon snap 

beans <0.01 <0.1 0.01 <0.01 <0.1 <0.5 <0.5 <0.5 <0.5 <0.5 

Florida cabbage <0.01 <0.1 0.01 <0.01 <0.1 <0.5 <0.5 <0.5 <0.5 <0.5 

Florida 

cucumber, 1 lb 

ai/acre 

<0.01 <0.1 0.07 <0.01 <0.1 <0.5 <0.5 <0.5 1.7 0.9 

Florida 

cucumber, 0.75 

lb ai/acre 

<0.01 <0.1 0.06 <0.01 <0.1 <0.5 <0.5 <0.5 1.3 0.6 

Florida 

cucumber, 0.5 lb 

ai/acre 

<0.01 <0.1 0.04 <0.01 <0.1 <0.5 <0.5 <0.5 0.9 <0.5 

Florida 

cucumber, 0.25 

lb ai/acre 

<0.01 <0.1 0.02 <0.01 <0.1 <0.5 <0.5 <0.5 <0.5 <0.5 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 58 

Estuarine 

Fish 

EstuarineInvertebrates Vascular 

Plants 

Non-Vascular 

Plants 

Acute1 Chronic2 Acute1 Acute1 Chronic2 NonList3 Listed4 NonList3 Listed4

PRZM Scenario 

(standard unless 

noted) 

Without Without Without With Without Without Without Without Without With 

Oregon mint, 0.5 

lb/A <0.01 <0.1 <0.01 <0.01 <0.1 <0.5 <0.5 <0.5 <0.5 <0.5 

1 Acute RQ = (acute EEC, ppb) / (acute LC50 or EC50, ppb), where the EEC is the 1-in-10 year 24-h peak value without 

(Without) and with (With) volatilization (Table 52) and the fish 96-h LC50 is 40,600 ppb ai for sheepshead minnow (Table 

23) and the mysid shrimp 96-h LC50 is 566 ppb (Table 24). A with volatilization analysis was included only where at least 

one RQ value exceeded a LOC under the conservation assumption that there was no volatilization from the land.

2 Chronic RQ = (chronic EEC, ppb) / (chronic NOAEC, ppb), where the EEC is the 1-in-10 60-d peak for fish and the 1-in10 year 21-d peak for invertebrates without volatilization (Table 52) and the fish chronic NOAEC is 4,850 ppb and the 

invertebrate chronic NOAEC is 68 ppb (Table 23 and Table 24, respectively). 

3 Non-listed Plant RQ = (EEC, ppb) / (IC50, ppb), where the EEC is the 1-in-10 year 24-h peak without volatilization (Table 

52) and the IC50 is 30,200 ppb for vascular plants and 167 ppb for non-vascular plants (Table 25).

4 Listed Plant RQ = (EEC, ppb) / (NOAEC, ppb), where the EEC is the 1-in-10 year 24-h peak without and with 

volatilization (Table 52) and the NOAEC is 7,300 ppb for vascular plants and 23.6 ppb for nonvascular (Table 25).

4.1.4.2 RICE USE

Wet Seeded 

Table 38 contains the acute RQ values for freshwater and estuarine-marine fish and 

invertebrates and aquatic plants for wet-seeded rice use with holding times spanning from the 

day of application to 32 days post-treatment. This analysis assumes that clomazone is 

applied directly to flood water or that the field is flooded immediately after treatment. Two 

of the Section 3 labels require applications be on fields with levees and that the levees, and 

application timing in terms of a rain event be such that runoff does not occur. Additionally, 

the water-seeded label prohibits release of the water from the treated field for 14 days posttreatment, unless the water is contained within a tailwater recovery system, or other system 

appropriate for preventing discharge from rice fields. The other Section 3 labels do not have 

these prohibitions. 

Table 38 Wet Seeded Rice Non-Listed and Listed Aquatic Plant RQs and Acute Fish 

and Aquatic Invertebrate RQ Values for Clomazone Residues in Rice Paddy Water at 

Various Holding Times 

Freshwater Estuarine Vascular Nonvascular 

Days after Fish Invertebrates Fish Invertebrates 

treatment 

EEC 

Clomazone 

ppb 

Acute 1

 Acute 1 Acute 1 Acute 1

Non-Listed 

and Listed 

Plants2, 3

NonListed 

Plants2

Listed 

Plants3

0 546 0.19 0.10 0.01 0.96 0.1 3.3 23.1 

15 352 0.12 0.07 <0.01 0.62 <0.1 2.1 14.9 

20 304 0.10 0.06 <0.01 0.54 <0.1 1.8 12.9 

24 271 0.09 0.05 <0.01 0.48 <0.1 1.6 11.5 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 59 

Freshwater Estuarine Vascular Nonvascular 

Days after Fish Invertebrates Fish Invertebrates 

treatment 

EEC 

Clomazone 

ppb 

Acute 1 Acute 1 Acute 1 Acute 1

Non-Listed 

and Listed 

Plants2, 3

NonListed 

Plants2

Listed 

Plants3

25 263 0.09 0.05 <0.01 0.46 <0.1 1.6 11.1 

28 241 0.08 0.04 <0.01 0.43 <0.1 1.4 10.2 

30 227 0.08 0.04 <0.01 0.40 <0.1 1.4 9.6 

32 214 0.07 0.04 <0.01 0.38 <0.1 1.3 9.1 

1 Acute RQ = (EEC, ppb) / (acute LC50 or EC50, ppb), where the EEC is the paddy water residue (Section 2.1.2.1) and the 

fish 96-h LC50 is 2,900 ppb for silver catfish (Table 23), the freshwater invertebrate 48-h EC50 is 5.400 ppb for D. magna

(Table 24), the estuarine fish 96-h LC50 is 40,600 ppb ai (Table 23), and the estuarine-marine invertebrate

2 Non-listed Plant RQ = (EEC, ppb) / (IC50, ppb), where the EEC is the paddy water residue (Section 2.1.2.1) and the IC50 is 

30,200 ppb for vascular plants and 167 ppb for non-vascular plants (Table 25).

3 Listed Plant RQ = (EEC, ppb) / (NOAEC, ppb), where the EEC is the paddy water residue (Section 2.1.2.1) and the 

NOAEC is 7,300 ppb (Table 25). 

Dry Seeded 

Modeled estimates of clomazone in rice flood water using a provisional modification of input 

to the Tier I Rice model (Section 2.1.2.2) to account for clomazone loss in soil from time of 

application till permanent flood results in a conservative estimate in the floodwater of 364 

ppb on the day of flood. However, as demonstrated in aquatic field dissipation studies 

performed in Texas and Arkansas the level appears to more realistically not exceed 5 to 7 

ppb (see Section 2.1.2.2.). Additionally, assuming release of floodwater on the day of 

application is highly conservative given that rice cultural practices should result in the water 

being retained at least for a few days without release to surface waters, unless a flood event 

occurs. The use of modeled versus measured values to assess risk makes a significant 

difference in the conclusion of risk. Measured levels in floodwater are below acute and 

chronic LOC values for freshwater and marine and estuarine fish and invertebrates, and 

below listed and nonlisted plant LOC values (Table 39). Modeled results indicate potential 

concern for acute risks to freshwater fish and invertebrates and for estuarine-marine 

invertebrates and exceedance of listed and non-listed LOC values for nonvascular and 

vascular plants. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 60 

Table 39 Dry Seeded Non-Listed and Listed Aquatic Plant RQs and Acute Fish and 

Aquatic Invertebrate RQ Values for Clomazone Residues in Rice Paddy Water at 

Permanent Flood 

Freshwater Estuarine Vascular Nonvascular 

Fish Invertebrates Fish Invertebrates 

EEC 

Clomazone 

ppb 

Acute 1

 Acute 1 Acute 1 Acute 1

Non-Listed 

and Listed 

Plants2, 3

NonListed 

Plants2

Listed 

Plants3

Modeled 364 0.13 0.07 0.01 0.64 <0.1 2.2 15.4 

Measured 7 <0.01 <0.01 <0.01 <0.01 <0.1 <0.1 0.3 

1 Acute RQ = (EEC, ppb) / (acute LC50 or EC50, ppb), where the EEC is the paddy water residue (Section 2.1.2.2) and the 

fish 96-h LC50 is 2,900 ppb for silver catfish (Table 23), the freshwater invertebrate 48-h EC50 is 5.400 ppb for D. magna

(Table 24), the estuarine fish 96-h LC50 is 40,600 ppb ai (Table 23), and the estuarine-marine invertebrate

2 Non-listed Plant RQ = (EEC, ppb) / (IC50, ppb), where the EEC is the paddy water residue (Section 2.1.2.2), and the IC50 is 

30,200 ppb for vascular plants and 167 ppb for non-vascular plants (Table 25).

3 Listed Plant RQ = (EEC, ppb) / (NOAEC, ppb), where the EEC is the paddy water residue (Section 2.1.2.2), and the 

NOAEC is 7,300 ppb (Table 25). 

4.1.5 Probit Slope Response Analysis of LOC Values and Acute RQ Values 

As part of the risk estimation, the probability of mortality associated with the endangered 

acute LOC values is estimated along with the probability of acute mortality occurring if 

exposure at the estimated environmental concentration actually occurs. The probability of 

mortality calculations are based on the probit slope dose response relationship. The 

probability of mortality for an exposed individual are calculated using an Excel spreadsheet 

tool IECV1.1 (Individual Effect Chance Model Version 1.1) developed by EFED of the U.S. 

EPA, OPP, Environmental Fate and Effects Division (June 22, 2004). 

There were two probit slopes from clomazone toxicity results. The acute mysid results fit a 

probit dose-response model with a probit slope of 6.7259 (95% CL 3.8914, 9.5605). The 

silver catfish acute results were not a good probit dose-response fit, but a slope estimated 

from the data is 3.1657 (95% CL -0.3946, 6.726). Because the lower 95% CL of the slope 

for the silver catfish is not a valid slope value, the default lower 95% CL of 2 is used for fish. 

The other acute toxicity results either consisted of limit dose type tests (e.g., avian acute dose 

and dietary results) which do not provide data which can be used to calculate a probit doseresponse slope, the data did not fit a probit model and no probit slope could be estimated 

from the data, or the results available from a study were limited to the LD50. In these cases 

the default slope of 4.5 (95% CL 2.9, 9) was used. 

The odds of mortality for an individual at the endangered species LOC for terrestrial and 

aquatic animals based on the slopes discussed in the previous paragraph are provided in 

Table 40. The odds of mortality for an individual at a range of acute RQ values between 0.05 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 61 

and 0.4 are provided in Table 42, using the slopes for the taxonomic groups listed in Table 

40. 

Table 40 Probit Slope and Odds of Mortality for an Individual at the LOC

Aquatic Invertebrates (TGAI) Listed 

Species 

Birds and 

Mammals 

(TGAI) 

Mammals 

(Commence 

EC) 

Fish 

(TGAI) Freshwater Estuarine/Marine 

LOC 0.1 0.1 0.05 0.05 0.05

Slope 4.5* 12.679 3.1657 6.7259 9.7919 

Slope 95% 

CL 2, 9* 2.121, 23.237 2**, 6.726 3.8914, 9.5605 5.1804, 13.7658 

Odds of 

individual 

effect 

1 in 294,000 <1 in 1036 1 in 52,500 <1 in 1017 <5 in 1036

95% CL 1 in 44, <1 in 

1018

1 in 59, <1 in 

10119 1 in 22, <1 in 1017 <1 in 106

, in 1035 <1 in 1011, <1 in 1071

*Default mean slope value (4.5) and 95% CL (2, 9) is used due to specific dose-response information for this group. 

**The probit model is a poor fit for the tested doses and response, therefore the lower 95% CL for the data was not a valid 

slope and the default lower 95% CL of 2 was used. 

Table 41 Odds of Mortality for an Individual Bird or Mammal at a Given Acute RQ 

Value for the TGAI and Commence EC 

Birds and Mammals, TGAI 

(Slope =4.5) 

Birds and Mammals, Commence EC 

RQ (Slope =12.679) 

Odds 95% CL Odds 95% CL 

0.01 <1 in 1018 1 in 31,600; <1 in 1072 <1 in 10141 1 in 20,300; <1 in 10273

0.02 <1 in 1013 1 in 2,950; <1 in 1052 <1 in 10102 1 in 6,370; <1 in 10273

0.03 <1 in 1011 1 in 862; <1 in 1042 <1 in 1082 1 in 1,620; <1 in 10273

0.04 <1 in 109

 1 in 386; <1 in 1035 <1 in 1069 1 in 661; <1 in 10231

0.05 <1 in 108

 1 in 216; <1 in 1031 <1 in 1060 1 in 345; <1 in 10200

0.06 <1 in 107

 1 in 138; <1 in 1027 <1 in 1053 1 in 209; <1 in 10176

0.07 <1 in 106

 1 in 96; <1 in 1024 <1 in 1048 1 in 140; <1 in 10158

0.08 <1 in 106

 1 in 70; <1 in 1024 <1 in 1043 1 in 100; <1 in 10142

0.09 1 in 791,000 1 in 55; <1 in 1020 <1 in 1039 1 in 75; <1 in 10130

0.10 1 in 294,000 1 in 44; <1 in 1018 <1 in 1036 1 in 59; <1 in 10119

0.11 1 in 125,000 1 in 36; <1 in 1017 <1 in 1033 1 in 47; <1 in 10109

0.12 1 in 58,500 1 in 30; <1 in 1016 <1 in 1031 1 in 39; <1 in 10101

0.13 1 in 29,900 1 in 26; <1 in 1015 <1 in 1028 1 in 33; <1 in 1093

0.15 1 in 9,563 1 in 20; <1 in 1013 <1 in 1025 1 in 24; <1 in 1081

0.19 1 in 1,710 1 in 13; <1 in 1010 <1 in 1019 1 in 16; <1 in 1062

0.20 1 in 1,210 1 in 12; <1 in 109

, <1 in 1018 1 in 14; <1 in 1058, 

0.23 1 in 491 1 in 9.9; <1 in 108

 <1 in 1015 1 in 11; <1 in 1049
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 62 

Table 42 Odds of Mortality for an Individual Fish or Aquatic Invertebrate at a Given 

Acute RQ Value 

Fish 

(Slope =3.1657) 

Freshwater Aquatic Invertebrates 

(Slope = 6.7259) 

Marine (Mysid) Invertebrates 

RQ (Slope = 9.7919) 

Odds 95% CL Odds 95% CL Odds 95% CL 

0.01 <1 in 109 1 in 31,600; <1 in 1040 <1 in 1040 <1 in 1014; <1 in 1040 <1 in 1084 <1 in 1024; <1 in 10166

0.05 1 in 52,500 1 in 216; <1 in 1017 <1 in 1017 <1 in 106; <1 in 1035 <5 in 1036 <1 in 1011; <1 in 1071

0.06 1 in 18,200 1 in 138; <1 in 1015 <1 in 1015 1 in 106; <1 in 1031 <3 in 1032 <1 in 109; <1 in 1063

0.07 1 in 7,810 1 in 96; <1 in 1014 <1 in 1014 1 in 286,000; <1 in 1027 <1 in 1029 <1 in 108

; <1 in 1056

0.09 1 in 2,150 1 in 55; <1 in 1011 <1 in 1011 1 in 42,400; <1 in 1023 <1 in 1024 <1 in 107

; <1 in 1046

0.10 1 in 1,290 1 in 44; <1 in 1011 <1 in 1011 1 in 20,100; <1 in 1021 <1 in 1022 <1 in 106

; <1 in 1042

0.11 1 in 831 1 in 36; <1 in 1010 <1 in 1010 1 in 10,500; <1 in 1019 <1 in 1020 <1 in 106

; <1 in 1039

0.12 1 in 562 1 in 30; <1 in 109 <1 in 109 1 in 5,890; <1 in 1018 <1 in 1019 <1 in 106

; <1 in 1036

0.13 1 in 397 1 in 26; <1 in 108 <1 in 108 1 in 3,540; <1 in 1016 <4 in 1017 1 in 451,000; <1 in 1033

0.15 1 in 220 1 in 20; <1 in 107 <1 in 107 1 in 1,490; <1 in 1014 <2 in 1015 1 in 101,000; <1 in 1029

0.19 1 in 89 1 in 13; 1.6 in 106 1.6 in 106 1 in 400; 1.6 in 106 <1 in 1012 1 in 10,700; <1 in 1022

0.20 1 in 74 1 in 12; 1 in 774,000 1 in 773,000 1 in 306; <1 in 1010 <1 in 1011 1 in 6,810; <1 in 1021

0.23 1 in 46 1 in 9.9; 1 in 113,000 1 in 113,000 1 in 154; <1 in 109 <1 in 109 1 in 2,120; <1 in 1018

0.25 1 in 35 1 in 8.8; 1 in 39,000 1 in 38,900 1 in 105; <1 in 108 <5 in 108 1 in 1,100; <1 in 1016

0.30 1 in 20 1 in 6.8; 1 in 4,580 1 in 4,580 1 in 48; <1 in 106 <1 in 106 1 in 296; <1 in 1012

0.35 1 in 13 1 in 5.5; 1 in 924 1 in 924 1 in 26; 1 in 153,000 1 in 249,000 1 in 110; <1 in 109

0.40 1 in 9.6 1 in 4.7; 1 in 269 1 in 269 1 in 16; 1 in 14,100 1 in 20,500 1 in 50; <1 in 107

0.43 1 in 8.1 1 in 4.3; 1 in 14.6 1 in 146 1 in 13; 1 in 4,370 1 in 6,030 1 in 34.7; <1 in 106

0.47 1 in 6.7 1 in 3.9; 1 in 72.9 1 in 72.9 1 in 9.9; 1 in 1,160 1 in 1,510 1 in 22.4; 1 in 314,000 

0.62 1 in 3.9 1 in 2.9; 1 in 12.3 1 in 12.3 1 in 4.8; 1 in 42 1 in 45 1 in 7.1; 1 in 469 

0.64 1 in 3.7 1 in 2.9; 1 in 10.4 1 in 10.4 1 in 4.4; 1 in 24.7 1 in 34 1 in 6.3; 1 in 262 

0.96 1 in 2.0 1 in 2.0; 1 in 2.2 1 in 2.2 1 in 2.1; 1 in 2.3 1 in 2.3 1 in 2.1; 1 in 2.5 

4.2 Risk Description 

The following risk description explains the overall conclusions regarding the potential 

ecological risk from the various uses of clomazone. The risk conclusions take into 

consideration the risk estimates, includes RQ values and information on odds of acute 

mortality, in conjunction with other information such as monitoring data, field studies, and 

incident data that may provide additional insights into the likelihood of exposure, and other 

factors that modify the likelihood of exposure such as timing of application, overlap of area 

affected and the degree of effect with the presence/absence of taxa, species sensitivity 

distribution, and presence/absence of dietary items. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 63 

4.2.1 Non-Listed Species 

4.2.1.1 DIRECT EFFECTS TO TERRESTRIAL PLANTS AND ANIMALS

Primary Productivity and Habitat Integrity 

Environmental baseline conditions from putting the land into crop production includes a 

temporary loss of primary production or existing standing crop on the field at the time of 

preparation of the seed or transplant bed and loss of habitat that the previous year’s crop trash 

provided or loss of habitat or plant diversity that a fallow field provided before being put 

back into production. Herbicides like clomazone provide a chemical alternative to physical 

methods in eliminating target undesirable plants from the field. Clomazone is applied as a 

preemergent with preparation of the field generally occurring when the plant growth curve is 

low (e.g., <5% production, or coming out of dormancy). For soybean and cotton, clomazone 

is registered for use under conservation tillage, which refers to a number of strategies and 

techniques for establishing crops with minimal cultivation of the soil. 

Runoff and Spray Drift

Plant RQ exceedences of non-listed and listed species LOC values (Table 33) indicate that 

loading of clomazone from combined runoff and spray drift to terrestrial and semi-aquatic 

areas adjacent to a treated field potentially poses unreasonable risks to off-field plants at all 

existing clomazone use rates (0.251 to 1.5 lbs ai/A). Effects from spray drift loading were 

below the LOC for non-listed monocots at all application rates (0.251 to 1.5 lbs ai/A), for 

listed monocots at application rates at and below 0.6 lbs ai/A, and for non-listed dicots at and 

below 0.375 lbs ai/A. Distances from the field that were estimated to exceed non-listed and 

listed species LOC values due to loading from spray drift are listed in Table 43. Except for 

the Cerano 5 MEG label for aerial or ground application to rice fields, labels specifically 

state to use spray drift conditions that meet a droplet size classification of coarse or greater or 

to use a drift reducing additive. The distances for ground spray were calculated using the 

largest droplet size available in the terrestrial Tier I model in AGDRIFT, which is ASAE fine 

to median-coarse (there is no Tier 2 ground spray model). The coarser droplet size called for 

on all labels, except the Cerano 5 MEG, would result in smaller distances than listed in the 

table. The Cerano 5 MEG label does not provide guidance on droplet size for ground or 

aerial spray so distances in table for ground spray would not necessarily be overestimated. 

Aerial spray was modeled for two droplet size categories available in the Tier I Aerial 

AGDRIFT model, ASAE coarse to very coarse and ASAE medium to coarse. 

Based on modeling the distance extending from the field where spray drift loading exceeds 

non-listed or listed species LOC extends out over 1,000 feet for aerial applications, for 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 64 

droplet size ASAE medium to coarse; distances are lower for coarser droplet size. For 1.5 

lbs ai/A, spray drift out to 470 feet exceeds the listed LOC for dicots and out to 215 feet for 

non-listed dicots; spray drift loading does not exceed the non-listed LOC for monocots. For 

1.0 lb ai/acre, spray drift exceeding the listed species LOC extends out to 300 ft for dicots. 

Table 43 Distance from Treated Field Spray Drift Loading Exceeds LOC 

Distance in feet (Acres) affected 

Application Aerial Spray Rate 

Load below 

LOC 

lbs ai/A 

Ground Spray 

Fine to Medium Coarse

Droplet Size Coarse to Very 

Coarse 

Medium to Coarse 

Monocot Listed 0.0062 110 (0.52 A)* N/A N/A 

Dicot Non-Listed 0.0037 215 (~1 A) N/A N/A 

1.5 lbs ai/A 

Dicot Listed 0.0019 470 (2.25 A) N/A N/A 

1.0 lb ai/acre Monocot Listed 0.0062 63 (0.30 A) N/A N/A 

Dicot Non-Listed 0.0037 127 (0.61 A) N/A N/A 

Dicot Listed 0.0019 300 (1.4 A) N/A N/A 

0.75 lbs ai/A Monocot Listed 0.0062 43 (0.21 A) N/A N/A 

Dicot Non-Listed 0.0037 88 (0.42 A) N/A N/A 

Dicot Listed 0.0019 209 (1.0 A) N/A N/A 

Monocot Listed 0.0062 N/A 270 (1.3 A) 425 (2.0 A) 

Dicot Non-Listed 0.0037 63 (0.30 A) 450 (2.2 A) 810 (3.9 A) 

0.6 lbs ai/A 

Dicot Listed 0.0019 155 (0.74 A) 900 (4.3 A) >1000 (>4.8) 

0.251 lbs ai/A Dicot Listed 0.0019 49 (0.23 A) N/A N/A 

*The acreage affected is calculated using the distance off the edge-of-field and the length of the treated field (208.7 ft along 

a side for a square acre). 

N/A: Not applicable because aerial application is not registered for the use or because the EEC was below the LOC. 

Non-Target Plant Incidents 

Clomazone incident data for non-target plant effects provide some records with reported 

distance to affected site or the farthest distance effects were observed (Appendix G). The 

incident data for clomazone does not include for a majority of records a definitive 

determination of the specific route of exposure (i.e., spray drift, volatilization, runoff or a 

combination). Most incidents reported as adjacent to treated sites are identified as 

attributable to spray drift. However, there are some records where volatilization is a definite 

contributing factor even to adjacent properties rather than spray drift or in addition to spray 

drift. An analysis of the incident distance information for the ME formulations (N = 52) 

indicates that 76.9% of incidents were within 300 feet, 21.1% were between 300 and 1,500 

feet, and one incident (1.9%) was one mile from the nearest treated field (Figure 5). 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 65 

Distances reported as adjacent to treated field were categorized as within 300 feet for this 

analysis. For the EC formulations (N = 160) 84.4% of incidents were within 300 feet, 10.0% 

were between 300 and 1,500 feet, 5.1% were between 1,500 and 4,500 feet, and two 

incidents (1.9%) were greater than or equal to one mile from the nearest treated field (Figure 

5). Most incidents were reported in 1986, 1993, and 1997 (Figure 6). The 1986 incidents 

were associated with the use of the newly registered EC formulation on soybeans. Most of 

the incidents associated with distances above 1,500 ft with the EC formulation (8 of 12) 

occurred during this time. Within a few months buffer zones, soil incorporation, and spray 

drift reducing label language (e.g., use of additive to reduce spray drift, use of low boom and 

coarse spray) were added to the label. There are no incident reports between 1987 and 1991. 

Several minor uses such as pumpkins and peas (succulent) were added to the label during this 

time period; additionally clomazone use on soybeans declined during this time period. In 

1993, cotton use was added and there was a major resurgence in reported incidents. Cotton 

use did not require soil incorporation. Emergency meetings were held to address non-target 

plant effects from registered use on cotton; the state of Tennessee put an emergency ban on 

use of clomazone through the summer of 1993. Except for an incident in 1994, the 

remaining incidents above 1,500 feet associated with the EC occur during this time. The ME 

formulation was registered for use in 1995, with no soil incorporation required. The 

following year, 1996, there were reported incidents with the ME formulation. Another 

resurgence of incidents occurred in 1997 along with increased use that year. After 1997 a 

number of minor incidents were reported but not much information is provided. This is in 

part due to a change in 1997 of the FIFRA 6(a)(2) reporting requirements changing the 

definition for minor non-target plant incidents to being defined as those affecting <45% of 

the area treated. For example, if 100 acres are treated 45 acres off the treated field would 

need to be affected before it is required to be reported as a major incident. Although use of 

clomazone on Figure 6 appears to reduce below 100,000 lbs annually in 2001, this in part is 

likely due to reduced monitoring by USDA-NASS. Alternative sources for 2001 to 2007 

indicate that combined cotton and soybean use is over 100,000 pounds (100 x 1,000 lbs) 

annually during this time. Use on rice, which was first registered for use with the ME 

formulation in 2000 has become the highest use of clomazone annually (approximately 

600,000 lbs). Approximately 13.5 percent of clomazone is used on soybeans and cotton 

annually (2001-2007). 

Discounting the 1986 data when spray drift reduction language was not on labels, the 

incident information supports that most effects are within the expected spray drift effect area. 

Additionally, during the 1993 meetings about incidents that year, it was identified that except 

for some cases in Tennessee, effects occurred well within the label buffer zones in place on 

the label. The degree of clomazone effects associated with various distances from a treated 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 66 

field, however, is not readily identifiable from the incident data base. Effects span the whole 

spectrum from minor whitening with all plants surviving, to severe effects with some plant 

mortality. 

Figure 5 Distribution of Distances from a Treated Field to Clomazone Effects 

0

10

20

30

40

50

60

70

80

90

< 300 301-500 501-1,000 1,001-1,500 1,501-2,500 2,501-3,500 3,501-4,500 > 5,280

Distance (feet)

Percent of Incidents

ME (N = 52)

EC (N = 160)

Unknow n (N = 128)

Total (N = 340)

_ _

0

10

20

30

40

50

60

70

80

90

< 300 301-500 501-1,000 1,001-1,500 1,501-2,500 2,501-3,500 3,501-4,500 > 5,280

Distance (feet)

Percent of Incidents

ME (N = 52)

EC (N = 160)

Unknow n (N = 128)

Total (N = 340)

_ _

The >5,280 feet group is comprised of two incidents, one incident occurred for the ME out to 1 mile from the site of application and one 

incident occurred for the EC out to 2 miles. 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 67 Figure 6 Reported Number of Non-Target Plant Incidents Associated with Clomazone Formulations and Decisions 

0

102030405060708090

100110120

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2

Year

Number of Reported Incidents

Unknow n (minorUnknow nME (minor)MEEClbs ai x 1,000

EC Soybean Use Registered Feb ‘86

>88*

GR soybean introduced ‘96

Must soil incorporate, and buffer zones** added to label in Nov ‘86

*Official reported incidents, but white areas were reported as observed to be widespread througout the midwest** Do not apply within 100 ft of desirable plants and 1,000 ft from towns, greenhouses and nurseries, commercial vegetable, fruit, and nut production** Request for registrant to submit 1-800 clomazone hotline incidents

EC Pumpkin and Peas (Succulent) Uses Registered ‘88

>1,173*** EC Cotton Use Registered ‘93

EC Fallow Land Use Registered ‘89

WP Pumpkin and Pea Use Registered ‘91

ME Registered ’95 for all EC Uses except Fallow Land

ME Dry-Seeded Rice Use Registered ’00

0

102030405060708090

100110120

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2

Year

Number of Reported Incidents

Unknow n (minorUnknow nME (minor)MEEClbs ai x 1,000

EC Soybean Use Registered Feb ‘86

>88*

GR soybean introduced ‘96

Must soil incorporate, and buffer zones** added to label in Nov ‘86

*Official reported incidents, but white areas were reported as observed to be widespread througout the midwest** Do not apply within 100 ft of desirable plants and 1,000 ft from towns, greenhouses and nurseries, commercial vegetable, fruit, and nut production** Request for registrant to submit 1-800 clomazone hotline incidents

EC Pumpkin and Peas (Succulent) Uses Registered ‘88

>1,173*** EC Cotton Use Registered ‘93

EC Fallow Land Use Registered ‘89

WP Pumpkin and Pea Use Registered ‘91

ME Registered ’95 for all EC Uses except Fallow Land

ME Dry-Seeded Rice Use Registered ’00

Although use of clomazone on Figure 6 appears to reduce below 100,000 lbs annually in 2001, this in part is likely due to reduced monitoring by USDA-N2001 to 2007 indicate that combined cotton and soybean use is over 100,000 pounds (100 x 1,000 lbs) annually during this time. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 68 

Currently all Section 3 labels either explicitly prohibit application to non-field areas 

including fence rows, waterways, ditches, and road sites or implicitly by restricting 

application to only those surfaces that will be soil incorporated. Section 3 labels for EC and 

WP registrations prohibit preemergent soil incorporated applications within 1,000 ft of towns 

and subdivisions, commercial vegetable production35, commercial fruit production, 

commercial nurseries, and commercial greenhouses and 100 ft of desirable plants whereas 

soil unincorporated applications are prohibited within 1,500 ft of towns and subdivisions, 

commercial vegetable production, commercial fruit production, commercial nurseries, 

commercial greenhouses and desirable plants. Section 3 labels for ME registrations prohibit 

applications with 1,200 ft of towns and subdivisions, commercial vegetable production, 

commercial fruit production, commercial nurseries, commercial greenhouses and some labels 

have 200 ft and others 300 ft of desirable plants. These buffers are not based on protection 

for any specific endangered species but were set to reduce effects to off-field plants from 

spray drift and redeposition of volatilized clomazone. 

Non-target Terrestrial Plant Effects 

A diagram of the species IC25 sensitivity distribution for dry weight, plant height, and 

survival with lines referencing the combined sheet runoff and spray drift EEC values for 

selected applications ranging from the highest application rate (sweet potato), to the lowest 

label application rate (succulent beans) are provided in Figure 7. Such a comparison 

provides insight as to the number of species likely to be affected, as represented by the test 

species, within the area adjacent to the treated site. At application rates of 0.5 lbs ai/A and 

lower, RQ values for non-listed monocot populations are at or below the LOC, indicating 

effects are within acceptable levels. For dicot species at the lowest application rate the IC25

for seedling survival was exceeded for only one test species, lettuce. Considering all species 

together, the lowest application rate indicates the potential for at least 25% inhibition in plant 

height for about 12% of species and at least 25% inhibition in dry weight biomass of 

surviving plants for about 20% of species. At ground spray applications of 0.375 to 0.6 lbs 

ai/A, the situation does not appear to differ much. Seedling survival is still only exceeded for 

one test species, the dicot lettuce, and the potential for at least 25% inhibition in plant height 

has increased to about 15% of species, and dry weight to about 25% of species. At the 

highest application rate of 1.5 lbs ai/A by ground spray, survival is reduced by at least 25% 

in approximately 15% of tested species, with at least 25% inhibition in height in about 45% 

of species and dry weight in about 35% of species. 

 

35 Most labels have at least an exception for sweet corn some labels also include exceptions which include one 

or more of the following: peppers, pumpkins, succulent peas, sweet potato, and winter squash.
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 69 

Figure 7 Terrestrial Plant EEC Values Compared to Seedling Emergence IC25 Species 

Sensitivity Distribution for Clomazone 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.001 0.01 0.1 1 10

Concentration (lb ai/A)

Probability

Dry Weight

Height

Survival

ltc ltc ltc

tom oni oni

oni tom

rye

car

tom

crn

cab

cuc

oat

soy

rye

car

cab

oat

crn

cuc

rye

car

oat

cab

crn

cuc

IC25 Sensitivity Distribution

0.03 lbs ai/A: total load for 

adjacent dry land – CB, MT, P

0.015 lbs ai/A: total load for 

adjacent dry land -- SB

Concentration at or below the non-listed monocot LOC

0.09 lbs ai/A: total load for 

adjacent dry land – SP

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.001 0.01 0.1 1 10

Concentration (lb ai/A)

Probability

Dry Weight

Height

Survival

ltc ltc ltc

tom oni oni

oni tom

rye

car

tom

crn

cab

cuc

oat

soy

rye

car

cab

oat

crn

cuc

rye

car

oat

cab

crn

cuc

IC25 Sensitivity Distribution

0.03 lbs ai/A: total load for 

adjacent dry land – CB, MT, P

0.015 lbs ai/A: total load for 

adjacent dry land -- SB

Concentration at or below the non-listed monocot LOC

0.09 lbs ai/A: total load for 

adjacent dry land – SP

cab = cabbage; car = carrot; crn = corn; cuc = cucumber; ltc = lettuce; oni = onion; rye = ryegrass; soy = soybean; tom = tomato 

CB: cabbage use; MT: mint use; P: succulent pea use; SB: succulent bean use; SP: sweet potato use 

These estimates of impact are based solely on seedling emergence data. Other lines of 

evidence indicate that there are mitigating factors that will reduce the overall impact to plant 

communities. Such factors include that clomazone applied to surface soil or incorporated in 

the upper 1-3 inches of soil does not readily leach to lower soil depths. With a couple of 

exceptions, crops that are planted on fields treated with clomazone are typically planted at 

depths below clomazone treatment. This allows the crop to germinate and grow without 

exposure in the root zone at levels that impact the crop. Warnings on labels for EC and WP 

formulated products for crops such as succulent peas, peppers, and pumpkins state that “seed 

or transplants should be planted below the herbicide barrier to minimize” bleaching. As such 

impacts are expected to be lower for plants that have already emerged or those which are 

dormant with roots, or with corms or tubers below clomazone deposition in areas adjacent to 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 70 

the field. Additionally, emerged plants may not be as sensitive to clomazone as seedlings. 

While there is only supplemental information on mortality available from vegetative vigor 

studies as discussed in Section 3.5.1, levels where mortality occurs, garnered from one such 

study, were higher than in seedling emergence tests. Mortality in vegetative studies were not 

observed until concentrations exceeded 0.137 lbs ai/A, whereas in seedling emergence 

increased mortality was observed at concentrations exceeding 0.0052 lbs ai/A. Because 

mortality does not appear to occur in emerged vegetation until after 0.137 lbs ai/A, while the 

loading from sheet runoff and spray drift may reduce the height and weight of a limited 

number of species and result in some bleaching, plant survival would be at acceptable levels. 

At rates of around 0.5 lbs ai/A clomazone labels indicate that clomazone only suppresses 

some annual grasses but does control velvetleaf and spurred anoda. Rates of 1.0 lb ai/acre or 

higher are recommended to control fields that have previously been in conservation 

programs, or where heavy weed pressure is anticipated. Therefore while some bleaching 

would be observed on terrestrial plants adjacent to the field, loss of the terrestrial plant 

community and its structural integrity is not considered likely at ground spray application 

rates at and below 0.6 lbs ai/A. This is based on consideration that combined runoff and 

spray drift loading to areas adjacent to the field are below the LOC for non-listed monocots, 

only species as sensitive as lettuce would have reduction in survival, most plants would be 

expected to survive, and of those limited species impacted the effect would be limited 

inhibition in growth. 

Non-target Semi-Aquatic (Wetland/Riparian) Plants 

A diagram of the species IC25 sensitivity distribution for dry weight, plant height, and 

survival with lines referencing the combined channel runoff and spray drift EEC values for 

selected applications ranging from the highest application rate (sweet potato), to the lowest 

label application rate (succulent beans) are provided in Figure 8. As indicated at the lowest 

label application rate at least 50% of plants are expected to have decreased height of at least 

25%, in addition to about 40% of them having reduced dry weight by at least 25%, and about 

20% having at least 25% mortality. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 71 

Figure 8 Wetland or Semi-Aquatic Plant EEC Values Compared to Seedling 

Emergence IC25 Species Sensitivity Distribution for Clomazone 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.001 0.01 0.1 1 10

Concentration (lb ai/A)

Probability

Dry Weight

Height

Survival

ltc ltc ltc

tom oni oni

oni tom

rye

car

tom

crn

cab

cuc

oat

soy

rye

car

cab

oat

crn

cuc

rye

car

oat

cab

crn

cuc

IC25 Sensitivity Distribution

0.13 lbs ai/A: total 

load for adjacent 

semi-aquatic area –

SB

Concentration at or below the non-listed monocot LOC

0.765 lbs ai/A: total 

load for adjacent semiaquatic area – SP

0.306 lbs ai/A: total 

load for adjacent semiaquatic area – R

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.001 0.01 0.1 1 10

Concentration (lb ai/A)

Probability

Dry Weight

Height

Survival

ltc ltc ltc

tom oni oni

oni tom

rye

car

tom

crn

cab

cuc

oat

soy

rye

car

cab

oat

crn

cuc

rye

car

oat

cab

crn

cuc

IC25 Sensitivity Distribution

0.13 lbs ai/A: total 

load for adjacent 

semi-aquatic area –

SB

Concentration at or below the non-listed monocot LOC

0.765 lbs ai/A: total 

load for adjacent semiaquatic area – SP

0.306 lbs ai/A: total 

load for adjacent semiaquatic area – R

cab = cabbage; car = carrot; crn = corn; cuc = cucumber; ltc = lettuce; oni = onion; rye = ryegrass; soy = soybean; tom = tomato 

SB: succulent bean use; SP: sweet potato use 

Volatilization

Plant RQ exceedences of non-listed and listed species LOC values (Table 34and Table 35) 

indicate that loading due to redeposition of volatilized clomazone to near-field areas 

potentially poses risks to off-field plants at all existing label clomazone use rates (0.251 to 

1.5 lbs ai/A). The level of risk, however, is very uncertain due to a number of unknowns 

such as the relationship between environmental factors and volatility of clomazone, the 

impact the ME formulation has on volatilization as compared to the EC and WP 

formulations, how much dispersal and degradation occurs between volatilization and 

deposition, and the use of seedling emergence results to estimate vegetative vigor risks. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 72 

Loading attributable to volatilization under 100% humidity for two assumptions (1) 

formulation has no effect therefore 65% volatilizes and (2) 50% reduction in volatility due to 

ME formulation for sweet potato (SP) applications are provided as references lines in a graph 

of seedling emergence IC25 sensitivity values (Figure 9). At the highest application rate (use 

on sweet potato) and assuming that formulation (ME vs EC) does not reduce the 

volatilization rate (EEC=0.975 lbs ai/A), biomass (dry weight and seedling height) and 

seedling survival are reduced by at least 25% for all tested species except soybean. Under 

the assumption that the ME formulation does result in a 50% reduction in the volatility rate 

(EEC=0.488 lbs ai/A) there is still 70% of plant species having 25% or more reduction in 

height and weight of surviving plants, and about 65% of species having 25% or greater 

mortality. 

Also provided as references in Figure 9 are loading attributable to volatilization off treated 

succulent bean fields under the same assumptions discussed above for sweet potatoes. Under 

the assumption that formulation does not reduce the volatilization rate (EEC=0.082 lbs ai/A) 

biomass of surviving plants, as measured by plant height is reduced by at least 25% in over 

50% of species, and dry weight is also reduced by at least 25% in over 40% of species. 

Seedling survival is reduced by at least 25% for about 25% of species. Assuming that the 

ME formulation results in a 50% reduction in the volatility rate (EEC = 0.163 lbs ai/A), still 

results in an estimated 45% of plant species having 25% or more reduction in plant height 

and 35% of species having 25% or more reduction in dry weight. However, survival is 

reduced by at least 25% in only about 15% of species. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 73 

Figure 9 Terrestrial, Wetland, and Semi-Aquatic Plant EEC Values Due to 

Volatilization Compared to Seedling Emergence IC25 Species Sensitivity Distribution 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.001 0.01 0.1 1 10

Concentration (lb ai/A)

Probability

Dry Weight

Height

Survival

ltc ltc ltc

tom oni oni

oni tom

rye

car

tom

crn

cab

cuc

oat

soy

rye

car

cab

oat

crn

cuc

rye

car

oat

cab

crn

cuc

IC25 Sensitivity Distribution

0.975 lbs ai/A, 100% 

humidity -- SP

0.163 lbs ai/A, 100% 

humidity -- SB

0.488 lbs ai/A (ME), 

100% humidity -- SP

0.082 lbs ai/A, 100% 

humidity -- SB

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.001 0.01 0.1 1 10

Concentration (lb ai/A)

Probability

Dry Weight

Height

Survival

ltc ltc ltc

tom oni oni

oni tom

rye

car

tom

crn

cab

cuc

oat

soy

rye

car

cab

oat

crn

cuc

rye

car

oat

cab

crn

cuc

IC25 Sensitivity Distribution

0.975 lbs ai/A, 100% 

humidity -- SP

0.163 lbs ai/A, 100% 

humidity -- SB

0.488 lbs ai/A (ME), 

100% humidity -- SP

0.082 lbs ai/A, 100% 

humidity -- SB

car: carrot; cab: cabbage; crn: corn; cuc: cucumber; ltc: lettuce; oat: oats; oni: onion; rye: rye; soy: soybean; tom: tomato 

SB: succulent bean use; SP: sweet potato use 

For this assessment it was conservatively assumed that the portion of clomazone that 

volatilized did not degrade nor did the mass of volatilized clomazone dilute before being 

redeposited. No air model for prediction of mass loading and spatial redeposition was 

identified that could be applied using the available information, for the degree of 

parameterization required for the models. Incident data (Appendix G), with reported distance 

to effects, were analyzed to provide the best available information regarding the degree of 

spatial displacement of volatilized clomazone. In the incident database the farthest distance 

associated with the ME formulation is 1 mile (I005879-009) and the farthest with the EC 

formulation is 2 miles (B000602-001). There was a report of 30 incidents (I009610-009) 

where it was claimed that soil particles off EC treated cotton fields traveled for several miles 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 74 

across state lines, and redeposited at sufficient loading to result in clomazone damage. There 

is a great deal of uncertainty in this claim as there were no measurements made of soil levels 

of clomazone, or any other investigation to support that clomazone on soil particles was the 

transport mechanism. Therefore, these 30 incidents were not included in the assessment of 

distance from field for clomazone effects. 

According to non-target plant incident data, most clomazone incidents occur within a 300 

foot area from treated fields. Within this area sensitive plant species among plant 

communities may be adversely affected by reduced growth and biomass, mortality, or 

reduced reproduction. These sensitive species because of adverse effects may be replaced 

due to competition with other plant species and thereby shifting the distribution of the plant 

species in the ecosystem. As plant species are replaced by other plant species, such as an 

invasive species which may have an advantage due to a lost plant or reduced growth 

competition, there is potential for change in habitat, degree of cover, or wildlife food items. 

Invasive plant species often come into plant communities to fill a niche vacated by weaker 

plant species, thereby contributing to more serious ecological stresses. 

Also, incident data indicate that beyond 300 foot from edge of treated field, the numbers of 

incidents fall off and thereby, the adverse effect on plant communities may be less uniform 

among sensitive species. This may contribute to less disruption to plant communities as a 

whole, however there may continue to be some adverse effects to more sensitive species 

further out from treated fields in a more random fashion. 

Birds, Reptiles, and Amphibians 

Based on several lines of evidence acute and chronic direct risks to nonlisted birds and the 

taxonomic groups for which they are surrogates (reptiles and amphibians) are considered to 

be within acceptable levels (i.e., below levels of concern for unreasonable adverse effects) 

for all existing labeled clomazone uses. The lines of evidence include that predicted on-field 

upper bound dietary residue levels at the highest application rate, banded application on 

sweet potatoes36 (Table 27), did not exceed the chronic LOC for any use, which means all 

other use rates are also below the chronic LOC and therefore no measureable effect on 

survival and reproduction of individuals is predicted from chronic exposure to clomazone at 

label rates. The acute risks were not evaluated using acute RQ values because of a lack of 

definitive acute study results (i.e., highest dietary level tested and highest oral dose tested did 

not result in mortality or sublethal effects). However, the subacute dietary data still provides 

evidence that predicted acute dietary residue levels are within acceptable risk levels. The use 

 

36 1.5 lbs a.i./A broadcast is equivalent to 3.0 lbs a.i./A banded for clomazone registered use
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 75 

of subacute dietary limit test data to evaluate acute dietary risks is described in detail in the 

following paragraph. Because clomazone is applied as a preemergent soil treatment or early 

preemergent (i.e., applied at the one- or two-leaf stage of target plants) dietary items such as 

short grass, tall grass, small broadleaf plants, fruits and pods would be essentially minimal to 

non-existent on conventional tilled fields resulting in reduced or nonexistent exposure to the 

predicted upper bound residue levels for these items. Clomazone can be applied as a soil 

treatment under conservation-tillage systems (minimum-till or no till) for cotton and soybean. 

In these systems, there is some potential for these dietary items to be represented on-field. 

The highest acute dose and dietary concentrations tested with both mallard ducks and 

bobwhite quail did not result in any mortality or sublethal effect in exposed birds (sample 

size = 50 in each test). A provisional method described in Appendix C was used to 

extrapolate subacute dietary LC50 values for this data (Table 20). Because no concentrationresponse relationship data is available from a limit toxicity study, LC50 values were modeled 

for a minimum slope of 2, a maximum slope of 9, and typical slope of 4.5 observed for 

pesticides in general (EPA, 1986; EPA, 2004). Then EEC residue levels were compared to 

one-tenth of each modeled LC50 value (0.1 x LC50), the factor applied as the effects threshold 

for the acute toxicity LOC for an endangered avian species (EPA, 2004). Except for short 

grass residues under the assumption that the concentration response slope is 9, predicted onfield upper bound dietary residue levels are lower than one-tenth of extrapolated LC50 values 

(Table 44) indicating that no direct acute effects are expected. Additionally although the 

short grass residue level exceeds one-tenth of the extrapolated LC50 of 6,905 ppm ai, based 

on its probit slope response of 9 the residue level of 720 ppm ai would be equivalent to an 

individual risk of mortality of less than 1 in 2 x 1018. Therefore measureable direct acute 

mortality is not expected at the highest application rate, which means all other use rates 

would also not be expected to result in acute effects. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 76 

Table 44 Upper bound avian dietary levels for a banded application on sweet 

potatoes compared to extrapolated dietary acute toxicity values 

Comparison of EEC to One-Tenth of the Extrapolated 

LC50 values2

Dietary Item EEC (mg/kgdiet) 1

22,940 ppm ai @ 

slope 2 

9,731 ppm ai @ 

slope 4.5 

6,905 ppm ai @ 

slope 9 

Short Grass 720 <2,294 ppm ai <973 ppm ai >690 ppm ai3

Tall Grass 330 <2,294 ppm ai <973 ppm ai <690 ppm ai 

Broadleaf plants/sm Insects 405 <2,294 ppm ai <973 ppm ai <690 ppm ai 

Fruits/pods/seeds/lg insects 45.0 <2,294 ppm ai <973 ppm ai <690 ppm ai 

Bolded values are above one-tenth of the LC50 value. 

1 Dietary-based residue levels for a banded application on sweet potatoes from Table 8.

2 Extrapolated bobwhite quail and mallard duck subacute dietary LC50 values from Appendix C. 3

Based on a LC50 of 6,905 ppm with a probit slope response of 9 the residue level of 720 ppm ai would 

correspond to odds of mortality for an exposed individual of less than 1 in 2 x 1018.

A comparison of modeled acute oral LD50 values from the bobwhite quail study with 

predicted residue dose levels were not performed because the estimated dietary equivalent 

level is less than half of that tested in the dietary administration test. The acute oral dose 

tested for the bobwhite quail of 2,230 mg ai/kg-bw is estimated to be equivalent to a dietary 

level of 4,221 mg ai/kg-diet37 using Nagy’s generalized food ingestion rate. This test value is 

less than the highest level tested in the bobwhite quail 8-day subacute dietary study (9,731 

mg ai/kg-diet). While compounds administered as an acute oral bolus (single, large dose) 

may potentially be more toxic than when administered in the diet, in this case as neither 

study produced mortality or sublethal effects at the highest treatment the use of the acute oral 

limit test dose is indicated to overestimate risk. Additionally, for pesticides it has been noted 

that the LC50 value may be a better indicator of acute toxicity to birds than the LD50 when the 

LD50 values are greater than 50 mg/kg-bw and the pesticide persists in the environment with 

a foliar half-life greater than one-day (USEPA, 2000). 

Dietary- and dose-based EEC values for the highest application rate (1.5 lbs ai/A broadcast 

which is equivalent to 3.0 lbs ai/A in band) due to off-site movement and deposition of 

volatilized clomazone (Table 13 and Table 14, respectively) are lower than on-field residues 

at the highest application rate. Because measureable direct acute or chronic risks are not 

expected at the highest application rate from residue levels on field as discussed in the 

 

37 ( ) ( ) 4,221 0.094

2,230 0.178 , /

, . ./ , , . ./ 50 50 = × = −

− × − − = FIR kg diet day

LD mg ai kg bw BW kg bw LC mg ai kg diet ; 

where the bobwhite quail body weight (BW) is 0.178 kg, and the food ingestion rate (FIR) is 0.094 kg-diet, 

ww/day = ((0.648)(BW, g)0.65 ÷ 0.2) ÷ 1000 g/kg, where 0.2 converts the dry weight (dw) to wet weight (ww). 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 77 

preceding paragraphs, residue levels due to volatilization and redeposition would also not be 

expected to result in acute or chronic effects of concern. 

Mammals 

Based on an evaluation of all lines of evidence acute and chronic risks based on clomazone 

exposure were considered overall not to pose unreasonable adverse acute or chronic effects 

for nonlisted species. The lines of evidence include that chronic dietary-based RQ values for 

predicted upper bound residue levels at the highest application rate, banded application on 

sweet potatoes, were below the chronic LOC for all dietary items (Table 27). No acute 

dietary study was performed but there was a 90-d dietary study with rats. Because a 90-d 

LC50 value is very likely lower than a subacute 5-d dietary study value (or at most equal if an 

asymptotic response with time was reached at 5 days), the use of a 90-d value in place of a 5-

d value to assess risk is expected to be conservative. In the 90-d dietary study there was no 

mortality observed at any test group, the highest dietary concentration was 8,000 ppm (1,200 

mg ai/kg-bw) and there were 240 test animals/dose (120 males and 120 females) (Acc # 

00132586, Appendix B). A provisional method described in Appendix C was used to 

extrapolate a dietary 90-d LC50 value for this data. Because no concentration-response 

relationship data is available from a limit study, LC50 values were modeled for a minimum 

slope of 2, a maximum slope of 9, and typical slope of 4.5 observed for pesticides in general 

(USEPA, 1986; USEPA, 2004). Then EEC residue levels were compared to one-tenth of 

each modeled LC50 value (0.1 x LC50), the factor applied as the effects threshold for the acute 

toxicity LOC for an endangered avian species (USEPA, 2004). Predicted on-field upper 

bound dietary residue levels are lower than one-tenth of extrapolated LC50 values (Table 45) 

indicating that no direct acute effects are expected either on a dietary basis. This means all 

other use rates are also below the acute endangered species and chronic LOC and therefore 

no measureable effect on survival and reproduction of individuals in these dietary groups is 

predicted from acute or chronic dietary exposure to clomazone at label rates. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 78 

Table 45 Upper bound mammalian dietary levels for a banded application on sweet 

potatoes compared to extrapolated dietary acute toxicity values 

Comparison of EEC to One-Tenth of the Extrapolated 

LC50 values2

Dietary Item EEC (mg/kgdiet) 1

85,102 ppm ai @ 

slope 2 

22,880 ppm ai @ 

slope 4.5 

13,529 ppm ai @ 

slope 9 

Short Grass 720 <8,510 ppm ai <2,288 ppm ai <1,353 ppm ai 

Tall Grass 330 <8,510 ppm ai <2,288 ppm ai <1,353 ppm ai 

Broadleaf plants/sm Insects 405 <8,510 ppm ai <2,288 ppm ai <1,353ppm ai 

Fruits/pods/seeds/lg insects 45.0 <8,510 ppm ai <2,288 ppm ai <1,353ppm ai 

Bolded values are above one-tenth of the LC50 value. 

1 Dietary-based residue levels for a banded application on sweet potatoes from Table 8.

2 Extrapolated rat 90-d dietary LC50 value from Appendix C. 

The dose-based assessment resulted in agreement that acute and chronic effects to mammals 

at the highest label application rate were not expected from residues on some dietary items, 

such as granivores eating primarily seeds, herbivores feeding on fruits and pods, insectivores 

feeding primarily on small insects, and agreement that acute and chronic effects are not 

expected for large (1000 grams bw and greater) mammalian herbivores, insectivores, or 

granivores from on-field residues on their dietary items, except for herbivores feeding on 

short grass (Table 28). However unlike the dietary-based assessment the acute and chronic 

dose-based RQ values for mammals below 1000 grams bw, exceeded the acute endangered 

species LOC and chronic LOC indicating a potential for residues on short grass, tall grass, 

broadleaf plants, and small insects to pose adverse acute and chronic effects. Further 

evaluation of the individual risk of mortality for the acute exceedances and the type and 

degree of effects for chronic effects are interpreted in this case as not significant for nonlisted 

species, these refinements are discussed in the following paragraphs. 

The individual risk of mortality based on the typical dose response slope (4.5) is 1 in 29,900 

(RQ=0.013; 15 gram bw) to <1 in 10 million (RQ=0.06; 1000 gram bw) for residues on 

broadleaf plants and small insects; 1 in 294,000 (RQ=0.10; 15 gram bw) to <1 in 100 million 

(RQ=0.05; 1000 g bw) for residues on tall grass; and 1 in 491 (RQ=0.23; 15 gram bw) to 1 in 

294,000 (RQ=0.10; 1000 g bw) for residues on short grass (Table 28 and Table 41). 

Therefore, although the dose-based acute RQ values exceeded for some mammals and 

dietary items indicating a potential for acute mortality effects of concern, an evaluation of the 

individual mortality risk indicate such risks are unlikely to occur or be measureable for 

nonlisted species in this case. Another consideration is that acute dose-based values were 

calculated using results from an acute oral gavage study where clomazone was administered 

as a single, large oral dose, which may potentially be more toxic than when administered in 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 79 

the diet. The dose-based approach assumes that the uptake and absorption kinetics of a 

gavage toxicity study approximate the absorption associated with uptake from a dietary 

matrix. While no acute dietary study was performed the lack of mortality in a 90-d dietary 

study with rats at 8,000 ppm ai with a calculated dose level of 1,200 mg ai/kg-bw would 

appear to support that clomazone administered in a dietary matrix is less toxic than that 

administered by gavage as the LD50 of 1,369 mg/kg-bw is comparable to 1,200 mg ai/kg-bw 

but one study has 50% mortality at the dose level, the other none. 

Dietary items that have exceedances of the chronic LOC for dose-based residue levels at the 

highest application rate include short grass, tall grass, broadleaf plants, and small insects. 

The chronic RQ values that exceed the chronic LOC range from 1.4 to 3.5. A closer 

examination of the type and magnitude of effects associated at doses 1.4 to 3.5 times the 

NOAEC (NOAEL) in the 2-generation reproduction study are a modest decrease in body 

weight of female rats during premating in F2a and F2b generations and during lactation with 

no detectable effect to reproduction (e.g., fertility, number of young produced) or viability of 

young, and no consistent statistically significant impact or correlation between test group and 

growth of pups. Although statistical significance and dose responses are not consistent and 

there is no strict correlation between combined weights at day 4 and day 21 in the 2000 and 

4000 ppm (2 and 4 times the NOAEC (NOAEL)) dose groups for pups it was assumed that 

growth was indicated to be impacted at these levels for pups since growth effects on parental 

females were significant at these levels. The decrease in body weight in F2a and F2b 

premating females were 26% and 11%, respectively, at 2 times the NOAEC (NOAEL) and 

26% and 13%, respectively, at 4 times the NOAEC (NOAEL). No significant effects 

associated with clomazone exposure were detected at any of the test doses; the highest dose 

was 4 times the NOAEC (NOAEL) for effect on body weight of females. Therefore, 

although the dose-based chronic RQ values exceeded for some mammals indicating a 

potential for reproductive effects of concern further evaluation of the type and degree of 

expected effects indicate that reproductive effects are not expected at these residue levels. 

Another consideration is that the above evaluation of risk potential was based on upper 

bound residue levels. While there is some uncertainty in how representative predicted mean 

residue levels based on the Fletcher et al. (1994) modified Kenaga nomogram values are of 

actual mean residues, risks calculated using these mean residue values (Table 46) indicate no 

exceedance of the endangered species LOC (0.1) and exceedence of the chronic LOC only 

for small- to medium-sized mammals ingesting short grass. As discussed in the previous 

paragraph, such a degree of exceedance of the chronic LOC in this case is not expected to 

result in effects to reproduction. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 80 

Table 46 Mean acute and chronic dose-based RQ values for mammals for a banded 

application on sweet potatoes 

Functional Feeding Group 15 g mammal 35 g mammal 1000 g mammal 

 Dietary Item Acute1

 Chronic Acute1 Chronic Acute1 Chronic 

Herbivores/Insectivores 

 Short Grass 0.08 1.2 0.07 1.1 0.04 0.6 

 Tall Grass 0.03 0.5 0.03 0.5 0.02 0.2 

 Broadleaf plants/sm insects 0.04 0.7 0.04 0.6 0.02 0.3 

Bolded chronic RQ values exceed the chronic LOC of 1. 

1 Acute RQ = (dose-based mean EEC, mg/kg-bw) / (LD50; mg/kg-bw). The mean EEC values for a given body weight are 

from Table 11 and LD50 values adjusted for the given body weight are in Table 20.

2 Chronic RQ = (dose-based mean EEC, mg/kg-bw) / (chronic NOAEC, mg/kg-bw). The dose-based mean EEC values for a 

given body weight and dietary item are from Table 11 and the NOAEC values adjusted for a given body weight are in Table 

20.

Commence EC, a multiple active ingredient formulation, is registered for use on cotton and 

soybean sites. This formulation was identified as potentially being more toxic than 

consideration of the individual active ingredient components. For Commence EC an 

assessment of acute exposure based on the formulation was conducted in addition to the 

assessment conducted on the basis of the active ingredient discussed in the previous 

paragraphs. All dietary item dose-based levels for were below the acute endangered species 

LOC (0.1) except for mammals smaller than 1000 g ingesting short grass. Therefore except 

for short grass, no effect to listed species feeding primarily on tall grass, broadleaf plants, 

small or large insects, fruits, or pods are expected. In addition, while the acute RQ values of 

0.17 and 0.14 exceed the endangered species LOC for 15-g and 35-g mammals, respectively, 

the odds of mortality at these RQ levels for an individual are considered discountable (<1 in 

1022 at the study probit slope of 12.679). 

Dietary- and dose-based EEC values for the highest application rate (1.5 lbs ai/A broadcast 

which is equivalent to 3.0 lbs ai/A in band) due to off-site movement and deposition of 

volatilized clomazone (Table 13) are lower than on-field residues at the highest application 

rate. Because measureable direct acute or chronic risks are not expected at the highest 

application rate from dietary residue levels on field as discussed in the preceding paragraphs, 

residue levels due to volatilization and redeposition would also not be expected to result in 

acute or chronic effects of concern on a dietary-basis. Conservative dose-based EEC values 

due to volatilization are also lower (Table 15) than those estimated for the 

Beneficial Insects 

In terms of the environmental baseline risk, agricultural fields are not in general managed to 

maintain terrestrial invertebrate populations to support populations of other animals. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 81 

Agronomic practices, irrespective of pesticides, affect the diversity and magnitude of 

terrestrial invertebrates on a field. As such, except for the potential for a listed species to be 

directly affected, the loss of populations of terrestrial invertebrates on a treated field are not 

considered here for indirect effects due to loss of a prey population for other species. 

Because on-field upper bound residue levels at the highest application rate, banded 

application on sweet potatoes, were below the LOC off-field residues for all uses due to 

spray drift or redeposition of volatilized clomazone will also be at acceptable risk levels for 

nonlisted species. 

4.2.1.2 DIRECT EFFECTS TO AQUATIC PLANTS AND ANIMALS

Label Uses Other Then Rice 

Primary Productivity and Habitat Integrity

Risks to non-listed aquatic plants both vascular and nonvascular are considered to be within 

acceptable levels for all registered clomazone uses. Therefore, no direct effects on primary 

productivity and habitat integrity in aquatic systems are expected. All vascular and 

nonvascular aquatic plant RQ values based on exposures estimated assuming no 

volatilization from soil occurs and 1-in-10 year peak concentrations in surface water were 

below the nonlisted species LOC (Table 36). The EEC values used in this assessment are 

considered to be conservative as most expected exposures in surface waters will be below 

this level as some volatilization is expected to occur from surface soil. 

Freshwater Fish, Invertebrates, and Amphibians (Aquatic-Phase)

Direct effect risks to freshwater fish, and the amphibians for which they are surrogates, and 

freshwater invertebrates are considered to be within acceptable levels for all registered 

clomazone uses. All acute and chronic freshwater fish and invertebrate RQ values based on 

1-in-10 year peak concentrations in surface water, assuming no volatilization occurs from 

soil were below the acute endangered species LOC and chronic LOC, respectively (Table 

36). The EEC values used in this assessment are considered to be conservative as most 

expected exposures in surface waters will be below this level as some volatilization is 

expected to occur from soil. 

Estuarine-Marine Fish

Direct effect risks to estuarine-marine fish are considered to be within acceptable levels for 

all registered clomazone uses. All acute and chronic estuarine-marine fish acute and chronic 

RQ values based on 1-in-10 year peak concentrations in surface water were below the acute 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 82 

endangered species LOC and chronic LOC, respectively (Table 37). Additionally, the EEC 

values used in this assessment are considered to be conservative as most expected exposures 

in surface waters will be below this level as some volatilization is expected to occur. 

Estuarine-Marine Invertebrates

Based on an evaluation of all lines of evidence risks to non-listed estuarine-marine 

invertebrates were considered to be within acceptable levels for all registered clomazone 

uses. All chronic estuarine-marine invertebrate RQ values in surface water were below the 

chronic LOC (Table 37). While some uses in a given region of the U.S. had acute RQ values 

that exceeded the acute endangered species LOC, this was for exposure estimates based on 

conservatively assuming no volatilization of clomazone off the treated field. The odds of 

mortality if exposure occurred at these levels (acute RQ values <0.07) is <1 in 1029 under the 

estimated mean mysid clomazone response slope (Table 41), which is considered to be 

within acceptable risk levels for populations of marine invertebrates. Additionally, if 

volatilization is considered, none of the acute RQ values exceed the acute endangered species 

LOC (0.05) (Table 37); the odds of mortality for an individual are <1 in 1036. 

Rice Uses—Wet Seeded 

Primary Productivity and Habitat Integrity

Water seeded labels range from requiring no holding time (Section 24(c) labels) to a holding 

time of 14 days (Section 3 label), before release of rice flood waters. Risks to non-listed 

vascular aquatic plants are considered to be within acceptable levels for wet seeded rice 

applications since the vascular plant RQ values from day of application are below the 

endangered species LOC (Table 38). However, the nonvascular plant RQ values exceed the 

non-listed aquatic plant LOC from day of application to the 14 day post-application holding 

time indicating the potential to affect listed vascular aquatic plants. Additionally, there is a 

potential to reduce biomass levels of nonvascular plants if rice paddy flood waters are 

released into surface waters during this time. It was estimated that up to 42 days of hold time 

is needed to reduce levels below the nonlisted species LOC. An evaluation of the range of 

nonvascular plants likely to be affected indicated only one of the four species (freshwater 

diatom, N. pelliculosa) tested had a 4-d IC50 value of above the nonlisted species LOC. The 

other three nonvascular test species (cyanobacteria, green algae, and a saltwater diatom) have 

4-d IC50 values of >950 ppb which are below the nonlisted species LOC and for two of these 

the concentration is lower than their NOAEC. Given 75% of the tested nonvascular species 

are below LOC, overall nonvascular, algal biomass and therefore primary productivity is not 

likely to be unreasonably adversely affected but the diversity of species has the potential to 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 83 

be affected. Additionally, given that some form of fertilizer has likely been added to the rice 

field, and in some cases is added concurrent with clomazone, clomazone is not likely to 

reduce nonvascular plant biomass in waters receiving rice flood water as those nonvascular 

plants not impacted are expected to take advantage of the additional nutrient resources. The 

diversity of species may be impacted, but this would also be expected to occur due to cultural 

practices in rice fields. Therefore the clomazone effect on nonvascular plants in receiving 

waters is considered to likely be brief and transient, and be within the background level of 

effects to nonvascular biomass and diversity attributable to such cultural practices. For 

example, the creation of standing or slow moving, shallow waters of a rice field will favor 

production of certain species of nonvascular plants over those present in a deeper, faster 

moving aquatic system and the added nutrients will favor some species over others both 

within the rice field and in the receiving water. 

Freshwater Fish and Amphibians (Aquatic-Phase)

Freshwater fish and the amphibians for which they are surrogates were indicated to be 

potentially at risk acutely from release of rice paddy flood water into surface waters. The 

acute endangered species LOC is exceeded in rice paddy water from the day of application to 

14 days post-application indicating the potential for acute effects if released into surface 

waters. It was estimated that up to 43 days of hold time is needed to reduce levels to the 

acute endangered species LOC exceedence criteria. The odds of mortality for an individual 

fish are 1 in 89 if rice paddy water is released on the day of application (acute RQ = 0.19), by 

day 14 post-application the odds of mortality are 1 in 397 (acute RQ = 0.12) (Table 41), 19 

days post-application the odds of mortality of individual decline to <1,000 (acute RQ = 0.10). 

Therefore individual listed freshwater and amphibian species may be directly affected and 

these prey bases may be acutely impacted. 

There is a high degree of uncertainty in this risk determination. The silver catfish acute value 

used for this assessment was from a study testing a clomazone formulation. Because there is 

no TGAI study with the silver catfish, and no formulation testing with the freshwater fish 

species that were tested with the TGAI, it is not possible to determine if the difference is due 

to a difference in sensitivity to clomazone, to the formulation, or both. If the rainbow trout 

value (19,000 ppb), which is the more sensitive freshwater species tested with the TGAI, is 

used to assess risk rather than the silver catfish, the acute RQ (0.03) would be below the 

listed species LOC on the day of application. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 84 

Freshwater Invertebrates

Freshwater invertebrates were indicated to be potentially at risk acutely from release of rice 

paddy flood water into surface waters. The acute endangered species LOC is exceeded in 

rice paddy water from the day of application to 14 days post-application indicating 

potentially unacceptable mortality effects. It was estimated that up to 24 days of hold time is 

needed to reduce levels to the nonlisted species LOC exceedence criteria. However, the odds 

of mortality at the concentration in rice paddy water on the day of application is <1 in 1011

(acute RQ = 0.10) (Table 41). Based on this information, risks to nonlisted freshwater 

invertebrates prey are considered to be within acceptable levels. 

Estuarine-Marine Fish

Acute effects to estuarine-marine fish species were considered to be within acceptable levels 

for release of rice paddy water as the acute RQ was below the acute endangered species LOC 

on the day of application. The odds of mortality of an individual are <1 in 109

. 

Estuarine-Marine Invertebrates

Estuarine-marine invertebrates were indicated to be potentially at risk acutely from release of 

rice paddy flood water into estuarine-marine surface waters. The acute restricted LOC is 

exceeded in rice paddy water from the day of application to 14 days post-application 

indicating potentially unacceptable mortality effects if water is released during this time. It 

was estimated that up to 98 days of hold time is needed to reduce levels to the acute 

endangered species LOC. However, given that the mean concentration response slope for the 

mysid study is 9.7919, the individual odds of mortality are <1 in 1,000 at 28 days postapplication for an acute RQ of 0.48 (Table 42). 

Concentrations as high as that in the rice paddy will be highly transitory in flowing water 

systems. These acute effects are expected to be relatively localized as a 50% dilution of rice 

paddy water, if released on the day of application or a 25% dilution if released 14 days postapplication reduces effects to these as a prey base to insignificant (e.g., odds of mortality <1 

in 1,000). 

Rice Uses—Dry Seeded 

Primary Productivity and Habitat Integrity

Based on measured concentrations in flood water from two field studies, no effect is 

expected to primary productivity in surface waters if flood waters are released. Estimated 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 85 

concentrations using a provisional model which is expected to result in conservative results 

for flood water also do not exceed the vascular plant LOC for non-listed species (Table 39) 

but do exceed the nonlisted LOC for nonvascular plants indicating the potential for concern. 

An evaluation of the range of nonvascular plants likely to be affected at the modeled 

concentration of 364 ppb indicated only one of the four species (freshwater diatom, N. 

pelliculosa) tested had a 4-d IC50 value of <364 ppb. The other three nonvascular test species 

(cyanobacteria, green algae, and a saltwater diatom) have 4-d IC50 values of >950 ppb which 

are below the nonlisted species LOC and for two of these the concentration is lower than 

their NOAEC. Given 75% of the tested nonvascular species are below LOC, overall 

nonvascular, algal biomass and therefore primary productivity is not likely to be 

unreasonably adversely affected but the diversity of species has the potential to be affected. 

Even without the presence of clomazone rice cultural practices are expected to modify the 

biomass and species diversity of nonvascular aquatic plants. For example, the creation of 

standing or slow moving, shallow water of a rice field will favor production of certain 

species of nonvascular plants over those present in a deeper, faster moving aquatic system, 

and fertilizer or nutrients added to a rice field will favor some aquatic plant species over 

others within the rice field in addition to providing nutritional resources that may not be as 

available in the same quantity in the receiving water. Clomazone effects to nonvascular plant 

biomass and diversity is expected to insignificant as compared to impacts on diversity and 

biomass from these cultural practices. 

Additionally because of the number of uncertainties and conservative accounting for 

dissipation in the provisional model, the field results are considered more environmentally 

representative of levels in flood water than the provisional model results. These field 

concentrations are below listed LOC resulting in a conclusion of no unreasonable adverse 

effects to primary productivity and associated habitat it provides from clomazone use on dry 

seeded rice. 

Freshwater Fish, Amphibians (Aquatic-Phase), and Invertebrates

Based on measured concentrations in flood water from two studies, no acute effect is 

expected to freshwater fish, the amphibians for which they are surrogates or invertebrates in 

surface waters if flood waters are released on the day of flood. Acute RQ values are below 

the endangered acute LOC (Table 39). Estimated concentrations using a provisional model 

which is expected to result in conservative results for flood water do exceed the endangered 

acute species LOC and acute restricted LOC for fish, and the endangered species LOC for 

invertebrates indicating the potential for concern (Table 39). However, consideration of the 

individual risk of mortality for the fish acute RQ (0.13) is 1 in 397 at the mean concentration-
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 86 

response slope estimate of 3.1657 (Table 41), and the individual risk of mortality for the 

invertebrate acute RQ (0.07) is <1 in 1014 at the mean concentration-response slope estimate 

of 6.7259 (Table 43). These rates would indicate that even at the conservative modeled 

levels acute effects appear not to result in unreasonable adverse effects. Because of the 

number of uncertainties and conservative accounting for dissipation in the provisional model 

the field results are considered more environmentally representative of levels in flood water 

than the provisional model results resulting in a conclusion of no unreasonable adverse 

effects to primary productivity and associated habitat it provides from clomazone use on dry 

seeded rice. 

Estuarine-Marine Fish and Invertebrates

Based on measured concentrations in flood water from two studies, no acute effect is 

expected to marine and estuarine fish or invertebrates in surface waters if flood waters are 

released on the day of flood. Additionally although conservative, provisional modeled 

results are also below the endangered acute LOC for fish (Table 39). However, the 

provisional model results do exceed the endangered species acute LOC and the restricted use 

LOC for invertebrates indicating the potential for concern (Table 39). The individual risk of 

mortality for the invertebrate acute RQ (0.64) is approximately 1 in 34 at the mean 

concentration-response slope estimate of 9.7919 (Table 43). While the modeled results 

indicate a concern for invertebrates because of the number of uncertainties and conservative 

accounting for dissipation in the provisional model the field results are considered more 

environmentally representative of levels in flood water than the provisional model results 

resulting in a conclusion of no unreasonable adverse effects to primary productivity and 

associated habitat it provides from clomazone use on dry seeded rice. 

4.2.2 Listed Species Effects Analysis 

The direct and indirect effects analysis for listed species is summarized in Table 47 for uses 

other than rice, and in Table 48 and Table 49 for wet seeded and dry seeded rice uses, 

respectively. The analysis is based on the direct and indirect effects concluded from the 

analysis of risk estimates. A co-occurrence analysis (Section 4.2.2.8) is conducted to 

evaluate if species within groups preliminarily identified in Table 47 are within the Action 

Area (Section 4.2.2.1). 

4.2.2.1 ACTION AREA

Clomazone was formulated as an ME to reduce volatilization of clomazone from soil during 

events (e.g., rain, high humidity) that would otherwise displace clomazone applied as the EC 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 87 

or WP. These formulation differences have resulted in notable differences observed in at 

least the off-field distal extent of clomazone effects reported in incident reports. Because the 

action area for the endangered species effects determination is based on the furthest distal 

extent for any effect, the difference in distal extent between these formulations result in 

different conclusions about the size of the affected area. Because formulation type in the 

case of clomazone plays a role in the size of the area affected, and may play a role for 

consideration for potential options for mitigation, if any are needed, a separate analysis for 

the extent of effects for the ME from EC and WP formulations was performed. Additionally 

because in the case of clomazone, rice versus nonrice uses have a significant impact on the 

downstream distal extent of aquatic animal and plant effects, a separate analysis was 

performed for the downstream extent of effects for rice versus nonrice uses. 

The furthest distal extent for expected direct or indirect effects to terrestrial plants and 

animals from spray drift and volatilization was determined based on an assessment of 

reported distances from treated fields in the incident report data base. The furthest distance 

for which effects to plants have been observed is 2 miles for the EC formulation (the WP 

formulation is assumed to have the same potential), and 1 mile for the ME formulation. For 

nonrice uses and dry seeded rice use aquatic organisms are not affected or effects do not 

extend beyond the field or edge-of-field. For wet seeded rice the extent of downstream 

effects is assumed to extend to the ocean. This extent is highly conservative given that for 

wet seeded rice the estimated concentration will only occur on the day of release but the tools 

to evaluate downstream dilution from releases off an agricultural field has not been 

developed for the contiguous United States, Hawaii, Alaska, or its territories. 

For convenience these areas of effect are referred to as action areas within the assessment but 

it is recognized that for the overall review the action area is the furthest extent considering all 

uses simultaneously. Specifically the effect areas evaluated for the co-occurrence analysis 

included (1) for EC and WP formulation nonrice uses, a 2 mile radius out from clomazone 

use sites; (2) for ME formulation nonrice uses and dry seeded rice use, a 1 mile radius out 

from clomazone use sites; and (3) for ME formulation wet seeded rice uses, a 1 mile radius 

out from use sites for effects to the terrestrial environment and all downstream miles 

extending from rice sites to the ocean. The action area for the clomazone registration review 

action consists of a 2 mile radius out from any clomazone use site which then further extends 

for adjacent flowing waters downstream to the ocean for rice uses. Operationally locations 

where clomazone is used are defined as all cultivated land that has had one or more of the 

crops for which clomazone is registered cultivated on it, except California where the only 

registered clomazone use is wet seeded rice. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 88 

The geographical extent of a 2 mile radius out from a use site is based on the farthest 

physical extent direct or indirect effects from exposure to clomazone are predicted to occur 

based on best available information. This action area should not be misconstrued as 

automatically meaning that unreasonable adverse effects to nonlisted species or the 

environment is occurring. The extent of the action area is based on any effect adverse or not, 

the effect may be to a single individual, and may not result in decreased survival, growth, or 

reproduction. The effects determination considers all available lines of evidence including 

estimates of risk, probability of mortality, type, degree, and magnitude of indirect and direct 

effects to listed and nonlisted species within the action area to make an effects determination 

for a specific listed species and potential for habitat modification to critical habitat. 

Crop use sites are determined from the “best available information” on locations where the 

crop is grown, the 1997 and 2002 Census of Agriculture (USDA 2002). The geographical 

extent of a 2 mile radius out from a use site is based on the farthest physical extent direct or 

indirect effects from exposure to clomazone are predicted to occur. Volatilization of 

clomazone results in the greatest spatial extent off a treated site that exposure potentially 

results in any effects. The spatial extent of volatilization effects was set based on an 

evaluation of information available on distance from treated field effects on plants from 

clomazone were observed (see Volatilization discussion under Section 4.2.1.1). Except for 

aerial spray applications, spray drift effects were determined to extend out less than 500 feet 

from a use site. Aerial spray applications extend out just over 1,000 feet. No direct effects to 

insects above baseline are expected on-site. Direct effects to birds, reptiles, terrestrial-phase 

amphibians, and mammals would be restricted to on-site or at most extend a few feet off the 

field. Except for rice, potential direct effects to freshwater fish, invertebrates, and aquatic 

plants, and estuarine-marine fish and vascular plants are at the edge of field. 

The geographical extent of a 2 mile radius out from a use site is based on the farthest 

physical extent direct or indirect effects from exposure to clomazone are expected to occur. 

In the case of clomazone this is identified as effects on non-target plants in terrestrial or 

semi-aquatic areas from potential off-site transport of clomazone due to volatilization. 

Volatilization of clomazone results in the greatest spatial extent off a treated site that 

exposure potentially results in any effects. The spatial extent of volatilization effects was set 

based on an evaluation of information available on distance from treated field effects on 

plants from clomazone were observed (see Volatilization discussion under Section 4.2.1.1). 

Except for aerial spray applications, spray drift effects were determined to extend out less 

than 500 feet from a use site. Aerial spray applications extend out just over 1,000 feet. No 

direct effects to insects above baseline are expected on-site. Direct effects to birds, reptiles, 

terrestrial-phase amphibians, and mammals would be restricted to on-site or at most extend a 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 89 

few feet off the field. Except for wet seeded rice, potential direct effects to freshwater fish, 

invertebrates, and aquatic plants, and estuarine-marine fish and vascular plants are at the 

edge of field. Operationally locations where clomazone is used are defined as all cultivated 

land that has had one or more of the crops for which clomazone is registered (Table 1) 

cultivated on it, except California where the only registered clomazone use is wet seeded 

rice. Crop use sites are determined from the “best available information” on locations where 

the crop is grown, the 1997 and 2002 Census of Agriculture (USDA 2002 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 90 Table 47 Summary of Indirect and Direct Effects to Listed Species and Preliminary Effects DeterminatioUses Other Than Rice 

Clomazone Use 

Listed species group(1)

Direct Effect 

Assessment Endpoints

Direct Effects Indirect 

Effects 

All uses Nonvascular 

Aquatic Plant Species 

Primary productivity assessed from effects on biomass as measured using cell count (density), and dry weight

N/A N/A 

All uses Vascular 

Aquatic Plant Species 

Effects on biomass as measured using frond count, dry weight, and area under the growth curve 

No Effect – Listed RQ values below listed LOC 

Plants in terrestrial and semi-aquatic areas – May affect because of potaffects to non-target plants in terrestrial and semi-aquatic areas and subsequent secondary or indirect effects to aquatic systems. Because th

main effect on terrestrial and semi-aquatic vegetation off-field is expecbe short-term inhibition in growth, erosion is not expected to be above baseline and canopy cover is not expected to be significantly reduced compared to baseline therefore these are considered insignificant effectNLAA listed aquatic vascular plants. Nonvascular (nonlisted) and vascular aquatic plants – No effect. 

All uses Freshwater 

Fish and Amphibians 

Acute mortality and reproductive effects (hatching success, growth, and survival) 

No Effect – acute and chronic RQ values below acute endangered species LOC and chronic LOC, respectively 

Plants in terrestrial and semi-aquatic areas – Main effect on terrestrial asemi-aquatic vegetation off-field is expected to be short-term inhibition growth, therefore erosion is not expected to be above baseline and canocover is not expected to be significantly reduced compared to baseline the application timing of clomazone – insignificant effects Effects on primary productivity, and fish, amphibian and invertebrate ppopulations are within acceptable levels – no effect 

All uses Freshwater 

Invertebrates 

Acute mortality and reproductive effects (fecundity, growth, survival) 

No Effect – acute and chronic RQ values are below acute endangered species LOC and chronic LOC, respectively 

Main effect on terrestrial and semi-aquatic vegetation off-field is expecbe short-term inhibition in growth, therefore erosion is not expected to 

above baseline and canopy cover is not expected to be significantly redcompared to baseline during the application timing of clomazone – insignificant effects Effects on primary productivity, and fish, amphibian, and invertebrate populations are within acceptable levels – no effect 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 91 

Clomazone Use 

Listed species group(1)

Direct Effect 

Assessment Endpoints

Direct Effects Indirect 

Effects 

All uses Estuarinemarine Fish Acute mortality and reproductive effects 

(hatching success, growth, and survival) 

No Effect – acute and chronic RQ values are below acute endangered species LOC and chronic LOC, respectively 

Main effect on terrestrial and semi-aquatic vegetation off-field is expecbe short-term inhibition in growth, effects are not on the order to affect estuarine or marine environments – insignificant effects Effects on primary productivity, and fish and invertebrate prey populatare within acceptable levels – no effect 

All uses Marine 

Invertebrates 

Acute mortality and reproductive effects (fecundity, growth, survival) and additionally for mollusks, shell deposition 

No Effect -- acute and chronic RQ values are below acute endangered species LOC and chronic LOC, respectively 

Main effect on terrestrial and semi-aquatic vegetation off-field is expecbe short-term inhibition in growth at distances not immediately adjacenthe field, effects are not on the order to affect estuarine or marine environments – insignificant effects Effects on primary productivity, fish and invertebrate populations are wacceptable levels – no effect 

All uses Marine 

Mammals 

Qualitative assessment of acute mortality and reproductive effects 

No Effect – extrapolated from no acute or chronic effects from dietary residues for large land mammals 

Main effect on terrestrial and semi-aquatic vegetation off-field is expecbe short-term inhibition in growth at distances not immediately adjacenthe field, effects are not on the order to affect estuarine or marine environments – insignificant effects Effects on primary productivity, fish and invertebrate populations are wacceptable levels – no effect 

All uses Freshwater 

Mammal 

Qualitative assessment of acute mortality and reproductive effects 

No Effect – extrapolated from no acute or chronic effects from dietary residues for large land mammals 

Main effect on terrestrial and semi-aquatic vegetation off-field is expecbe short-term inhibition in growth at distances not immediately adjacenthe field, effects are not on the order to affect estuarine or marine environments – insignificant effects Effects on primary productivity, fish and invertebrate populations are wacceptable levels – no effect 

All uses Marine Birds Qualitative assessment 

of acute mortality and reproductive effects 

No Effect -- extrapolated from no acute or chronic effects from dietary residues for land birds 

Main effect on terrestrial and semi-aquatic vegetation off-field is expecbe short-term inhibition in growth at distances not immediately adjacenthe field, effects are not on the order to affect estuarine or marine environments – insignificant effects Effects on primary productivity, fish and invertebrate populations are wacceptable levels – no effect 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 92 

Clomazone Use 

Listed species group(1)

Direct Effect 

Assessment Endpoints

Direct Effects Indirect 

Effects 

All uses Marine 

Reptiles 

Qualitative assessment of acute mortality and reproductive effects 

No Effect – extrapolated from no acute or chronic effects from dietary residues for land birds 

Main effect on terrestrial and semi-aquatic vegetation off-field is expecbe short-term inhibition in growth at distances not immediately adjacenthe field, effects are not on the order to affect estuarine or marine environments – insignificant effects Effects on primary productivity, fish and invertebrate populations are wacceptable levels – no effect 

EC uses Nontarget 

plants in terrestrial and semi-aquatic areas 

Primary productivity assessed from effects on biomass as measured using survival, length, and dry weight. 

1,500 ft from site:May affect and LAA 2 miles from site: : May affect and LAA

Main effect on terrestrial and semi-aquatic vegetation is expected to be term inhibition in growth. Listed plant species, which are reliant or obon another plant species or in a habitat that is dominated by a few speci(e.g., prairie grassland) are expected to be indirectly affected. Effects on primary productivity, mammals, birds, reptiles, and insects within acceptable levels – no effect 

ME uses Nontarget 

plants in terrestrial and semi-aquatic areas 

Primary productivity assessed from effects on biomass as measured using survival, length, and dry weight. 

1,500 ft from site:May affect and LAA 1 mile from site : May affect and LAA

Main effect on terrestrial and semi-aquatic vegetation is expected to be term inhibition in growth. Listed plant species, which are reliant or obon another plant species or in a habitat that is dominated by a few speci(e.g., prairie grassland) are expected to be indirectly affected. Effects on primary productivity, mammals, birds, reptiles, and insects within acceptable levels – no effect 

1.5 lbs ai/A broadcast applied as a banded application (~3.0 lbs ai/A in band) 

Birds, Reptiles, and (Land-Phase) Amphibians 

Acute mortality and reproductive effects 

May affect but NLAA – Dietary chronic RQ values did not exceed chronic LOC but 

potentially there is an exceedance of acute dietary levels, but this was considered discountable as the individual risk of mortality is <1 in 2 x 1018

Main effect on terrestrial and semi-aquatic vegetation is expected to be term inhibition in growth. Effects to off-site diverse plant communities emergent vegetation are expected to be mainly reduction of growth. Therefore, indirect effects to food resources or habitat requirements of 

to large carnivorous or omnivorous listed birds, reptiles, and terrestrial-amphibians and large herbivores with large home ranges are expected tinsignificant. Species which are reliant or obligate on a limited number of plant specihabitat that is dominated by a few species (e.g., prairie grassland), have 

localized populations, or are highly reliant on camouflage during late M

through late July may potentially be indirectly affected from impacts to vegetation. 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 93 

Clomazone Use 

Listed species group(1)

Direct Effect 

Assessment Endpoints

Direct Effects Indirect 

Effects 

Application Rates <1.5 lbs ai/A broadcast (or applied as a band) 

Birds, Reptiles, and (Land-Phase) Amphibians 

Acute mortality and Reproductive effects 

No Effect – Dietary chronic RQ values do not exceed the chronic LOC and acute effects are below levels of concern 

Main effect on terrestrial and semi-aquatic vegetation is expected to be term inhibition in growth. Effects to off-site diverse plant communities emergent vegetation are expected to be mainly reduction of growth. Therefore, indirect effects to food resources or habitat requirements of to large carnivorous or omnivorous listed birds, reptiles, and terrestrial-amphibians and large herbivores with large home ranges are expected tinsignificant. Species which are reliant or obligate on a limited number of plant specihabitat that is dominated by a few species (e.g., prairie grassland), have localized populations, or are highly reliant on camouflage during late Mthrough late July may potentially be indirectly affected from impacts to 

vegetation 

Application Rates >0.75 lbs ai/A broadcast (or banded equivalent) 

Land Mammals 

Acute mortality and reproductive effects 

Mammals that eat 

short grass, tall grass, broadleaf plants, or small insects May 

affect but NLAA –see Land mammal discussion in text (Section 4.2.1.1) Mammals that eat 

seeds, fruits, pods, or large insects: No 

Effect -- dietary and dose-based acute and chronic RQ values below acute and chronic LOC 

Main effect on terrestrial and semi-aquatic vegetation is expected to be term inhibition in growth. Effects to off-site diverse plant communities emergent vegetation are expected to be mainly reduction of growth. Therefore, indirect effects to food resources or habitat requirements of to large carnivorous or omnivorous listed mammals and large herbivorelarge home ranges are expected to be insignificant. Species which are reliant or obligate on a limited number of plant specihabitat that is dominated by a few species (e.g., prairie grassland), have 

localized populations, or are highly reliant on camouflage during late M

through late July may potentially be indirectly affected from impacts to vegetation. 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 94 

Clomazone Use 

Listed species group(1)

Direct Effect 

Assessment Endpoints

Direct Effects Indirect 

Effects 

Application Rates <0.75 lbs ai/A broadcast (or banded equivalent) 

Land Mammals 

Acute mortality and Reproductive effects 

No Effect – dietary and dose-based acute and chronic RQ values below acute and chronic LOC 

Main effect on terrestrial and semi-aquatic vegetation is expected to be term inhibition in growth. Effects to off-site diverse plant communities emergent vegetation are expected to be mainly reduction of growth. Therefore, indirect effects to food resources or habitat requirements of to large carnivorous or omnivorous listed mammals and large herbivore

large home ranges are expected to be insignificant. Species which are reliant or obligate on a limited number of plant specihabitat that is dominated by a few species (e.g., prairie grassland), have localized populations, or are highly reliant on camouflage during late Mthrough late July may potentially be indirectly affected from impacts to vegetation. 

All uses Subterranean 

Ground and Mold Insects and Subterranean Ground Arachnids 

Acute mortality No Effect -- no 

direct exposure 

No indirect effects expected. The base of the food web for these cave dwellers (troglobites) is driven by nutrients in the form of leaf litter, ordebris, plant roots, feces, eggs, or dead bodies of other cave dwellers or visitors. 

All uses Surface Insects 

and Arachnid 

Acute mortality No Effect Listed insects and arachnids reliant on a limited number of plant specie

be indirectly affected from impacts to vegetation 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 95 Table 48 Summary of Indirect and Direct Effects to Listed Species and Preliminary Effects DeterminatioUses On Wet Seeded Rice 

Listed species attribute group(1)

Direct Effect 

Assessment 

Endpoints 

Direct Effects Indirect 

Effects 

Nonvascular Aquatic Plant Species 

Primary productivity assessed from effects on biomass as measured using cell count (or density), and dry weight 

N/A – no listed nonvascular listed species 

N/A – no listed nonvascular listed species. 

Vascular Aquatic Plant Species 

Primary productivity assessed from effects on biomass as measured using frond count, and dry weight 

NE – plant RQ values are below listed LOC at 0.6 lbs ai/A 

Nontarget plants in terrestrial and semi-aquatic areas – Clomazone does not have the potential to volatize from water as it does from soil, exposure of off-site plants is from 

spray drift. No effect to monocot primary productivity or biomass is expected as spray 

drift exposure is estimated to be below the nonlisted species LOC for both ground and 

aerial spray applications using coarse droplet size. Indirect affects may occur because the listed LOC is exceeded for dicots for ground spray applications and monocots and dicots for aerial spray applications. Because the main effect to non-target plants in terrestrial and 

semi-aquatic areas is expected to be short-term inhibition in growth on limited species (i.esurvival not affected in vegetation vigor study at estimated spray drift load for rice, survival and recovery of plants is high in field studies, the IC25 for growth effects is only 

exceeded for 1 out of the 10 plants tested for ground spray applications and 2 out of 10 species tested for aerial applications) with no effect to exposed monocot population, 

erosion is not expected to be above baseline and canopy cover is not expected to be significantly reduced compared to baseline, therefore these are considered insignificant effects and NLAA listed aquatic vascular plants. Nonvascular aquatic plants – May affect because RQ exceeds the nonlisted LOC. 

However, the effect would be short term and a majority of species tested (75%, 3 out of th4 studies) are below the LOC such that overall biomass is not likely to be affected, main effect is likely to be species diversity but this would also be expected to occur due to the addition of fertilizer and effects to water quality due to the creation of standing or slow moving, shallow ponded areas which favor different species over others and therefore indirect effects are considered insignificant and NLAA over baseline rice field release water impacts. 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 96 

Listed species attribute group(1)

Direct Effect 

Assessment 

Endpoints 

Direct Effects Indirect 

Effects 

Freshwater Fish and Amphibians 

Acute mortality May affect and 

LAA– No hold to 

14-day postapplication hold time: acute RQ 

exceeds acute LOC (Note: uncertainty about silver catfish value affects risk conclusions) 

Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see discussion for vascular aquatic plant species) Nonvascular aquatic plants – May affect but NLAA (see discussion for vascular aquatic 

plant species) Freshwater aquatic invertebrates –Loss of this prey base is considered insignificant as the individual odds of mortality is <1 in 1011 and therefore NLAA. 

Freshwater fish and amphibians – At release on day of application LAA fish prey base. A14-days post-application LAA for species obligate on another fish or amphibian as individuals may be affected but NLAA for nonobligate species as loss of this prey base is considered insignificant 

Freshwater Invertebrates 

Acute mortality May affect but 

NLAA – No hold 

to 14-day postapplication hold time: acute RQ 

exceeds acute LOC but risk of mortality of an individual (<1 in 1011) is considered discountable 

Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see discussion for vascular aquatic plant species) Nonvascular aquatic plants – May affect but NLAA (see discussion for vascular aquatic 

plant species) Freshwater aquatic invertebrates –Loss of this prey base is considered insignificant (individual odds of mortality <1 in 1011), NLAA 

Freshwater fish and amphibians – At release on day of application LAA fish prey base. A14-days post-application LAA for species obligate on another fish or amphibian as individuals may be affected but NLAA for nonobligate species as loss of this prey base is considered insignificant. 

Estuarinemarine Fish Acute mortality No Effect – acute RQ is below 

listed species acute LOC 

Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see discussion for vascular aquatic plant species) Nonvascular aquatic plants – May affect but NLAA (see discussion for vascular aquatic 

plant species) Marine and estuarine invertebrates – No hold to 14-day post-application hold time: May 

affect and LAA because the acute RQ value exceeds the nonlisted acute LOC and effect level is considered potentially significant to this prey base. These acute effects are expected to be relatively localized as a 50% dilution of rice paddy water, if released on thday of application or a 25% dilution if released 14 days post-application drops these effecto insignificant (e.g., odds of mortality <1 in 1,000). Estuarine and marine fish – No effect 
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Registration Review Ecological Risk Assessment Clomazone 97 

Listed species attribute group(1)

Direct Effect 

Assessment 

Endpoints 

Direct Effects Indirect 

Effects 

Marine Invertebrates 

Acute mortality and additionally for mollusks, shell deposition 

May affect and LAA– No hold to 

14-day postapplication hold time: acute RQ 

exceeds acute listed species LOC 

Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see discussion for vascular aquatic plant species) Nonvascular aquatic plants – May affect but NLAA (see discussion for vascular aquatic 

plant species) Marine and estuarine invertebrates – No hold to 14-day post-application hold time: May 

affect and LAA because effects are considered potentially significant to this prey base as nonlisted species acute LOC is exceeded. Estuarine and marine fish – No effect 

Marine Mammals 

Qualitative assessment of acute mortality 

No Effect Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see 

discussion for vascular aquatic plant species) Nonvascular aquatic plants – May affect but NLAA (see discussion for vascular aquatic 

plant species) Marine and estuarine invertebrates – No hold to 14-day post-application hold time: May 

affect and LAA because effects are considered potentially significant to this prey base as nonlisted species acute LOC is exceeded. Estuarine and marine fish – No effect 

Freshwater Mammal 

Qualitative assessment of acute mortality 

No Effect Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see 

discussion for vascular aquatic plant species) Nonvascular aquatic plants – May affect but NLAA (see discussion for vascular aquatic 

plant species) Freshwater aquatic invertebrates – May affect but NLAA (see discussion for Freshwater Aquatic Species) Freshwater fish and amphibians – NLAA for nonobligate species as loss of this prey base is considered insignificant (no freshwater listed mammal obligate on a fish or amphibian species) 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 98 

Listed species attribute group(1)

Direct Effect 

Assessment 

Endpoints 

Direct Effects Indirect 

Effects 

Marine Birds Qualitative assessment 

of acute mortality 

No Effect Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see 

discussion for vascular aquatic plant species) Nonvascular aquatic plants – May affect but NLAA (see discussion for vascular aquatic 

plant species) Marine and estuarine invertebrates – No hold to 14-day post-application hold time: May 

affect and LAA because effects are considered potentially significant to this prey base as nonlisted species acute LOC is exceeded. Estuarine and marine fish – No effect 

Marine Reptiles Qualitative assessment 

of acute mortality 

No Effect Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see 

discussion for vascular aquatic plant species) Nonvascular aquatic plants – May affect but NLAA (see discussion for vascular aquatic 

plant species) Marine and estuarine invertebrates – No hold to 14-day post-application hold time: May 

affect and LAA because effects are considered potentially significant to this prey base as nonlisted species acute LOC is exceeded. Estuarine and marine fish – No effect 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 99 

Listed species attribute group(1)

Direct Effect 

Assessment 

Endpoints 

Direct Effects Indirect 

Effects 

Nontarget plants in terrestrial and semi-aquatic areas 

Survival of individuals and effects on growth (length, and dry weight). 

Ground spray 

applications

MA/LAA dicots (dicot plant RQ exceeds listed species LOC) Aerial spray 

applications: 

MA/LAA monocots and dicots (listed species LOC is exceeded for monocots and dicots) 

Nontarget plants in terrestrial and semi-aquatic areas – Clomazone does not have the 

potential to volatilize from water as it does from soil, exposure of off-site plants will be primarily due to spray drift and potentially some subsequent volatilization from these spradrift impacted areas. Ground spray applications: May affect because of potential affects t

dicot plants (nonlisted species LOC is exceeded). However exposure levels are below the listed species LOC for monocots, and the main effect on dicot plants off the field is expected to be primarily short-term inhibition in growth for a limited number of species (estimated spray drift exposure levels are below levels exhibiting mortality in a vegetative 

vigor study and the IC25 for growth effects is only exceeded for 1 out of the 10 plants 

tested). For listed plant species dependent on a community of plants these levels of effectare considered insignificant and therefore NLAA, but for listed plants obligate on a plant (dicot) species, LAA. For the coarse droplet size required by the label, direct spray drift effects to the limited dicot species drop below the listed species LOC at 155 ft from the field margin. Aerial spray applications: May affect because of potential affects to dicot plants (nonliste

species LOC is exceeded for dicots, listed species LOC is exceeded for monocots). However, the main effect on plants off the field is expected to be primarily short-term inhibition in growth for a limited number of species (estimated spray drift exposure levels are below levels exhibiting mortality in a vegetative vigor study and the IC25 for growth 

effects is only exceeded for 2 out of the 10 plants tested). For listed plant species dependent on a community of plants these levels of effects are considered insignificant antherefore NLAA, but for listed plants obligate on a plant species, LAA. Birds, reptiles, amphibians (land-phase), mammals and insects – No Effect 

Birds, Reptiles, and (LandPhase) Amphibians 

Acute mortality and reproductive effects 

No Effect – drift residues below LOC 

Nontarget plants in terrestrial and semi-aquatic areas – May affect and NLAA, for listed species not obligate on a plant species but LAA for listed species obligate on a plant species (see discussion of Nontarget plants in terrestrial and semi-aquatic areas indirect effects) Birds, reptiles, amphibians (land-phase), mammals and insects – No Effect 

Land Mammals Acute mortality and 

Reproductive effects 

No Effect – drift residues below LOC 

Nontarget plants in terrestrial and semi-aquatic areas – May affect and NLAA, for listed species not obligate on a plant species but LAA for listed species obligate on a plant species (see discussion of Nontarget plants in terrestrial and semi-aquatic areas indirect effects) Birds, reptiles, amphibians (land-phase), mammals and insects – No Effect 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 100 

Listed species attribute group(1)

Direct Effect 

Assessment 

Endpoints 

Direct Effects Indirect 

Effects 

Insects and arachnids 

Acute mortality No Effect – drift 

residues below LOC 

Nontarget plants in terrestrial and semi-aquatic areas – May affect and NLAA, for listed species not obligate on a plant species but LAA for listed species obligate on a plant species (see discussion of Nontarget plants in terrestrial and semi-aquatic areas indirect effects) Birds, reptiles, amphibians (land-phase), mammals and insects – No Effect 

Table 49 Summary of Indirect and Direct Effects to Listed Species and Preliminary Effects DeterminatioUses On Dry Seeded Rice 

Listed species attribute group(1)

Direct Effect 

Assessment 

Endpoints 

Direct Effects Indirect 

Effects 

Nonvascular Aquatic Plant Species 

Primary productivity assessed from effects on biomass as measured using cell count (or density), and dry weight 

N/A – no listed nonvascular listed species 

N/A – no listed nonvascular listed species. 

Vascular Aquatic Plant Species 

Primary productivity assessed from effects on biomass as measured using frond count, and dry weight 

None – Listed RQ values are 

below listed LOC for field measured residues 

Nontarget plants in terrestrial and semi-aquatic areas – May affect because of potential affects to non-target plants in terrestrial and semi-aquatic areas and subsequent secondary 

or indirect effects to aquatic systems. Because the main effect on terrestrial and semiaquatic vegetation off-field is expected to be short-term inhibition in growth, erosion is noexpected to be above baseline and canopy cover is not expected to be significantly reducecompared to baseline therefore these are considered insignificant effects and NLAA listed aquatic vascular plants. Nonvascular aquatic plants – No effect 

Freshwater Fish and Amphibians 

Acute mortality No Effect – acute 

RQ values are below listed LOC for field measured residues 

Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see discussion for vascular aquatic plant species) Vascular and nonvascular aquatic plants – No effect Freshwater aquatic fish, amphibian, and invertebrate prey base – No effect 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 101 

Listed species attribute group(1)

Direct Effect 

Assessment 

Endpoints 

Direct Effects Indirect 

Effects 

Freshwater Invertebrates 

Acute mortality No Effect – acute 

RQ values are 

below listed LOC for field measured residues 

Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see discussion for vascular aquatic plant species) Vascular and nonvascular aquatic plants – No effect Freshwater aquatic fish, amphibian, and invertebrate prey base – No effect 

Estuarinemarine Fish Acute mortality No Effect – acute RQ values are 

below listed LOC for field measured residues 

Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see discussion for vascular aquatic plant species) Vascular and nonvascular aquatic plants – No effect Marine and estuarine fish and invertebrate prey base – No effect 

Marine Invertebrates 

Acute mortality No Effect – acute 

RQ values are 

below listed LOC for field measured residues 

Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see discussion for vascular aquatic plant species) Vascular and nonvascular aquatic plants – No effect Marine and estuarine fish and invertebrate prey base – No effect 

Marine Mammals 

Qualitative assessment of acute mortality 

No Effect Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see 

discussion for vascular aquatic plant species) Vascular and nonvascular aquatic plants – No effect Marine and estuarine fish and invertebrate prey base – No effect 

Freshwater Mammal 

Qualitative assessment of acute mortality 

No Effect Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see 

discussion for vascular aquatic plant species) Vascular and nonvascular aquatic plants – No effect Freshwater aquatic fish, amphibian, and invertebrate prey base – No effect 

Marine Birds Qualitative assessment 

of acute mortality 

No Effect Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see 

discussion for vascular aquatic plant species) Vascular and nonvascular aquatic plants – No effect Marine and estuarine fish and invertebrate prey base – No effect 
Chapter 4 Res

Registration Review Ecological Risk Assessment Clomazone 102 

Listed species attribute group(1)

Direct Effect 

Assessment 

Endpoints 

Direct Effects Indirect 

Effects 

Marine Reptiles Qualitative assessment 

of acute mortality 

No Effect Nontarget plants in terrestrial and semi-aquatic areas --May affect but NLAA (see 

discussion for vascular aquatic plant species) Vascular and nonvascular aquatic plants – No effect Marine and estuarine fish and invertebrate prey base – No effect 

Nontarget plants in terrestrial and semi-aquatic areas 

Survival of individuals and effects on growth (length, and dry weight). 

1,500 ft from 

site:

May Effect and LAA 1 mile from site :

May Effect and LAA 

Nontarget plants in terrestrial and semi-aquatic areas – May affect because of potential affects to non-target plants in terrestrial and semi-aquatic areas and subsequent secondary or indirect effects to terrestrial system. Because the main effect on terrestrial and semiaquatic vegetation off-field is expected to be short-term inhibition in growth, erosion is noexpected to be above baseline and canopy cover is not expected to be significantly reducecompared to baseline therefore these are considered insignificant effects, NLAA, for listed plants not obligate on another species but LAA for listed plants obligate on another species. Bird, reptile, amphibian (land-phase), mammal and insect prey base – No Effect 

Birds, Reptiles, and (LandPhase) Amphibians 

Acute mortality and Reproductive effects 

No Effect – onfield and drift residues below LOC 

Nontarget plants in terrestrial and semi-aquatic areas – May affect and NLAA, for listed species not obligate on a plant species but LAA for listed species obligate on a plant species (see discussion of Nontarget plants in terrestrial and semi-aquatic areas indirect effects) Bird, reptile, amphibian (land-phase), mammal and insect prey base – No Effect 

Land Mammals Acute mortality and 

Reproductive effects 

No Effect – onfield and drift residues below LOC 

Nontarget plants in terrestrial and semi-aquatic areas – May affect and NLAA, for listed species not obligate on a plant species but LAA for listed species obligate on a plant species (see discussion of Nontarget plants in terrestrial and semi-aquatic areas indirect effects) Bird, reptile, amphibian (land-phase), mammal and insect prey base – No Effect 

Insects and arachnids 

Acute mortality No Effect – drift 

residues below LOC 

Nontarget plants in terrestrial and semi-aquatic areas – May affect and NLAA, for listed species not obligate on a plant species but LAA for listed species obligate on a plant species (see discussion of Nontarget plants in terrestrial and semi-aquatic areas indirect effects) Bird, reptile, amphibian (land-phase), mammal and insect prey base – No Effect 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 103 

4.2.2.2 DIRECT EFFECTS TO LISTED SPECIES FOR USES OTHER THAN RICE

Aquatic Plants 

There are no listed aquatic nonvascular plants so no further endangered species determination 

is performed on this taxonomic functional group. There are vascular freshwater and 

estuarine-marine listed species. Direct effects to listed aquatic vascular plants in receiving 

waters are considered to be within acceptable levels for all uses. All vascular aquatic plant 

RQ values based on 1-in-10 year peak concentrations in surface water, were below the listed 

species LOC (Table 36 and Table 37). Therefore, a no effect determination is made for 

direct effects to listed vascular aquatic plants. 

Freshwater Fish and Amphibians (Aquatic-Phase) 

Direct effects to listed freshwater fish and amphibians (aquatic-phase) in receiving waters are 

considered to be within acceptable levels for all uses. All freshwater fish acute and chronic 

RQ values based on 1-in-10 year peak concentrations in the modeled receiving water, were 

below the listed species acute LOC and the chronic LOC (Table 36). Therefore, a no effect 

determination is made for direct effects to listed freshwater fish and amphibians. 

Freshwater Invertebrates 

Direct effects to listed freshwater invertebrates in receiving waters are considered to be 

within acceptable levels for all uses. All freshwater invertebrate acute and chronic RQ 

values based on 1-in-10 year peak concentrations in the modeled receiving water, were below 

the endangered species acute LOC and the chronic LOC (Table 36). Therefore, a no effect 

determination is made for direct effects to listed freshwater fish and amphibians. 

Estuarine-Marine Fish 

Direct effects to listed estuarine or marine species in receiving waters are considered to be 

within acceptable levels for all uses. All estuarine/marine acute and chronic RQ values based 

on 1-in-10 year peak concentrations in the modeled receiving water, were below the 

endangered species acute LOC and the chronic LOC (Table 37). Therefore, a no effect 

determination is made for direct effects to listed estuarine or marine fish species. 
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Registration Review Ecological Risk Assessment Clomazone 104 

Estuarine-Marine Invertebrates 

Direct effects to listed estuarine/marine invertebrate species in receiving waters are 

considered to be within acceptable levels for all uses. All estuarine/marine invertebrate acute 

and chronic RQ values based on 1-in-10 year peak concentrations in the modeled receiving 

water, were below the endangered species acute LOC and the chronic LOC (Table 37). 

Therefore, a no effect determination is made for direct effects to listed estuarine or marine 

invertebrate species. 

Marine Mammals 

No direct effects from any registered clomazone use are expected to listed marine mammal 

species. This conclusion is based on an evaluation of several lines of evidence. 

Exposure routes for contaminants in water are expected to be passive uptake from water 

across the dermis, oral exposure from residues in food items and incidental ingestion of 

water. Unlike fish, marine mammals breathe air and do not have gills by which uptake of 

clomazone from water would occur. Additionally ingestion of sea water is expected to be 

incidental as marine mammals, except for the manatee, meet their water needs from water in 

their food and from formation of water during metabolic processes to maintain ionic control. 

The listed manatee is found in freshwater, brackish, and marine environments and may ingest 

freshwater from runoff, seeps, springs, streams, and rivers. Therefore the toxicity values in 

water for fish and invertebrates based on exposure to water alone with gills being the primary 

uptake route are not expected to be directly applicable to marine mammals. 

In terms of dietary exposure, the oral and dietary acute toxicity of clomazone to mammals is 

considered practically nontoxic. Modeled maximum residues on-dietary items on the field at 

the highest use rate were below endangered species acute LOC and chronic LOC, for 1,000 

gram mammals (Table 29). Clomazone residue levels in aquatic dietary items would be 

expected to be lower due to a number of factors and even if the dietary residue levels were 

the same, risks to mammals above 1000 g such as sea otters, sea-lions, and whales, would be 

expected to be lower than that of a 1000 g mammal. Secondary residues in the aquatic food 

chain due to bioaccumulation are expected to be low to not detectable since clomazone 

rapidly metabolizes as observed in both laboratory mammals and fish and residues on aquatic 

plants would also be less. Therefore, no effects are expected. 
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Registration Review Ecological Risk Assessment Clomazone 105 

Freshwater Mammal 

No direct effects from any registered clomazone use are expected to the one listed freshwater 

mammal, the manatee, which also occurs in brackish, and marine waters (see section on 

marine mammals above). The manatee breathes air and does not have gills by which uptake 

of clomazone from water would occur. 

Exposure routes for contaminants in water are expected to be passive uptake from water 

across the dermis, oral exposure from residues in food items and ingestion of freshwater. 

Unlike fish, the manatee breathes air and does not have gills by which uptake of clomazone 

from water would occur. The manatee may ingest freshwater from runoff, seeps, springs, 

streams, and rivers. Therefore the toxicity values in water for fish and invertebrates based on 

exposure to water alone with gills being the primary uptake route are not expected to be 

directly applicable to the manatee. 

In terms of dietary exposure, the oral and dietary acute toxicity of clomazone to mammals is 

considered practically nontoxic. Modeled maximum residues on-dietary items on the field at 

the highest use rate were below endangered species acute LOC and chronic LOC, for 1,000 

gram mammals (Table 29). Clomazone residue levels in aquatic dietary items would be 

expected to be lower due to a number of factors and even if the dietary residue levels were 

the same, risks to the manatee which is much larger than 1000 g, would be expected to be 

lower than that of a 1000 g mammal. Secondary residues in the aquatic food chain due to 

bioaccumulation are expected to be low to not detectable since clomazone rapidly 

metabolizes as observed in both laboratory mammals and fish and residues on aquatic plants 

would also be less. Additionally, the movement of a manatee during a day may be 

considerable, such that the assumption of 100% of the diet would be from an affected 

location is highly conservative. Therefore, no effects are expected. 

Marine Birds 

No direct effects from any registered clomazone use are expected to listed marine birds. This 

conclusion is based on an evaluation of several lines of evidence. The acute toxicity of 

clomazone to birds is considered practically nontoxic. Modeled maximum residues ondietary items on the field at the highest use rate are below the chronic LOCs (Table 27) and 

below levels of acute concern (see Section 4.2.1.1 and Table 44). Secondary residues in the 

aquatic food chain due to bioaccumulation would be expected to be significantly below onfield residue levels since clomazone rapidly metabolizes as observed in both laboratory 

mammals and fish. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 106 

Marine Reptiles 

No direct effects from registered clomazone nonrice uses are expected to listed marine reptile 

species. This conclusion is based on an evaluation of several lines of evidence. The acute 

toxicity of clomazone to birds, which are a surrogate for reptiles, is considered practically 

nontoxic. Modeled maximum residues on-dietary items on the field at the highest use rate 

are below the chronic LOCs (Table 27) and below levels of acute concern (see Section 

4.2.1.1 and Table 44). Secondary residues in the aquatic food chain due to bioaccumulation 

would be expected to be significantly below on-field residue levels since clomazone rapidly 

metabolizes as observed in both laboratory mammals and fish. Residues on aquatic plants 

would be lower than on-field. Additionally, the movement of sea turtles during a day may be 

considerable, such that the assumption of 100% of the diet would be from an affected 

location is highly conservative further. Therefore, no effects are expected. 

Nontarget plants in terrestrial and semi-aquatic areas 

Direct effects to listed plants are expected off the treated field for all clomazone uses. Off 

field transport may occur from runoff, and drift from spray at time of application and drift of 

vapors post-application. Plant RQ values from runoff and spray drift loading exceed the 

listed species LOC (Table 33) for terrestrial and semi-aquatic habitats adjacent to a treated 

field. Distances from the field that were estimated to exceed the listed species LOC due to 

loading from spray drift are listed in Table 43. These distances are based on label language 

to use a low boom and coarse spray to apply clomazone. Based on modeling, the distance 

extending from the field where spray drift loading exceeds the listed species LOC extends 

out from the edge-of-the treated field to a maximum of 470 feet for the highest application 

rate (1.5 lbs ai/A broadcast, 3.0 lbs ai/A in band application). There is a high degree of 

uncertainty in the magnitude of clomazone that will volatilize and drift off field, and in the 

distance that this volatilized clomazone will travel and be redeposited off field in 

concentrations that will affect plants. Risk estimates were made on the highly conservative 

assumption that of the estimated 65% that would volatilize off a field, 100% of the air mass 

would retain its integrity (i.e., not mix and become diluted vertically or laterally) before 

redepositing, which is recognized to be highly unlikely. An evaluation of incident data 

indicated that the vast majority of incidents appear to occur within about 300 feet of the site, 

followed by between 300 and 1,500 feet. There are distances out to 2 miles associated with 

the use of the EC formulation, but these occurred primarily in the first year clomazone was 

registered for use in 1986, and in 1993 with application to cotton. A number of label changes 

for the EC formulation were immediately instituted within a few months of registration and 

included buffer zones, soil incorporation in areas vulnerable to conditions leading to 

volatilization, and spray drift reducing label language (e.g., use of additive to reduce spray 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 107 

drift, use of low boom and coarse spray). From the incident data combined with insights 

provided by modeled spray drift effect distances, there is high degree of confidence that 

effects out to 1,500 feet are not uncommon for either the EC or ME formulations. No off-site 

incidents with the WP formulation have been reported but the volatility of clomazone for this 

formulation would not be expected to differ greatly from that of the EC. Even though most 

incidents that occurred out past 1,500 feet were associated with use in 1986, there are still 

incidents reported to occur out to 1 mile for the ME formulation and 2 miles for the EC. 

Land Birds, Reptiles, and Amphibians 

Based on an evaluation of all lines of evidence risks to listed birds and the taxonomic groups 

for which they are surrogates (reptiles and amphibians) were concluded to be no effect for 

application rates of less than 1.5 lbs ai/A applied in bands (~3.0 lbs ai/A) and may affect but 

NLAA for the highest application rate of 1.5 lbs ai/A applied in bands (~3.0 lbs ai/A). This 

conclusion was reached because clomazone applied at and above 1.5 lbs ai/A in a band (~3.0 

lbs ai/A) although not exceeding the chronic LOC (Table 27) did have one acute estimate for 

residues on short grass above 1/10th an extrapolated acute LC50 (Table 44). However, the 

individual risk of mortality is on the order of <1 in 1018 which is considered discountable. 

All other applications would not exceed chronic LOC values and extrapolated estimates of 

acute effects would also be below concern levels. 

Land Mammals 

Based on an evaluation of all lines of evidence risks to mammals were concluded to be no 

effect for application rates of 0.75 lbs ai/A or less (as a band or broadcast) and may affect but 

NLAA for rates resulting in greater than 0.75 lbs ai/A as a broadcast or within a band. This 

conclusion was reached because on a dietary-basis residue levels for all uses do not exceed 

chronic LOC and an evaluation of subchronic 90-d mortality results indicate acute effects 

should also be below acute concern levels (see Section 4.2.1.1). For application rates of 0.75 

lbs ai/A broadcast or banded applications which would be at this level within the band, dosebased acute and chronic RQ values are also below acute and chronic LOC values, 

respectively (Table 31 and see Section 4.2.1.1), and therefore these rates and lower are 

determined to have no direct effect on mammals. At the 1.0 lb ai/acre broadcast or banded 

applications which would be at this level within the band, one or more dose-based acute or 

chronic RQ values exceed either or both the acute endangered LOC or chronic LOC and 

therefore these uses are determined to may directly affect a listed mammal. At the highest 

application rate, dose-based acute and chronic RQ values for mammalian granivores eating 

primarily seeds, herbivores feeding on fruits and pods, insectivores feeding primarily on 

small insects are below acute and chronic LOC values, respectively, so these types of 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 108 

mammals would also not be directly affected. Further evaluation of the individual risk of 

mortality for the acute exceedances and the type and degree of effects for chronic effects are 

interpreted in this case as not significant for nonlisted species, these refinements are 

discussed in the following paragraphs. 

At the highest application rate, the chronic RQ values that exceed the chronic LOC range 

from 1.4 to 3.5. A closer examination of the type and magnitude of effects associated at 

doses 1.4 to 3.5 times the NOAEC (NOAEL) in the 2-generation reproduction study are a 

modest decrease in body weight of female rats during premating in F2a and F2b generations 

and during lactation with no detectable effect to reproduction (e.g., fertility, number of young 

produced) or viability of young, and no consistent statistically significant impact or 

correlation between test group and growth of pups. Although statistical significance and 

dose responses are not consistent and there is no strict correlation between combined weights 

at day 4 and day 21 in the 2000 and 4000 ppm (2 and 4 times the NOAEL) dose groups for 

pups it was assumed that growth was indicated to be impacted at these levels for pups since 

growth effects on parental females were significant at these levels. The decrease in body 

weight in F2a and F2b premating females were 26% and 11%, respectively, at 2 times the 

NOAEL and 26% and 13%, respectively, at 4 times the NOAEL. No significant 

reproductive effects associated with clomazone exposure were detected at any of the test 

doses; the highest dose was 4 times the NOAEL for effect on body weight of females. 

Therefore, although the dose-based chronic RQ values exceeded for some mammals 

indicating a potential for reproductive effects of concern further evaluation of the type and 

degree of expected effects indicate that reproductive effects are not expected at these residue 

levels. 

The individual risk of mortality based on the typical dose response slope (4.5) is 1 in 29,900 

(RQ=0.013; 15 gram bw) to <1 in 10 million (RQ=0.06; 1000 gram bw) for residues on 

broadleaf plants and small insects; 1 in 294,000 (RQ=0.10; 15 gram bw) to <1 in 100 million 

(RQ=0.05; 1000 g bw) for residues on tall grass; and 1 in 491 (RQ=0.23; 15 gram bw) to 1 in 

294,000 (RQ=0.10; 1000 g bw) for residues on short grass (Table 28 and Table 41). 

Therefore, although the dose-based acute RQ values exceeded the acute endangered species 

LOC for some mammals and dietary items indicating a potential for acute mortality effects of 

concern, except potentially for small mammals ingesting primarily short grass, an evaluation 

of the individual mortality risk indicate such risks are unlikely to occur or be measureable for 

nonlisted species in this case. Another consideration is that acute dose-based values were 

calculated using results from an acute oral gavage study where clomazone was administered 

as a single, large oral dose, which may potentially be more toxic than when administered in 

the diet. The dose-based approach assumes that the uptake and absorption kinetics of a 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 109 

gavage toxicity study approximate the absorption associated with uptake from a dietary 

matrix. While no acute dietary study was performed the lack of mortality in a 90-d dietary 

study with rats at 8,000 ppm ai with a calculated dose level of 1,200 mg ai/kg-bw would 

appear to support that clomazone administered in a dietary matrix is less toxic than that 

administered by gavage as the LD50 of 1,369 mg/kg-bw is comparable to 1,200 mg ai/kg-bw 

but one study has 50% mortality at the dose level, the other none. 

Taking into consideration that dietary-based acute and chronic LOC do not exceed even at 

the highest application rate, that dose-based chronic values while potentially resulting in a 

slight decline in weight will not affect the number of young produced or the viability of the 

young, and that dose-based acute risks of mortality are discountable or more reasonably 

represented by conclusions from dietary-based results as there is some indication that dietary 

administration is less toxic than a single, oral bolus in the case of clomazone, it was 

concluded that effects are not significant or are discountable at application rates above 0.75 

lbs ai/A broadcast or in a band. 

Insects and Arachnids 

No direct effects to listed insects or arachnids are expected to occur for any registered use 

because on-field upper bound residue levels at the highest application rate, banded 

application on sweet potatoes, were below the LOC for small and large insects. 

Clomazone use is determined to have no direct effect on subterranean ground and mold 

insects and subterranean arachnids. These animals spend their entire lives underground in 

subterranean voids (caves, sinkholes, fractures, interconnections, etc.) in areas with low air 

movement, and elevated relative humidity, and reduced temperature fluctuations. Therefore 

these animals would not be directly exposed to clomazone from spray drift, runoff, or 

redeposited volatilized clomazone. 

4.2.2.3 DIRECT EFFECTS TO LISTED SPECIES FOR USE ON WET SEEDED RICE

Aquatic Plants 

There are no listed aquatic nonvascular plants so no further endangered species determination 

is performed on this taxonomic functional group. There are vascular freshwater and 

estuarine-marine listed species. Direct effects to listed aquatic vascular plants in rice paddy 

water are below listed species LOC (Table 38). Therefore, a no effect determination is made 

for direct effects to listed vascular aquatic plants. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 110 

Freshwater Fish and Amphibians (Aquatic-Phase) 

Flood water clomazone levels are expected to exceed the acute listed species LOC (0.05) for 

42 days post-application at the 23.7 day half-life based on field studies (Table 38). The 

national Section 3 label for wet seeded rice (Cerrano 5 MEG) prohibits release of flood water 

for up to 14 days post-application. Section 24(c) labels for application of Command 3 ME to 

wet seeded rice in TX, MS, MO, LA, and AR do not provide any time restrictions on release 

of flood water after treatment. In either case, the odds of mortality at the acute RQ values on 

the day of application and 14-days post-application are 1 in 89 and 1 in 562, respectively. 

Therefore, listed species may be affected by use of clomazone on wet seeded rice and use is 

LAA listed species. 

There is a high degree of uncertainty in this risk determination. The silver catfish acute value 

used for this assessment was from a study testing a clomazone formulation. Because there is 

no TGAI study with the silver catfish, and no formulation testing with the freshwater fish 

species that were tested with the TGAI, it is not possible to determine if the difference is due 

to a difference in sensitivity to clomazone, to the formulation, or both. If the rainbow trout 

value (19,000 ppb), which is the more sensitive freshwater species tested with the TGAI 

alone, were used to assess risk rather than the silver catfish the acute RQ (0.03) would be 

below the listed species LOC on the day of application. 

Freshwater Invertebrates 

Flood water clomazone levels are expected to exceed the acute listed species LOC for 20 

days post-application (Table 38) at the 23.7 day half-life based on field studies. Therefore 

wet seed rice use may directly affect listed freshwater invertebrates. However, while the 

acute RQ (0.10) (Table 41) exceeds the acute LOC on the day of application the odds of 

mortality are <1 in 1011. Based on this information, direct effects to freshwater invertebrates 

are considered to be discountable and therefore not NLAA listed species. 

Estuarine-Marine Fish 

Acute RQ values are below the acute endangered species LOC (Table 38). Therefore no 

direct effects to listed species from wet seeded rice use are expected. 

Estuarine-Marine Invertebrates 

Estuarine-marine invertebrates were indicated to be potentially at risk acutely from release of 

rice paddy flood water into estuarine-marine surface waters. The acute restricted LOC is 

exceeded in rice paddy water from the day of application to 14 days post-application 

indicating potentially unacceptable mortality effects if water is released during this time. It 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 111 

was estimated that up to 98 days of hold time is needed to reduce levels to the acute 

endangered species LOC exceedence criteria. One rice label prohibits release of flood water 

for up to 14 days post-application; the others do not provided any time restrictions on release 

of flood water after treatment. Therefore, direct effects may affect and are considered LAA 

listed species. 

Marine Mammals 

No direct effects from any registered clomazone use are expected to listed marine mammal 

species. This conclusion is based on an evaluation of several lines of evidence. 

Exposure routes for contaminants in water are expected to be passive uptake from water 

across the dermis, oral exposure from residues in food items and incidental ingestion of 

water. Unlike fish, marine mammals breathe air and do not have gills by which uptake of 

clomazone from water would occur. Additionally ingestion of sea water is expected to be 

incidental as marine mammals, except for the manatee, meet their water needs from water in 

their food and from formation of water during metabolic processes to maintain ionic control. 

The listed manatee is found in freshwater, brackish, and marine environments and may ingest 

freshwater from runoff, seeps, springs, streams, and rivers. Therefore the toxicity values in 

water for fish and invertebrates based on exposure to water alone with gills being the primary 

uptake route are not expected to be directly applicable to marine mammals. 

In terms of dietary exposure, the oral and dietary acute toxicity of clomazone to mammals is 

considered practically nontoxic. Modeled maximum residues on-dietary items on the field at 

the highest use rate were below endangered species acute LOC and chronic LOC, for 1,000 

gram mammals (Table 29), clomazone residue levels in aquatic dietary items would be 

expected to be lower due to a number of factors and even if the dietary residue levels were 

the same, risks to mammals above 1000 g such as sea otters, sea-lions, and whales, would be 

expected to be lower than that of a 1000 g mammal. Clomazone rapidly metabolizes as 

observed in both laboratory mammals and fish. Therefore, no direct effects are expected. 

Freshwater Mammal 

No direct effects from any registered clomazone use are expected to the one listed freshwater 

mammal, the manatee, which also occurs in brackish, and marine waters (see section on 

marine mammals above). The manatee breathes air and does not have gills by which uptake 

of clomazone from water would occur. 

Exposure routes for contaminants in water are expected to be passive uptake from water 

across the dermis, oral exposure from residues in food items and ingestion of freshwater. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 112 

Unlike fish, the manatee breathes air and does not have gills by which uptake of clomazone 

from water would occur. The manatee may ingest freshwater from runoff, seeps, springs, 

streams, and rivers. Therefore the toxicity values in water for fish and invertebrates based on 

exposure to water alone with gills being the primary uptake route are not expected to be 

directly applicable to the manatee. 

In terms of dietary exposure, the oral and dietary acute toxicity of clomazone to mammals is 

considered practically nontoxic. Modeled maximum residues on-dietary items on the field at 

the highest use rate were below endangered species acute LOC and chronic LOC, for 1,000 

gram mammals (Table 29). Clomazone residue levels in aquatic dietary items would be 

expected to be lower due to a number of factors and even if the dietary residue levels were 

the same, risks to the manatee which is much larger than 1000 g, would be expected to be 

lower than that of a 1000 g mammal. Additionally, the movement of a manatee during a day 

may be considerable, such that the assumption of 100% of the diet would be from an affected 

location is highly conservative. Therefore, no effects are expected. 

Marine Birds 

No direct effects from any registered clomazone use are expected to listed marine birds. This 

conclusion is based on an evaluation of several lines of evidence. The acute toxicity of 

clomazone to birds is considered practically nontoxic. Modeled maximum residues ondietary items on the field at the highest use rate are below the chronic LOCs (Table 27) and 

below levels of acute concern (see Section 4.2.1.1 and Table 44), residues in the aquatic food 

chain due to bioaccumulation would be expected to be significantly below on-field residue 

levels since clomazone rapidly metabolizes as observed in both laboratory mammals and 

fish. 

Marine Reptiles 

No direct effects from any registered clomazone use are expected to listed marine reptile 

species. This conclusion is based on an evaluation of several lines of evidence. The acute 

toxicity of clomazone to birds, which are a surrogate for reptiles, is considered practically 

nontoxic. Modeled maximum residues on-dietary items on the field at the highest use rate 

are below the chronic LOCs (Table 27) and below levels of acute concern (see Section 

4.2.1.1 and Table 44). Residues in the aquatic food chain due to bioaccumulation would be 

expected to be significantly below on-field residue levels since clomazone rapidly 

metabolizes as observed in both laboratory mammals and fish. Residues on aquatic plants 

would be lower than on-field. Additionally, the movement of sea turtles during a day may be 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 113 

considerable, such that the assumption of 100% of the diet would be from an affected 

location is highly conservative further. Therefore, no effects are expected. 

Nontarget plants in terrestrial and semi-aquatic areas 

Clomazone applied to soil in a rice field and then immediately flooded or directly applied to 

a flooded field is not expected to have the same volatility issue as that observed for the 

nonrice uses and the rice fields per the labels should have berms such that transport by runoff 

should not be a viable pathway. However, clomazone may be transported to off-field sites 

from spray drift from ground or aerial applications to rice fields (see Sections 4.1.3.1 and 

4.2.1.1). Ground spray applications do not result in spray drift that exceeds the listed species 

LOC for monocots but does result in exceedance of the listed LOC for dicots (Table 33). 

Therefore, clomazone use may directly affect listed dicot plant species and is LAA. Aerial 

applications result in estimated spray drift exposure that exceeds listed plant LOC for both 

monocots and dicots. The furthest extent off-field for ground spay effects is estimated as 155 

feet and more than 1,000 feet for aerial applications as some labels do not restrict droplet 

size, the estimate for those that do are 900 feet (Table 43). Volatilization from these areas 

impacted by spray drift is a possibility. Because only ME formulations are registered for rice 

use, the furthest extent of any effect is considered 1 mile based on an evaluation of incident 

data. 

Land Birds, Reptiles, and Amphibians 

Residues due to spray drift will be lower than those estimated for the highest application rate 

on field. As on-field residue levels for the highest application rate resulted in no effect to 

listed avians, use on rice will also not effect birds and their surrogates. 

Land Mammals 

Off-field dietary- and dose-based residues are below acute endangered species LOC and 

chronic LOC at 0.6 lbs ai/A. Therefore clomazone use on rice is determined to have no 

effect on listed land mammals. 

Insects and Arachnids 

No direct effects to listed insects or arachnids are expected to occur for any registered use 

because on-field upper bound residue levels at the highest application rate, banded 

application on sweet potatoes, were below the LOC for small and large insects and therefore 

would be below off-field spray drift exposure. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 114 

Clomazone use is determined to have no direct effect on subterranean ground and mold 

insects and subterranean arachnids. These animals spend their entire lives underground in 

subterranean voids (caves, sinkholes, fractures, interconnections, etc.) in areas with low air 

movement, and elevated relative humidity, and reduced temperature fluctuations. Therefore 

these animals would not be directly exposed to clomazone from spray drift, runoff, or 

redeposited volatilized clomazone. 

4.2.2.4 DIRECT EFFECTS TO LISTED SPECIES FOR USE ON DRY SEEDED RICE

Aquatic Plants, Fish, Amphibians, and Aquatic Invertebrates 

Measured concentrations in flood water from field studies were used to make risk 

determinations as these results are considered more environmentally relevant. Aquatic plant 

RQ values were below listed LOC, and acute RQ values for freshwater fish (includes 

amphibians) and invertebrates, and marine-estuarine fish and invertebrates were below the 

acute endangered species LOC. Based on these results, clomazone use on dry seeded rice is 

determined to have no direct effect on listed vascular plants, freshwater fish (including 

amphibians) and invertebrates, and estuarine-marine fish and invertebrates. 

Marine Mammals and Freshwater Mammals 

Same conclusions as for wet seeded rice, for discussion see the discussions for marine 

mammals and the freshwater mammal in Section 4.2.2.3. 

Marine Birds 

No direct effects from any registered clomazone use are expected to listed marine birds. This 

conclusion is based on an evaluation of several lines of evidence. The acute toxicity of 

clomazone to birds is considered practically nontoxic. Modeled maximum residues ondietary items on the field at the highest use rate are below the chronic LOCs (Table 27) and 

below levels of acute concern (see Section 4.2.1.1 and Table 44), residues in the aquatic food 

chain due to bioaccumulation would be expected to be significantly below on-field residue 

levels since clomazone rapidly metabolizes as observed in both laboratory mammals and 

fish. 

Marine Reptiles 

No direct effects from any registered clomazone use are expected to listed marine reptile 

species. This conclusion is based on an evaluation of several lines of evidence. The acute 

toxicity of clomazone to birds, which are a surrogate for reptiles, is considered practically 

nontoxic. Modeled maximum residues on-dietary items on the field at the highest use rate 

are below the chronic LOCs (Table 27) and below levels of acute concern (see Section 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 115 

4.2.1.1 and Table 44). Residues in the aquatic food chain due to bioaccumulation would be 

expected to be significantly below on-field residue levels since clomazone rapidly 

metabolizes as observed in both laboratory mammals and fish. Residues on aquatic plants 

would be lower than on-field. Additionally, the movement of sea turtles during a day may be 

considerable, such that the assumption of 100% of the diet would be from an affected 

location is highly conservative further. Therefore, no effects are expected. 

Nontarget plants in terrestrial and semi-aquatic areas 

Clomazone applied to soil in a rice field and then not permanently flooded for several days 

potentially faces the same volatility issues as that observed for nonrice uses. Clomazone may 

also be transported to off-field sites from spray drift from ground or aerial applications (see 

Sections 4.1.3.1 and 4.2.1.1) but runoff should not be an issue as per the label rice fields 

clomazone is used on should have berms. Ground spray applications do not result in spray 

drift that exceeds the listed species LOC for monocots but does result in exceedance of the 

listed LOC for dicots (Table 33). Aerial spray exceeds listed LOC for monocots and dicots. 

Therefore, clomazone use may directly affect listed plant species and is LAA listed species. 

The furthest extent off-field for ground spay effects is estimated as 155 feet and more than 

1,000 feet for aerial applications as some labels do not restrict droplet size, the estimate for 

those that do are 900 feet (Table 43). 

Land Birds, Reptiles, and Amphibians 

Residues due to spray drift will be lower than those estimated for the highest application rate 

on field. As on-field residue levels for the highest application rate resulted in no effect to 

listed avians, use on rice will also not affect birds and their surrogates. 

Land Mammals 

At 0.6 lbs ai/A, on-field dietary- and dose-based residues are below acute endangered species 

LOC and chronic LOC (Table 28 and Table 30). Therefore clomazone use on rice is 

determined to have no effect on listed land mammals. 

Insects and Arachnids 

No direct effects to listed insects or arachnids are expected to occur for any registered use 

because on-field upper bound residue levels at the highest application rate, banded 

application on sweet potatoes, were below the LOC for small and large insects and therefore 

would be below off-field spray drift exposure. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 116 

Clomazone use is determined to have no direct effect on subterranean ground and mold 

insects and subterranean arachnids. These animals spend their entire lives underground in 

subterranean voids (caves, sinkholes, fractures, interconnections, etc.) in areas with low air 

movement, and elevated relative humidity, and reduced temperature fluctuations. Therefore 

these animals would not be directly exposed to clomazone from spray drift, runoff, or 

redeposited volatilized clomazone. 

4.2.2.5 INDIRECT EFFECTS TO LISTED SPECIES FOR USES OTHER THAN ON RICE

As discussed in Section 4.2.1, no unacceptable direct acute or chronic risks above levels of 

concern are expected for nonlisted birds, land mammals, beneficial or listed insects, 

freshwater or estuarine-marine animals or aquatic plants from registered clomazone nonrice 

uses. Therefore, no indirect effects to listed species that depend on these groups for food or 

habitat are expected. 

There are indirect effects expected to listed species that are obligate or depend on plants in 

terrestrial or semi-aquatic areas as a food resource or for habitat. However, this effect is 

expected to be restricted to relatively localized areas from a treated site, the highest 

occurrence and likelihood being within 300 feet of the site from combined runoff, spray drift 

and redeposition of volatilized clomazone, followed out to 1,500 feet from spray drift and 

drift of clomazone vapor. Redeposition events outside 1,500 feet are expected to be 

infrequent as indicated by the incident information, and be confined to relatively scattered 

small tracts of land. As discussed in Section 4.1.3, effects to off-site diverse plant 

communities with emergent vegetation are expected to be mainly reduction of growth rather 

than plant mortality. According to non-target plant incident data, most clomazone incidents 

occur within a 300 foot area from treated fields. 

Therefore, indirect effects to food resources or habitat requirements of large carnivorous or 

omnivorous land mammals and birds with large home ranges are expected to be insignificant. 

Similarly such degree of effects should be insignificant for species not dependent or having a 

low dependency on terrestrial vegetation or are not obligate or dependent on limited plant 

species based on species profiles or recovery plans such as the Shenandoah salamander 

(Plethodon shenandoa) which occurs in dry, rocky, talus slopes and feeds on soil 

invertebrates; the sand skink (Neoseps reynoldsi) which is adapted to an underground 

existence, occurs in sandy soils free of roots and feeds on soil invertebrates; the Houston toad 

(Bufo houstonensis) which uses rain pools, flooded fields, and natural or manmade ponds for 

breeding and as adults are reliant upon the presence of sand substrate inhabiting areas under 

logs, in leaf litter, in burrows, beneath undercut banks and in the actual sand substrate. Other 

listed land mammals, birds, reptiles, and land-phase amphibians, which are reliant or obligate 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 117 

on a limited number of plant species, a habitat that is dominated by a few species (e.g., 

prairie grassland), have small localized populations dependent on vegetation, or are highly 

reliant on vegetative camouflage during late May through late July may potentially be 

indirectly affected from impacts to vegetation. 

For listed marine mammals (including the manatee which may occur in freshwater, brackish, 

and marine environments), birds and reptiles indirect effects due to loss of food resources 

either plants or animals (fish, invertebrates) is not expected in aquatic systems. Additionally, 

for aquatic systems increased turbidity or erosion over baseline levels is not expected. 

Tillage of fields is expected to be the predominant factor affecting erosion, which clomazone 

use will not impact over baseline. Conservation tillage systems for cotton and soybeans will 

reduce erosion as compared to conventional tillage for these crops. But again the use of 

clomazone is not expected to impact the baseline erosion rate. Off-field decreased short-term 

growth in a diverse plant community is not expected to impact erosion over baseline levels. 

4.2.2.6 INDIRECT EFFECTS TO LISTED SPECIES FOR RICE USES

Wet Seeded Rice 

As discussed in Section 4.2.1, no unacceptable acute or chronic risks above levels of concern 

are expected for listed or nonlisted birds, land amphibians, reptiles, land mammals, and 

beneficial or listed insects. 

For freshwater systems, while effects to aquatic invertebrates and vascular aquatic plants do 

not exceed the nonlisted species LOC values, the nonlisted LOC is exceeded for nonvascular 

plants and freshwater fish on the day of application and at 14 days post-application. 

However, concentrations on these days do not exceed the nonlisted LOC for the other three 

nonvascular plants tested. Given that some form of fertilizer has likely been added to the rice 

field, and in some cases is added concurrent with clomazone, clomazone is not likely to 

reduce nonvascular plant biomass in waters receiving rice flood water as those nonvascular 

plants not impacted are expected to take advantage of the additional nutrient resources. The 

diversity of species may be impacted, but this would also be expected to occur due to cultural 

practices in rice fields. Therefore the clomazone effect on nonvascular plants in receiving 

waters is considered to likely be brief and transient, and be within the background level of 

effects to nonvascular biomass and diversity attributable to such cultural practices. For 

example, the creation of standing or slow moving, shallow waters of a rice field will favor 

production of certain species of nonvascular plants over those present in a deeper, faster 

moving aquatic system and the added nutrients will favor some species over others both 

within the rice field and in the receiving water. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 118 

As discussed in Section 4.1.3, effects to off-site diverse plant communities with emergent 

vegetation are expected to be mainly reduction of growth. Therefore, indirect effects to food 

resources or habitat requirements of large carnivorous or omnivorous land mammals and 

birds with large home ranges are expected to be insignificant. Other listed land mammals, 

birds, reptiles, and land amphibians, which are obligate on a plant species, or depends on a 

habitat that is dominated by a few species (e.g., prairie grassland), have small localized 

populations, or are highly reliant on camouflage during May through late July may 

potentially be indirectly affected from impacts to vegetation. For ground spray applications 

the species impacted is further reduced to those listed species that are obligate on either a 

dicot or a non-flowering plant (e.g., fern and its allies, conifers or cycads, lichen) as a food 

resource or for habitat. Additionally, for aquatic systems increased turbidity or erosion over 

baseline levels is not expected from this degree of effect. Tillage of fields is expected to be 

the predominant factor affecting erosion, which clomazone use is not expected to impact over 

baseline. Off-field decreased short-term growth in a diverse plant community is not expected 

to impact erosion over baseline levels. 

For ground spray applications direct spray drift effects to the limited dicot species drop 

below the listed species LOC at 155 ft from the field margin. For aerial applications, the 

spray drift impact is estimated to extend to 270 and 900 feet for monocots and dicots, 

respectively, when a coarse spray is used, and 425 and >1000 feet for monocots and dicots, 

respectively, when a median to coarse spray is used (Table 43). 

Clomazone has the potential to volatilize off the rice field or areas impacted by spray drift. 

For rice uses, the distance volatilization may impact plants is estimated to be 1 mile from the 

site of application based on all rice registrations being ME formulations and the use of 

incident data for distance of impact from a field for ME formulations. Frequency of 

redeposition events outside the spray drift impact area are expected to be infrequent as 

indicated by the incident information and be confined to relatively scattered small tracts of 

land. 

Dry Seeded Rice 

As discussed in Section 4.2.1, no unacceptable acute or chronic risks above levels of concern 

are expected for listed or nonlisted birds, land amphibians, reptiles, land mammals, and 

beneficial or listed insects. Based on clomazone concentrations measured in field studies, 

which are considered a more appropriate measure of exposure than modeled values in this 

case, no effects are expected for listed or nonlisted freshwater fish and aquatic invertebrates, 

estuarine-marine fish and invertebrates or to aquatic plants. 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 119 

For indirect effects from impacts to non-target plants in terrestrial and semi-aquatic areas see 

the discussion under wet seeded rice. However for aquatic plants there is no exceedance of 

nonlisted LOC for vascular or nonvascular plants so no indirect effects to listed species are 

expected from impacts to aquatic plants. 

4.2.2.7 EFFECTS TO CRITICAL HABITAT

Uses Other Than Rice and Dry Seeded Rice 

As discussed in Section 4.2.1, no unacceptable direct acute or chronic risks are expected to 

nonlisted mammals, birds, reptiles, amphibians (aquatic- or terrestrial-phases), beneficial or 

listed insects, fish, aquatic invertebrates or aquatic plants. Therefore, where these broad 

groups are principle constituent elements of critical habitat then no effect to these principle 

constituents are expected. Where critical habitat principle constituent elements consist of a 

listed plant species or is reliant or obligate on a limited number of nonlisted plant species, or 

is a habitat that is dominated by a few species (e.g., prairie grassland) then these principal 

constituent elements may potentially be affected. 

Clomazone is not expected to affect the physical structure or existence of subterranean caves. 

The base of the food web is driven by nutrients in the form of leaf litter and organic debris 

which washes or falls into caves through small holes and fissures, plant roots, feces, eggs or 

dead bodies of cave visitors (e.g., cave crickets, small mammals, and raccoons). The 

subterranean ground and mold insects and subterranean arachnids would not be directly 

exposed to clomazone from spray drift, runoff, or redeposited volatilized clomazone. 

Clomazone effects on surface plants is not expected to result in indirect effects to nutrient 

input below baseline conditions, or result in reduction of roots in the subterranean caves. 

Land use and cultural practices on a crop field (e.g., tilling, fertilizing, and crop type) should 

be the significant driver in terms of nutrient inputs and moisture to surface and subsurface 

drainage basins below and near the field. 

Wet Seeded Rice Use 

As discussed in Section 4.2.1, no unacceptable direct acute or chronic risks are expected to 

nonlisted mammals, birds, reptiles, amphibians (aquatic- or terrestrial-phases), beneficial or 

listed insects, or aquatic plants. There are potential affects on the freshwater fish and 

estuarine invertebrate prey base. Therefore, where these broad groups are principle 

constituent elements of critical habitat then no effect to these principle constituents are 

expected. Where critical habitat principle constituent elements consist of a listed plant 

species, a listed freshwater fish or amphibian species, or is reliant or obligate on a limited 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 120 

number of nonlisted plant species, or is a habitat that is dominated by a few species (e.g., 

prairie grassland) then these principal constituent elements may potentially be affected. 

Clomazone is not expected to affect the physical structure or existence of subterranean caves. 

The base of the food web is driven by nutrients in the form of leaf litter and organic debris 

which washes or falls into caves through small holes and fissures, plant roots, feces, eggs or 

dead bodies of cave visitors (e.g., cave crickets, small mammals, and raccoons). The 

subterranean ground and mold insects and subterranean arachnids would not be directly 

exposed to clomazone from spray drift, runoff, or redeposited volatilized clomazone. 

Clomazone effects on surface plants is not expected to result in indirect effects to nutrient 

input below baseline conditions, or result in reduction of roots in the subterranean caves. 

Land use and cultural practices on a crop field (e.g., tilling, fertilizing, and crop type) should 

be the significant driver in terms of nutrient inputs and moisture to surface and subsurface 

drainage basins below and near the field. 

4.2.2.8 CO-OCCURRENCE ANALYSIS

The FESTF IMS data base contains land use cover information, and agriculture census data 

(USDA 2002 Census of Agriculture) and species occurrence and critical habitat location 

information. This data base was used to identify counties with specific crop use and adjacent 

counties, and was used to perform an analysis of listed species that occur within 1 mile of use 

sites for the ME formulation and that occur within 2 miles of the use sites for the EC and WP 

formulations. Crop use is represented in the IMS by one-or more designated crop uses as 

shown in Table 50, taken from the IMS protocol document. The species locations in IMS are 

from EFED (June, 2003) and from the NatureServe Multi-Jurisdictional Database (MJD) 

licensed by FESTF (the FESTF MJD; McGaughey et al. 2007). 

There were some locations where NLCD cultivated land information was not available for 

incorporation into the version of the IMS used for this analysis such as HI, Puerto Rico (PR), 

U.S. Virgin Islands (VI), Guam (GU), and the Mariana Islands (MP). In the light of this lack 

of knowledge, species and their critical habitat at these locations were assumed to overlap 

with the clomazone action area with the following exceptions (1) if the location of the 

species was restricted to a small island which does not support an agribusiness, and/or the 

whole island is a nature reserve (e.g., Northwestern Hawaiian Islands, Mona Island, Monito 

Island), and (2) rice uses, USDA agricultural census for HI and these U.S. territories do not 

report rice as planted in these locations.38

 

38 American Somoa, www.nass.usda.gov/census/census97/amsomoa/intro.htm; Guam, 

www.nass.usda.gov/census/census97/guam/guam.htm; Puerto Rico, 
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Registration Review Ecological Risk Assessment Clomazone 121 

Table 50 Clomazone uses designated in problem formulation and corresponding IMS 

sites 

Crop IMS Site 

Agricultural Fallow/Idleland Assumed would be covered by use sites for specific crops that 

clomazone is registered as use of land for crops after application is 

limited to species tolerant of clomazone which consists of these 

crops. 

Arracacia (Persian Carrot) Vegetables, other 

Beans – Succulent (Snap) 

Beans – Succulent (Lima) 

Beans – snap 

Green lima beans 

Cabbage Cabbage - head 

Cotton (unspecified) Cotton, all 

Cucumber Cucumbers and pickles 

Manioc (cassava) Cassava (PR)1

Melons, musk Honeydew melons, cantaloupes 

Melons, water Watermelons 

Mint Mint for oil, all 

Peas, succulent Peas, green (excluding southern) 

Peas, green southern (cowpeas) - blackeyed, crowder, etc. 

Pepper Peppers, bell 

Peppers, chile (all peppers - excluding bell) 

Pimiento Pimientos 

Pumpkin Pumpkins 

Rice Rice 

Soybeans (unspecified) Soybeans for beans 

Squash, summer Squash 

Squash, winter Squash 

Sugarcane Sugarcane for seed 

Sugarcane for sugar 

Sweet potato Sweet potatoes 

Tobacco Tobacco 

Yam Yams (PR)1 

Yautia Taro2

1The designation of (PR) indicates that the data are for Puerto Rico. The 2002 Agricultural Census contains no records for 

cassava or yams outside Puerto Rico. 

2The crop Yautia is not specifically included as a crop in the 2002 Agricultural Census. The crop Taro will be used to 

represent this crop in the IMS.

The co-occurrence results were further evaluated to remove species that were identified in 

Section 4.2.2.2 to 4.2.2.6 as having NE or May affect but NLAA, independent of cooccurrence, the effects determination for each listed species is provided in Appendix I for 

nonrice uses, Appendix K for wet seeded rice, and Appendix J for dry seeded rice. Figures in 

Appendix I show the counties with listed species for all uses except rice identified as LAA. 

 

www.agcensus.usda.gov/Publications/2007/Full_Report/Outlying_Areas/prv1.pdf; Commonwealth 

of Northern Mariana Islands, www.nass.usda.gov/census/census97/cnmi/cnmi.htm; U.S. Virgin 

Islands, www.nass.usda.gov/census/cesus92/outlying/vi-54.pdf
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 122 

4.2.2.9 EFFECTS DETERMINATION

Listed Species Associated with All Uses Except Rice 

The action area for reregistration of clomazone consists of a 2 mile radius out from 

clomazone use sites. Operationally locations where clomazone is used are defined as all 

cultivated land that has had one or more of the crops for which clomazone is registered 

cultivated on it, except California where the only registered clomazone use is rice. The 

geographical extent of a 2 mile radius out from a use site is based on the farthest physical 

extent direct or indirect effects from exposure to clomazone are predicted to occur based on 

best available information. 

May affect but NLAA determinations were made for listed vascular aquatic plants (there are 

no nonvascular listed species), freshwater and marine listed fish and invertebrates, aquatic 

amphibians, and marine mammals. May affect but NLAA determinations were made for 

listed large land birds, mammals and reptiles not known to be reliant on one or a limited 

number of nonlisted or listed terrestrial or semi-aquatic plant species. This conclusion was 

based on a review of multiple lines of evidence for both direct and indirect effects, effects to 

critical habitat, and included, estimates of risk, odds of mortality, timing of application, and 

refinement of exposure levels where appropriate. Additionally, some species were identified 

as being outside the action area of clomazone, and a no effect determination was made for 

these. Species designated as may affect but NLAA, are listed in Appendix I of this 

document. 

May affect and LAA determinations were made for 10 amphibians, 1 arachnid, 17 birds, 1 

conifer, 174 dicots, 5 ferns and allies, 14 insects, 2 lichens, 14 mammals, 7 land snails, 23 

monocots, and 17 reptiles. This conclusion was based on a review of multiple lines of 

evidence for both direct and indirect effects, effects to critical habitat, and included, estimates 

of risk, odds of mortality, and refinement of exposure levels where appropriate. Based on the 

co-occurrence analysis there are locations for some of these species which fall outside the 

action area. Species designated as may affect and LAA, are listed in Appendix I of this 

document. 

Listed Species Associated with Wet Seeded Rice 

The action area for reregistration of clomazone use on wet seeded rice consists of all streams 

and rivers downstream of rice fields. Operationally rice field sites are defined as all 

cultivated land that has had rice cultivated on it. No direct effect to listed vascular aquatic 

plants, estuarine and marine fish, marine mammals and birds are expected. Direct effects to 

freshwater invertebrates are considered discountable as the risk of individual mortality is less 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 123 

than 1 in 1011. No direct effects to land mammals, birds, reptiles, or amphibians are 

expected. Direct effects to freshwater fish and amphibians, and estuarine and marine 

invertebrates are expected. There are direct effects to non-target vegetation off the field. For 

those labels which do not state use of a coarse droplet size, especially for aerial applications 

to rice this effect may potentially reach several thousand feet. For other labels with such 

restrictions in place, this effect is expected to be restricted to relatively localized areas off a 

treated site, the highest occurrence and likelihood being adjacent to the site from spray drift 

and/or vapor drift. Effects to off-site diverse plant communities with emergent vegetation are 

expected to be mainly reduction of growth, with mortality potentially amongst highly 

sensitive species. Therefore, for large mammalian and avian listed species not reliant on a 

listed plant species or not reliant on one or a limited number of nonlisted plant species either 

as prey or habitat indirect effects are expected to be insignificant. 

May affect but NLAA determinations were made for listed vascular aquatic plants (there are 

no nonvascular listed species), freshwater and marine listed fish and invertebrates, aquatic 

amphibians, and marine mammals. May affect but NLAA determinations were made for 

listed large land birds, mammals and reptiles not known to be reliant on one or a limited 

number of nonlisted or listed terrestrial or semi-aquatic plant species. This conclusion was 

based on a review of multiple lines of evidence for both direct and indirect effects, effects to 

critical habitat, and included, estimates of risk, odds of mortality, and refinement of exposure 

levels where appropriate. Additionally, some species were identified as being outside the 

action area of clomazone, and a no effect determination was made for these. Species 

designated as no effect and may affect but NLAA, are listed in Appendix K of this document. 

May affect and LAA determinations were made for flowering and nonflowering plants 

because of exceedance of endangered species LOC. May affect and LAA determinations 

were made for listed freshwater fish and amphibians, and estuarine and marine invertebrates. 

May affect and LAA determinations were made for listed freshwater invertebrates that are 

obligate on a specific freshwater fish species and for estuarine and marine fish that are 

obligate on a specific estuarine-marine invertebrate. May affect and LAA determinations 

were made for small mammals, reptiles, and amphibians that because of their size and home 

range may rely on a limited number of plant species for a food resource or habitat. 

Listed Species Associated with Dry Seeded Rice 

The action area for reregistration of clomazone use on rice consists of a 2 mile radius out 

from rice fields, except California where its use is prohibited by the label. Operationally rice 

field sites are defined as all cultivated land that has had rice cultivated on it. Based on 

measured field residue levels in flood water, direct effects to freshwater fish, amphibians, and 
Chapter 4 Results: Risk Characterization 

Registration Review Ecological Risk Assessment Clomazone 124 

invertebrates and marine-estuarine fish and invertebrates are below the acute endangered 

species LOC, and effects to aquatic plants are below the listed species LOC. No direct 

effects to birds, reptiles, amphibians (land phase), and mammals are expected. There are 

direct effects to non-target vegetation off the field. For those labels which do not state use of 

a coarse droplet size, especially for aerial applications to rice this effect may potentially 

reach several thousand feet. For other labels with such restrictions in place, this effect is 

expected to be restricted to relatively localized areas off a treated site, the highest occurrence 

and likelihood being adjacent to the site from spray drift and/or vapor drift. 

May affect but NLAA determinations were made for listed vascular aquatic plants (there are 

no nonvascular listed species), freshwater and marine listed fish and invertebrates, aquatic 

amphibians, and marine mammals. May affect but NLAA determinations were made for 

listed large land birds, mammals and reptiles not known to be reliant on one or a limited 

number of nonlisted or listed terrestrial or semi-aquatic plant species. This conclusion was 

based on a review of multiple lines of evidence for both direct and indirect effects, effects to 

critical habitat, and included, estimates of risk, odds of mortality, and refinement of exposure 

levels where appropriate. Additionally, some species were identified as being outside the 

action area of clomazone, and a no effect determination was made for these. Species 

designated as no effect and may affect but NLAA, are listed in Appendix J of this document. 

May affect and LAA determinations were made for flowering and nonflowering plants 

because of exceedance of endangered species LOC. May affect and LAA determinations 

were made for small mammals, reptiles, and amphibians that because of their size and home 

range may rely on a limited number of plant species for a food resource or habitat. 
Chapter 5 Uncertainties 

Registration Review Ecological Risk Assessment Clomazone 125 

Chapter 5 Uncertainties 

A description of assumptions, uncertainties, strengths, and limitations of the basic risk 

assessment performed is described in Chapter 6 of the Agency’s Overview Document (EPA, 

2004) and include those related to exposure for all taxa, those related to exposure for aquatic 

species, those related to exposure for terrestrial animals, those related to the effects 

assessment, and those associated with the acute LOC values. This Chapter discusses 

additional uncertainties associated with refinements made to the basic risk assessment. 

5.1 Exposure Assessment Uncertainties 

5.1.1 Volatilized Clomazone 

The major source of uncertainty in the exposure assessment for clomazone is the 

quantification of movement due to volatilization. Aquatic exposure estimates from PRZMEXAMS are reduced by approximately a factor of 2 when volatilization is included as a 

transport route. The magnitude of the loss to volatilization is supported by the formula given 

by Yang and Holmen (ES&T, in press) which indicates that over 50% of the applied mass 

could be lost to volatilization in the first 48 hours. 

There are considerable uncertainties, both positive and negative, in the estimates of 

volatilization mass loss. The Yang and Holmen formula was derived for only two soils and 

clomazone was not among the 7 tested chemicals, although its vapor pressure and Henry’s 

Law constant were within the tested range. Volatilization could be lower or higher using 

other soils with higher sorption capacity. 

The PRZM routine for volatilization is not sensitive to temperature, which should have 

considerable influence on the amount of chemical volatilized. The amount volatilized would 

increase with increasing temperature, and vice versa, as the model temperature deviates from 

nominal room temperature. 

Quantitative estimates of downwind exposures to volatilized clomazone are not given in this 

assessment. Such estimates should be possible with the development of computer models 

such as Calpuff for the specific requirements of OPP. However, given the estimate that up to 

65% of applied clomazone could volatilize at 100% relative humidity, downwind movement 

is considered to be a major exposure route. The importance of volatilization as an exposure 

route is demonstrated by the large number of plant bleaching incidents, their severity (up to 

plant death), the fact that effects were seen at distances up to 2 miles from application sites, 
Chapter 5 Uncertainties 

Registration Review Ecological Risk Assessment Clomazone 126 

and label statements that buffer zones of 1000 feet and greater are needed to protect nontarget plants. 

Aquatic exposure assessments are also affected by the magnitude of volatilization loss. 

PRZM-EXAMS estimates are given both with and without volatilization loss to give an 

estimate of the possible effect on aquatic exposures. Exposures should be no more than those 

calculated with the PRZM volatilization routine “off,” other factors being equal. The 

exposures calculated with the PRZM routine “on” give an estimate of the remaining aquatic 

exposure, although because the volatilization routine only operate at room temperature, the 

actual loss to air could be lesser or greater. 

5.1.2 Alternative Aquatic Exposure Estimates for Non-Rice Uses

For non-rice uses, EEC values for evaluating direct effects to aquatic listed species and 

indirect effects from affects to aquatic habitat and the aquatic prey base are performed using 

1-in-10 year peak daily values, and 21-day and 60-day averages for acute and chronic 

invertebrate and vertebrates exposures, respectively (Section 2.1.2). These provide one of 

numerous alternative expressions of exposure that can be generated using the model. The set 

of selected values are considered representative reasonable upper-end exposure estimates, but 

are not the maximum predicted concentration in water which is a value of interest to the 

Services. A hierarchy of maximum concentrations estimated with the model includes: 

• Edge-of-field runoff water exposures (maximum and 1-in-10 year maximum values), 

calculated from PRZM model “zts” time-series output files, specifically dissolved 

mass and runoff volume, using an Excel spreadsheet (M. Corbin, Feb. 2008). The 

spreadsheet calculates daily runoff water concentrations for each day of a 30-year 

simulation, by dividing the dissolved mass by the volume of runoff water. Model 

runs were made with (ON) and without (OFF) volatilization from soil. 

• The maximum exposures from the PRZM-EXAMS static pond analysis. These 

exposures were taken from the standard pe5 output tables. Model runs were made 

with (ON) and without (OFF) volatilization from soil. 

5.1.1.1 MAXIMUM EXPOSURE ANALYSIS: EDGE-OF-FIELD PRZM ANALYSIS

Exposure of organisms to runoff water as it leaves the field, and before it enters the EXAMS 

pond, is approximated by the edge-of-field exposure calculation. This analysis estimates the 

concentration in runoff water from parameters in the PRZM *.zts output file, by dividing the 

mass flux of chemical in runoff (RFLX) by the volume of runoff water (RUNF), using an 

Excel spreadsheet. This EEC represents the short-term (one to several days) exposure 
Chapter 5 Uncertainties 

Registration Review Ecological Risk Assessment Clomazone 127 

expected in areas where runoff water could collect just off the agricultural field. The *.zts 

files were the same as used in the 1-in-10 year exposure analysis (Table 6). The maximum 

and 1-in-10 year maximum concentration of clomazone in runoff at the edge-of-field are 

listed in Table 51, with and without volatilization from soil. The maximum edge-of-field 

modeled concentrations with and without volatilization from the soil, range from 48 to 300 

ppb and 63 to 360 ppb, respectively. The 1-in-10 year maximum edge-of-field 

concentrations range from 25 to 166 ppb with volatilization, and from 37 to 236 ppb without 

volatilization. 

Table 51 PRZM Edge-of-Field Exposure 

Edge-of-Field Exposure, ppb 

(Volatilization OFF)

Edge-of-Field Exposure, ppb 

(Volatilization ON) PRZM Scenario (standard unless 

noted) 1-in-10 years Maximum 1-in-10 years Maximum 

Florida sugarcane 190 249 166 237 

Louisiana sugarcane 170 290 141 230 

Mississippi cotton 172 210 127 150 

North Carolina cotton 177 200 152 160 

Florida peppers 180 210 121 166 

Mississippi soybeans 162 200 137 172 

North Carolina sweet potato 236 353 135 291 

North Carolina tobacco 120 190 82 137 

Florida potato (NMC)1 183 360 63 300 

Michigan bean 131 180 83 132 

Oregon snap beans 79 90 55 88 

Florida cabbage 81 124 53 92 

Florida cucumber (1 lb/A) 147 250 101 190 

Florida cucumber (0.75 lb/A) 110 188 76 143 

Florida cucumber (0.5 lb/A) 74 125 51 95 

Florida cucumber (0.25 lb/A) 37 63 25 48 

Oregon mint 63 74 31 56 

 (1) NMC: scenario developed for N-methyl carbamate cumulative assessment. This Florida potato scenario 

was used as a surrogate for tropical tuber crops such as yautia, tanier, cassava, and arracacia. 

5.1.1.2 MAXIMUM EXPOSURE ANALYSIS: AGRICULTURE

In the screening exposure analysis conducted in accordance with the Overview document 

(USEEPA, 2004), the reported EECs represent the maximum exposure expected in the 

EXAMS pond once every 10 years. Because the PRZM-EXAMS models are usually run for 

a thirty-year simulation, the 1-in-10 year EECs are not the highest calculated exposures (they 

are approximately the third-highest). The table below shows the maximum EECs from the 
Chapter 5 Uncertainties 

Registration Review Ecological Risk Assessment Clomazone 128 

PRZM-EXAMS runs given in Table 6. The maximum 24-h peak concentrations ranged from 

1.9 to 45.6 ppb and 4.1 to 71 ppb, with and without volatilization from soil, respectively. 

Table 52 Maximum Surface Water EEC Values from PRZM/EXAMS Model 

Maximum 24-h Peak, ppb

Without/With 

Volatilization 

Maximum 21-day 

Average, ppb 

Without/With 

Volatilization

Maximum 60-day 

Average, ppb 

Without/With 

Volatilization

PRZM Scenario (standard unless 

noted) 

Without With Without With Without With 

Florida sugarcane 64.4 38.9 36.5 19.9 15.9 9.5 

Louisiana sugarcane 35.3 33.2 25.1 23.7 13.3 11.9 

Mississippi cotton 53.2 45.6 29.6 25.3 13.8 11.8 

North Carolina cotton 25.1 13.2 13.2 8.6 6.4 4.4 

Florida peppers 53.2 34.2 27.3 18.2 12.2 8.1 

Mississippi soybeans 22.5 20.0 12.9 10.8 7.0 5.3 

North Carolina sweet potato 18.0 13.8 11.1 8.6 5.4 4.1 

North Carolina tobacco 6.7 3.4 3.7 2.1 1.7 1.1 

Florida potato (NMC)1 42.2 22.7 24.3 11.4 10.4 5.1 

Michigan bean 14.5 11.9 7.4 6.0 3.2 2.6 

Oregon snap beans 7.9 5.9 4.7 3.5 2.2 1.7 

Florida cabbage 9.3 5.5 5.8 2.7 2.7 1.2 

Florida cucumber, 1.0 lb ai/acre 71.0 53.1 37.0 25.4 16.4 10.5 

Florida cucumber, 0.75 lb ai/acre 54.7 39.8 28.5 19.0 12.6 7.9 

Florida cucumber, 0.5 lb ai/acre 35.5 26.5 18.5 12.7 8.2 5.3 

Florida cucumber, 0.25 lb ai/acre 17.8 13.3 9.3 6.3 4.1 2.6 

Oregon mint 4.1 1.9 2.4 1.1 1.2 0.56 

 (1) NMC: scenario developed for N-methyl carbamate cumulative assessment. This Florida potato scenario 

was used as a surrogate for tropical tuber crops such as yautia, tanier, cassava, and arracacia. 

5.2 Effects and Risk Assessment Uncertainties 

5.2.1 Lack of definitive avian acute oral and subacute dietary clomazone 

test results 

No definitive avian acute oral or subacute dietary toxicity test results were available to 

calculate acute risks for avian taxa. Available data consisted of limit test results with no 

mortality at the highest dosages tested. A provisional method using the number of animals 

tested, the Binomial Theorem, a rearrangement of the Hill et al. (1975) dose-response model, 

and a range of slopes from 2 to 9 were used to estimate a subacute dietary LC50 (Appendix 

C). These results were used to characterize the potential for acute effects from exposure to 

dietary residue levels. The estimated proportion of mortality based on the 95th upper 

confidence limit represents a high-end estimate of mortality for the study, and because the 
Chapter 5 Uncertainties 

Registration Review Ecological Risk Assessment Clomazone 129 

higher the mortality the lower the estimated LC50 value for a given slope this results in a lowend or conservative estimated LC50 value for a given slope. The probability that mortality is 

greater is highly unlikely. Because exposure levels, except for short grass at the highest 

banded application rate and a slope response of 9, were below 1/10th of estimated LC50

values a lower estimate of mortality for the current data, i.e., a higher LC50, would not affect 

the conclusions about acute risks to birds and the taxa for which they are surrogates. For 

short grass, EEC values were below 1/10th of estimated LC50 values from slopes at and below 

4.5 but were above 1/10th at the slope assumption of 9. At a slope of 9 the odds of mortality 

for an individual at the exposure level was considered discountable at 1 in 2 x 1018. A less 

conservative LC50 would further decrease the odds of mortality which would not affect the 

current risk conclusions. 

5.2.2 Lack of ELS or life cycle test results for the most acutely sensitive 

freshwater fish species 

No ELS or life cycle test result was available for the most acutely sensitive freshwater 

species, the silver catfish. An ACR approach was used to extrapolate a reproductive NOAEC 

for the silver catfish. For extrapolation of a NOAEC, the higher an ACR value is the lower 

the resulting extrapolated NOAEC value. The NOAEC for reproduction for the silver 

catfish, the most acutely sensitive freshwater species, was extrapolated using the highest 

aquatic animal ACR value calculated which was 8.297 for rainbow trout. One other aquatic 

animal ACR was available for clomazone, 2.454 for the freshwater invertebrate, Daphia 

magna. Unless circumstances provide evidence to the contrary, the general policy within 

EFED for pesticides is to use an ACR derived from within the same taxa because the mode 

of action of some pesticides may be taxa specific, and may also be affected by differences 

between freshwater and saltwater species. In this case the highest ACR is for a freshwater 

fish and is therefore consistent with this policy. However while the data is limited the 

relative agreement of the ACR values (e.g. within a factor of 10), and toxicity information 

that indicates that clomazone appears to be primarily a liver toxicant in animals provides 

evidence that ACR values for freshwater fish and invertebrates are likely representative 

across freshwater aquatic taxa. Given the likely range of ACR values, within a factor of 10, 

the resultant chronic NOAEC values would all be below chronic EEC values for all nonrice 

and dry seeded exposure scenarios for freshwater fish and wet seeded would still exceed 

therefore the degree of uncertainty associated with this limitation does not affect risk 

conclusions. 
Chapter 5 Uncertainties 

Registration Review Ecological Risk Assessment Clomazone 130 

5.2.3 Lack of ELS test results for the most acutely sensitive 

estuarine/marine fish 

No ELS or life cycle test result was available for an estuarine/marine species. There was an 

acute test with the sheepshead minnow and an ACR approach was used to estimate an ELS 

NOAEC for the sheepshead minnow. The NOAEC for sheepshead minnow, the most acutely 

sensitive freshwater species, was extrapolated using the highest aquatic animal ACR value 

calculated which was 8.297 for rainbow trout. One other aquatic animal ACR was available 

for clomazone, 2.454 for the freshwater invertebrate, Daphia magna. Unless circumstances 

provide evidence to the contrary, the general policy within EFED for pesticides is to use an 

ACR derived from within the same taxa because the mode of action of some pesticides may 

be taxa specific, and may also differ between freshwater and saltwater species. While there 

was an ACR ratio for a freshwater fish and a freshwater invertebrate by which differences 

between a fish and invertebrate could be evaluated, no ACR for at least a saltwater species 

was available to evaluate if there are differences between freshwater and saltwater ACR 

values. Therefore there is uncertainty in what, if any, effect salinity would have on the ACR 

for clomazone. 

5.2.4 Lack of life cycle test results for the most acutely sensitive 

estuarine/marine invertebrate 

The NOAEC for reproduction for the mysid shrimp was extrapolated using the highest 

aquatic animal ACR value calculated which was 8.297 for rainbow trout data. One other 

aquatic animal ACR was available for clomazone, 2.454 for the freshwater invertebrate, 

Daphia magna. Unless circumstances provide evidence to the contrary, the general policy 

within EFED for pesticides is to use an ACR derived from within the same taxa because the 

mode of action of some pesticides may be taxa specific, and may also differ between 

freshwater and saltwater species. While the data is limited the relative agreement of the ACR 

values (e.g. within a factor of 10), and toxicity information that indicates that clomazone 

appears to be primarily a liver toxicant in animals provides evidence that ACR values for 

freshwater fish and invertebrates are likely representative across at least freshwater aquatic 

taxa. While there was an ACR ratio for a freshwater fish and a freshwater invertebrate by 

which differences between a fish and invertebrate could be evaluated, no ACR for at least a 

saltwater species was available to evaluate if there are differences between freshwater and 

saltwater ACR values. 
Chapter 5 Uncertainties 

Registration Review Ecological Risk Assessment Clomazone 131 

5.2.5 Use of most sensitive species toxicity endpoint 

Although the assessment relies on a selected toxicity endpoint from the most sensitive 

species tested, it does not necessarily mean that the selected toxicity endpoints reflect 

sensitivity of the most sensitive species existing in a given environment. The relative 

position of the most sensitive species tested in the distribution of all possible species is a 

function of the overall variability among species to a particular chemical. In the case of 

listed species, there is uncertainty regarding the relationship of the listed species’ sensitivity 

and the most sensitive species tested, in some cases it may be less sensitive. For example, for 

clomazone the most sensitive endpoint for freshwater fish was the Silver Catfish but the 

Rainbow Trout also know as Steelhead when it is the ocean going and returning 

(anadromous) form was much less sensitive. Therefore, the potential direct risks of concern 

identified for salmonids may be overestimated. For nonrice and dry land rice uses as 

exposure levels were below the acute endangered and chronic LOC values risk conclusions 

are not impacted. For wet seeded rice, direct effects to listed salmonids may be 

overestimated. 

5.2.6 Single and multiple active ingredient formulations relative to TGAI 

effects 

In addition to registered single active ingredient clomazone formulations there are registered 

multiple active formulations of clomazone. An analysis of the available toxicity data for 

multiple active ingredient products and single active ingredient formulations relative to the 

technical grade active ingredient (TGAI) is provided in Appendix D. There was no available 

toxicity information on registered formulations for birds, aquatic animals, aquatic plants, or 

terrestrial plants. The silver catfish data, which is from a study with Gamit (single active 

ingredient formulation not registered in the US). There is no silver catfish study with TGAI, 

therefore it is unknown if formulation affected study results potentially resulting in an 

overestimation of risks to fish from runoff, or in wet seeded rice. Fish species tested with the 

TGAI did not have formulation testing which means can not look at a different aquatic 

organisms and get insight into formulation testing was compared to the TGAI. If difference 

between silver catfish and the bluegill and rainbow trout results is attributable to formulation 

versus TGAI then (i.e., TGAI less toxic than formulation) does not affect risk conclusions for 

nonrice and dry seeded rice because acute values are already below endangered species LOC, 

potential to affect risk conclusions for wet seeded rice. If TGAI, results more representative 

of range for blue gill and rainbow trout, after 14 days holding using rainbow trout LD50 

would be below acute endangered species LOC. This uncertainty affects risk conclusions for 

wet seeded rice. 
Chapter 5 Uncertainties 

Registration Review Ecological Risk Assessment Clomazone 132 

There was acute oral mammalian toxicity data for multiple active ingredient products, and 

single active ingredient formulations which is provided in Appendix B. An analysis of the 

relative toxicity of the formulations to the TGAI is provided in Appendix D. The results of 

this analysis show that except for the multiple active Commence EC formulation, an 

assessment based on the toxicity of the clomazone TGAI is appropriate. However, dietary 

effects on at least an acute basis may be underestimated for the Commence EC registered 

uses which include only cotton and soybeans. 

5.2.7 Lack of NLCD 82 (cultivated land) data for U.S. Territories, Hawaii, and 

Alaska 

Except for small islands which would not support an agribusiness and/or the island is a 

nature reserve (e.g., Northwestern Hawaiian Islands, Mona Island, Monito Island), where 

there was no cultivated land data available (e.g., U.S. territories, Hawaii), the action area was 

assumed to overlap with the listed species. This potentially results in more species identified 

as LAA. 
Chapter 6 Bibliography 

Registration Review Ecological Risk Assessment Clomazone 133 

Chapter 6 Bibliography 

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Conover, W. 1980. Practical Nonparametric Statistics, 2nd Edition. John Wiley & Sons, 

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Marchezan, E., and Baldisserotto, B. (2004). Lethal Concentration of Clomazone, 

Metsulfuron-Metil, and Quinclorac for Silver Catfish, Rhamdia quelen, Fingerlings. 

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Mayer, D. & C. Johansen. 1990. Pollinator Protection: A Bee & Pesticide Handbook. 

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United States Environmental Protection Agency (EPA). 1993. Wildlife Exposure 

Handbook, Volume I of II. Office of Research and Development, Washington, DC. 

EPA/600/R-93/187a. 

United States Environmental Protection Agency (EPA). 2000. Guidance for Conducting 

Screening Level Avian Risk assessments for Spray Applications of Pesticides. 

Environmental Fate & Effects Division, Office of Pesticide Programs, Washington, D.C. 

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United States Environmental Protection Agency (EPA). 2004. Overview of the Ecological 

Risk Assessment Process in the Office of Pesticide Programs, U.S. Environmental 

Protection Agency, Endangered and Threatened Species Effects Determination. Office of 

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D.C., January 23, 2004. 

United States Environmental Protection Agency (EPA). 2007a. Clomazone Summary 

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United States Environmental Protection Agency (EPA). 2007b. Revised Clomazone Final 

Work Plan Registration Review July 2007. Docket Number: EPA-HQ-OPP-2006-0113, 

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