EFED's Reregistration Eligibility Decision

Chapter for Sethoxydim

Michael Davy, Agronomist

William Eckel, Ph.D., Agronomist

Shannon Borges, Biologist

Environmental Fate and Ecological Effects Division (7507C)

Office of Pesticide Programs

Environmental Protection Agency

June 22, 2005

Secondary Review:

Dana Spatz, Risk Assessment Process Leader

Donna Randall, Senior Scientist

Environmental Risk Branch II

Environmental Fate and Effects Division (7507C)

Branch Chief Approval:

Tom Bailey, Ph.D., Chief

Environmental Risk Branch II

Environmental Fate and Effects Division (7507C)
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I. Executive Summary .................................................... -6-

A. Nature of Chemical Stressor ....................................... -6-

B. Potential Risks to Non-target Organisms .............................. -7-

C. Conclusions - Exposure Characterization ............................. -7-

D. Conclusions - Effects Characterization ............................... -8-

E. Uncertainties and Data Gaps ....................................... -9-

II. Problem Formulation ................................................ -10-

A. Stressor Source and Distribution ................................... -10-

1. Physical/Chemical/Fate and Transport Properties .................... -10-

2. Pesticide Type, Class, and Mode of Action ........................ -14-

3. Overview of Pesticide Formulation and Usage ...................... -14-

B. Receptors and Assessment Endpoints ............................... -14-

1. Measures of Ecological Effect .................................. -14-

a. Terrestrial Organisms ................................... -15-

Acute Toxicity to Birds .............................. -15-

Chronic Toxicity to Birds ............................ -16-

Acute Toxicity to Mammals .......................... -16-

Chronic Toxicity to Mammals ......................... -16-

Toxicity to Plants ................................... -16-

b. Aquatic Organisms ..................................... -17-

Acute Toxicity to Aquatic Organisms ................... -17-

Chronic Toxicity to Aquatic Organisms ................. -17-

c. Discussion ............................................ -17-

2. Measures of Exposure ........................................ -19-

Aquatic Systems .......................................... -20-

Terrestrial Systems ........................................ -21-

C. Conceptual Model .............................................. -21-

1. Risk Hypotheses .......................................... -21-

2. Diagram ................................................ -22-

a. Aquatic Exposure ...................................... -23-

b. Terrestrial Exposure .................................... -23-

D. Key Uncertainties and Information Gaps ............................. -25-

E. Analysis Plan .................................................. -26-

1. Assessment .............................................. -26-

(a) Fate and Exposure ..................................... -26-

(b) Toxicity ............................................. -26-

(c) Risk Quotient and Levels of Concern ...................... -27-

III. Analysis ............................................................ -29-

A. Use Characterization ............................................ -29-
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Characterization of Production Areas and Soils ....................... -31-

Soybeans ..................................................... -31-

Cotton and Peanuts ............................................. -31-

Other Cotton Areas ............................................. -31-

Other Crops ................................................... -32-

B. Exposure Characterization ........................................ -32-

1. Environmental Fate and Transport Characterization .............. -32-

Degradation Scheme for Sethoxydim ......................... -35-

2. Measures of Aquatic Exposure .................................. -37-

a. Aquatic Exposure Modeling .......................... -37-

Spray Drift Exposure from Formulated Sethoxydim ........ -39-

b. Aquatic Exposure Monitoring and Field Data ............... -40-

3. Measures of Terrestrial Exposure ............................... -42-

a. Terrestrial Exposure Modeling ............................ -42-

Residues on Avian Food Items ........................ -43-

Residues on Mammalian Food Items ................... -44-

Terrestrial Plant EEC ................................ -44-

C. Measures of Ecological Effects .................................... -46-

1. Aquatic Effects ........................................... -46-

a. Aquatic Animals ................................... -46-

Acute Effects .................................... -46-

Chronic Effects .................................. -47-

Field Studies .................................... -47-

b. Aquatic Plants ..................................... -47-

2. Terrestrial Effects ............................................ -48-

a. Terrestrial Animals ................................. -48-

Acute Effects .................................... -48-

Chronic Effects .................................. -48-

Field Studies ..................................... -48-

b. Terrestrial Plants ................................... -49-

3. ECOTOX Database ........................................... -49-

IV. Risk Characterization ................................................ -50-

A. Risk Estimation - Integration of Exposure and Effects Data .............. -50-

1. Non-target Aquatic Animals ................................ -50-

Runoff ................................................. -50-

2. Non-target Terrestrial Animal ............................... -51-

3. Non-target Plants ......................................... -53-

Terrestrial ............................................... -53-

Aquatic Plants ........................................... -54-

B. Risk Description ................................................ -54-

1. Risks to Aquatic Animals and Plants .......................... -54-

Summary of Major Conclusions ............................. -54-
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Discussion .............................................. -55-

Aquatic animals .......................................... -55-

Aquatic Plants ........................................... -57-

2. Risks to Terrestrial Organisms .................................. -57-

Summary of Major Conclusions ............................. -57-

Discussion .............................................. -58-

Terrestrial animals ........................................ -58-

3. Review of Incident Data ...................................... -61-

4. Endocrine Disrupter .......................................... -61-

C. Threatened and Endangered Species Concerns ........................... -62-

1. Taxonomic Groups Potentially at Risk ........................ -62-

2. Probit Slope Analysis ...................................... -65-

3. Indirect Effect Analyses .................................... -67-

4. Critical Habitats .......................................... -68-

D. Description of Assumptions, Uncertainties, Strengths, and Limitations ..... -69-

1. Assumptions and Limitations Related to Exposure for all Taxa ..... -69-

2. Assumptions and Limitations Related to Exposure for Terrestrial Species

....................................................... -70-

a. Location of Wildlife Species ........................... -70-

b. Routes of Exposure .................................. -70-

c. Residue Levels Selection ............................. -71-

d. Dietary Intake - Differences Between Laboratory and Field

Conditions ........................................ -71-

e. Estimated Environmental Concentrations to Non-Target Plants

................................................. -72-

f. Data gaps .......................................... -72-

3. Assumptions and Limitations Related to Aquatic Effects Assessment -72-

a. Age Class and Sensitivity of Effects Thresholds ............ -72-

b. Use of the Most Sensitive Species Tested ................ -73-

c. Data Gaps ......................................... -73-

d. Aquatic Environment Effects .......................... -73-

e. Aquatic Environment Exposure ........................ -74-

Appendix A. Environmental Fate Studies ................................. -75-

Appendix B Summary of Public Literature from ECOTOX database ........... -83-

Appendix C Summary of Public Literature that were excluded from ECOTOX database

....................................................... -90-

Appendix D List of Exclusion Terms Utilized under the ECOTOX Database 

...................................................... -128-

Appendix E Ecological Effects Data ................................... -130-

Appendix F. Input Parameters For GENEEC2 Runs for Sethoxydim Total Toxic

Residues ............................................... -140-

Appendix G Environmental Fate Data Requirements ...................... -142-

Appendix H Use Closure Memorandum ................................. -144-
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Appendix I Ecological Data Requirements .............................. -146-

Appendix J Structures of Degradates .................................. -148-

Appendix K Sethoxydim MRID Bibliography ............................ -156-

Appendix L Listed Species by State and Crops ........................... -166-

Appendix M Full list of listed species in states in which sethoxydim is applied. . . -187-

Appendix N AgDrift Aquatic Model Runs ............................... -205-

Appendix O AgDrift Model Runs for Plants ............................. -211-

Appendix P TerrPlant Model Results .................................. -219-
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I. Executive Summary

A. Nature of Chemical Stressor

There are two chemical stressors in this risk assessment. The first is the active ingredient,

sethoxydim (PC code 121001) and its degradates, and the second is the petroleum solvent (CAS

Registry Number 64742-94-5) that is used in the end-use formulations POAST® and POAST®

Plus. The solvent is known to contain several percent by weight of Naphthalene, which is also a

registered active ingredient (PC code 055801). Sethoxydim is responsible for all presumed risks

to plants and birds, and the petroleum solvent is responsible for presumed risks to fish and

aquatic invertebrates (fresh- and salt-water).

Sethoxydim. The first chemical stressor in this assessment is the cyclohexenone-class herbicide

sethoxydim and its eight degradates. The eight degradates are grouped into three sets. These are

(1) the sulfoxide (M-SO) and sulfone (M-SO2) of the parent chemical, (2) the hydrolysis product

M1-S, its sulfoxide (M1-SO), and sulfone (M1-SO2), and (3) the hydrolysis/ring closure product

(M2-S), and its sulfoxide (M2-SO) and sulfone (M2-SO2).

Structures of all the degradates are given in Appendix J, and the degradation scheme showing the

relationship of the parent to the degradates is given in section B.1.

The major degradates present in soil are the parent sulfoxide and sulfone (M-SO and M-SO2). 

These compounds may revert to parent sethoxydim by chemical reduction (loss of the added

oxygen atoms) under anaerobic conditions. Each of these compounds is mobile in soil-water

systems. 

The major degradates formed in water are the hydrolysis product (M1-S) and the resulting ringclosure product M2-S. As noted, each of these has a sulfoxide and a sulfone. The M1 series of

degradates are mobile in soil-water systems, as is parent sethoxydim and its sulfoxide and

sulfone. The M2 series, being a fused two-ring system (tetrahydrobenzoxazole) is less mobile.

For this assessment, all of the degradates are assumed to be equally as toxic as parent

sethoxydim. Therefore, a Total Toxic Residue approach is used for the assessment of risks to

aquatic organisms and plants from sethoxydim per se.

Petroleum Solvent. This solvent (CAS Registry Number 64742-94-5) is used to stabilize the

manufacturing use product (MUP) and is present in the end-use products POAST® and

POAST® Plus. It contains Naphthalene, a polycyclic aromatic hydrocarbon, which is known to

be toxic to fish and invertebrates. The toxicity of POAST® (18% active ingredient by weight) is

similar to the toxicity of Naphthalene, rather than that of pure sethoxydim. 

For this reason, the presumed risks to aquatic animals in this assessment are attributable to the

petroleum solvent, rather than sethoxydim itself. 
-7-

B. Potential Risks to Non-target Organisms

Risk from sethoxydim:

• Chronic risk to birds from sethoxydim is expected.

• It is expected that risk to non-target aquatic plants is limited to aquatic grasses, due to the

mode of action of sethoxydim and terrestrial plant data. The risk cannot be quantified,

because there are no toxicity testing data for aquatic grasses (family Poaceae). 

• The terrestrial listed plant LOC (family Poaceae) is exceeded from spray drift from

ground and aerial application and from runoff to low semi-aquatic areas.

No LOC Exceeded

• LOC for acute risk for fish and aquatic invertebrates from run-off or spray drift to the

pond is not exceeded.

• There are no LOC exceedances to algae or diatoms.

• LOC for chronic risk for aquatic estuarine animals from runoff to the standard farm pond

is not exceeded. 

• Avian or mammalian acute LOC is not exceeded.

• The chronic LOC is not exceeded for mammals.

• LOC for non-grass plant species are not exceeded.

C. Conclusions - Exposure Characterization

Terrestrial. The exposure of terrestrial animals and plants was assessed by standard methods

(Hoerger-Kenaga nomogram for dietary exposure to animals, 1% or 5% spray drift for plants in

adjacent fields and wetlands, and AgDrift for spray drift exposure of plants).

For animals, exposure on food items at maximum labeled rates, with minimum application

intervals, triggered only chronic (reproductive) risk to birds via dietary intake. This was true

with the assumption of either the 35-day default foliar dissipation half-life on food items, or a

refined assumption of a one-day foliar dissipation half-life for the Hoerger-Kenaga analysis (T-
-8-

REX model). The exposure that would not trigger this risk is not known, since a no-effect

concentration was not established.

For plants, the runoff exposure from spray drift was estimated as 1% or 5% of the application

rate for ground and aerial applications, respectively. Exposure to non-target plants from runoff

was estimated with 5% runoff factor that took into consideration the solubility of sethoxydim. 

Spray drift exposure of 1-5% and runoff exposure was sufficient to trigger risk concern for

terrestrial grasses.

Aquatic. The exposure of aquatic animals and plants was estimated by standard Tier 1 methods

(GENEEC model, version 2.0). Screening-level (Tier 1) exposure analysis was done because it

was not anticipated that sethoxydim would reach concentrations sufficient to trigger either acute

or chronic risk in animals. The Tier 1 EECs were below 100 ppb, which was indeed insufficient

to trigger acute or chronic risk to animals from sethoxydim alone. 

Screening-level estimates of exposure to the petroleum solvent from aerial spray drift to a 6-inch

wetland were up to 350 ppb. This was sufficient to trigger acute risk concerns in fish and

invertebrates. The exposure analysis was done with non-guideline fate data, however, and so is

more uncertain than a standard Tier 1 assessment.

It is not known whether the estimated exposure in the standard 6-foot deep farm pond (about 100

ppb) is sufficient to affect sensitive aquatic plants, since there are no toxicity data for aquatic

grasses.

D. Conclusions - Effects Characterization

• Sethoxydim technical is practically non-toxic to aquatic and terrestrial animals on an

acute basis.

• The petroleum solvent that contains Naphthalene is moderately toxic to aquatic animals

on an acute basis. The toxicity of POAST® is slightly less toxic than that of 

Naphthalene to aquatic animals.

• The sheepshead minnow early life stage study found no chronic effects (NOAEC > 98

ppm ai) and the mysid shrimp life cycle study found reproductive effects at 13 ppm ai

(NOAEC = 6.5 ppm a.i.). 

• Sethoxydim technical and TEP appear to be phytotoxic to grass species only.

• Birds may experience a decrease in normal hatchlings when dietary exposure is as low as

100 ppm ai. Mammals may show tail abnormalcies at dietary concentrations as low as

3000 ppm (NOAEL = 600 ppm).
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E. Uncertainties and Data Gaps

• Risk to non-target terrestrial plants from spray drift is underestimated because the

vegetative vigor study was conducted on the technical sethoxydim and not on the TEP

(POAST®).

• Risk to non-target aquatic grass species can not be certain or quantified due to lack of

data.

• The avian chronic reproduction study in the mallard duck did not achieve a NOAEL. 

Chronic avian toxicity is underestimated. 

• The estuarine Fish Early Life Stage and Estuarine Invertebrate Life Cycle tests were

conducted with 40 and 43% active ingredient formulations respectively, and it is unclear

whether the petroleum solvent was present in these test substances. It is uncertain as to

what the results of the studies represent. 

• The submitted Terrestrial Field Dissipation studies (164-1) were judged to be

unacceptable because total sethoxydim residues were analyzed, rather than the parent

compound and its eight degradates. However, due to the large uncertainty that exists for

the degradates of sethoxydim, additional data need to be generated to lower the level of

uncertainty on the fate of sethoxydim. If the assumption that the degradates are equally as

toxic as the parent is adequate for risk assessment, then the studies do not need to be

repeated. 

• Additional information is needed to determine if the submitted Aerobic Aquatic

Metabolism study (162-3) is acceptable. There is uncertainty in this study that needs to

be clarified.

• EEC for 6-inch shallow pond scenario was not attempted. Many species of fish and

invertebrates breed in shallow waters. There is uncertainty as to whether there may be

risk to aquatic animals that spend part of life cycle in these shallow water bodies.
-10-

II. Problem Formulation

A. Stressor Source and Distribution

1. Physical/Chemical/Fate and Transport Properties

Sethoxydim is a highly soluble compound (4700 mg/L in pH 7 water) with a low octanol/water

partition coefficient (Kow for Sethoxydim is 45.1). Sethoxydim is labile and mobile in the

environment. The transformation of parent Sethoxydim, primarily by photodegradation, aerobic

metabolism in water and soil, and acid-catalyzed hydrolysis, is rapid (half-lives on the order of

hours to days). Sethoxydim total residues (Sethoxydim plus Sethoxydim transformation

products) are determined using a common moiety method that cannot distinguish parent

Sethoxydim from its transformation products. These rearrangement products are more persistent

in the environment than Sethoxydim (half-lives on the order of weeks to months) and potentially

mobile. Because of its low vapor pressure (1.6x10-7 mm Hg) and Henry's Law Constant

(1.47x10-11 atm-m3/mol), Sethoxydim is not expected to be highly volatile. 

For the aquatic (but not terrestrial) ecological assessment of sethoxydim, we have assumed that

the stressor is sethoxydim Total Toxic Residues, that is, the sum of the parent and its eight

degradates. This assumption is justifiable on several bases.

First, in aerobic soil metabolism studies, the degradation half-life of parent sethoxydim is less

than one day. The resulting metabolites are the sulfoxide (M-SO) and the sulfone (M-SO2). 

Because the re-application interval is 14 days, and the degradates are more persistent, it is

reasonable to conclude that M-SO and M-SO2 are the forms that have biological activity.

Second, the oxidation process that forms M-SO and M-SO2 is in principle reversible, that is,

under chemically reducing (anaerobic) conditions, the degradates could be re-oxidized to the

parent compound. This behavior was observed in the unacceptable Anaerobic Aquatic

Metabolism study (MRID 42165603), in which the concentration of parent sethoxydim declined

to 16.5% at 0.5 months, and then rose again to 34.3% at 1 month. So, in a sense, parent

sethoxydim is not irreversibly lost by aerobic soil metabolism.

Third, the water exposure models that EFED uses do not account for reversible (or equilibrium)

processes such as reversible sulfoxidation. To avoid underestimation of exposure, M-SO and MSO2 were considered to be the same chemical entity, since their reversion to parent could not be

modeled with the computer.

Fourth, the submitted Terrestrial Field Dissipation studies (MRID 41510608, 41510609, and

41510610) used a common moiety analytical method that reported total residues, rather than the

parent and degradates individually ( the studies were judged to be unacceptable for this reason). 

This is an indication that the registrant considers soil degradates to be as important as the parent.

For the aquatic assessment, the decision to use parent-only or total toxic residues (TTR) does not
-11-

affect the outcome of the assessment, since the TTR exposure estimates were well below toxicity

endpoints (LC50s) for the technical compound. However, it does result in a more conservative

assessment.

For the terrestrial assessment, the standard assumption of a 35-day half-life on food items was

used, as well as the refined assumption of a one-day half-life. Under either assumption, chronic

risk concerns in birds were triggered. Neither of these assumptions has any relationship to the

formation of the eight degradates of sethoxydim, since the half-lives only refer to the rate of

disappearance of the parent. However, the 35-day half-life is not unreasonable given that

sethoxydim TTR has a half-life of about one month in aerobic soil. 
-12-

CH3

N O

CH3 CH OH

3

S

H3C

O

Below is the chemical structure of Sethoxydim (Figure 1) and Table 1 showing the PhysicalChemical Properties of Sethoxydim

Figure 1: Structure of Sethoxydim. CAS Reg. No. 74051-80-2, Formula C17H29NO3S
-13-

Table 1: Physical-Chemical Properties of Sethoxydim

Property Value Reference

Solubility in Water 257 mg/L at pH 5 and 25°C

4700 mg/L at pH 7 and 25°C

16,200 mg/L at pH 9 and

25°C

25 mg/L at pH 4 and 20°C

EFED one-liner database

9/14/1993

EFGWB update memo on

Reregistration package

12/26/1990

Tomlin, C (1997) cited in

EPI-Suite v3.12

Log Kow 3.51 at pH 5

1.65 at pH 7

-0.032 at pH 9

EFED one-liner database

9/14/1993

EFGWB update memo on

Reregistration package

12/26/1990

Vapor Pressure 1.6x10-7 mm Hg EFED one-liner database

9/14/1993

Henry’s Law Constant 1.47x10-11 atm-m3/mol

Molecular Weight

Molecular Formula

327.48 g/mol

C17H29NO3S

pKa 

(acid dissociation constant)

4.62

SMILES code CCCC(=NOCC)C1=C(O)CC

(CC(C)SCC)CC1=O

EPI-Suite v3.12

Physical State amber oily liquid EFED one-liner database

9/14/1993

CAS Registry number 71441-80-0

74051-80-2

EPI-Suite v3.12

EFED one-liner database

9/14/1993; 

Merck Index 10th Ed.
-14-

2. Pesticide Type, Class, and Mode of Action

Sethoxydim is a member of the Cyclohexanedione class of chemicals. The mode of action for

this herbicide is Lipid Biosynthesis Inhibition. Selectivity is shown to be due to the greater

susceptibility at acetyl-coenzyme A carboxylase (ACCase) of grassy species. Susceptible grassy

species are killed by inhibition of the ACCase which is a key enzyme in the lipid biosynthetic

pathway. 

3. Overview of Pesticide Formulation and Usage

Sethoxydim is a systemic herbicide which is registered for selective postemergence control of a

wide spectrum of annual and perennial grasses. It is applied either by aerial or ground methods

and is registered for many different agricultural and residential uses throughout the United States. 

Sethoxydim is formulated with Naphthalene-containing solvents. Material Safety Data Sheets

for POAST® (Reg. No. 7969-58-51036) http://www.cdms.net/ldat/mp5KM007.pdf and

POAST® Plus (Reg. No. 7969-88-51036) http://www.cdms.net/ldat/mp5KN001.pdf indicate

that POAST® contains 74% petroleum solvent including 7% Naphthalene, and that POAST®

Plus contains 87% inerts including 1.3% Naphthalene.

Due to its high content of Naphthalene, toxicity data for Naphthalene will be used as surrogate

toxicity data for the solvent. It is likely that the other constituents of Naphthalene-containing

solvents (petroleum distillates) have toxicity similar to that of Naphthalene, especially if the toxic

effect is simple narcosis. 

B. Receptors and Assessment Endpoints

Assessment endpoints are defined as “explicit expressions of the actual environmental value that

is to be protected.” Two criteria are used to select the appropriate ecological assessment

endpoints: 1) identification of the valued attributes of the environment that are considered to

be at risk, and 2) the operational definition of assessment endpoints in terms of an ecological

entity (i.e., a community of fish and aquatic invertebrates) and its attributes (i.e., survival and

reproduction). Therefore, the selection of assessment endpoints is based on valued entities (i.e.,

ecological receptors), the ecosystems potentially at risk, the migration pathways of pesticides,

and the routes by which ecological receptors are exposed to pesticide-related contamination. The

selection of clearly defined assessment endpoints is important because they provide direction and

boundaries in the risk assessment for addressing risk management issues of concern.

1. Measures of Ecological Effect

Ecosystems potentially at risk are expressed in terms of the selected assessment endpoints. The

typical assessment endpoints for screening-level pesticide ecological risks are reduced survival

and reproductive and growth impairment for both aquatic and terrestrial receptors. A set of
-15-

surrogate species is used to provide an estimation of ecological risk. Ecological relevance and

sensitivity are essential for selecting receptors and assessment endpoints that are scientifically

defensible. Surrogate species include freshwater fish and invertebrates, estuarine/marine fish

and invertebrates, and amphibians. In the absence of toxicity data on amphibians, it is assumed

that aquatic-phase amphibians are approximately as sensitive as fish to the potential effects of a

pesticide. Terrestrial animals used as test species include birds and small mammals. The risk

assessment assumes that reptiles and terrestrial-phase amphibians are approximately as sensitive

to pesticide-induced effects as are birds in the absence of toxicity data. Potential effects to

benthic organisms may also be evaluated if environmental fate data indicate that the assessed

chemical may partition to the sediment or if sediment organisms are particularly sensitive

species. For both aquatic and terrestrial receptors, direct acute and direct chronic effects are

considered. Although these endpoints are measured at the individual level, they provide insight

about risks at higher levels of biological organization (i.e., populations and communities). For

example, pesticide effects on individual survivorship have important implications for both

population-level effects as well as habitat carrying capacity. Indirect effects on critical habitat

are only considered in the risk characterization for the potential effects on endangered species,

provided that the risk quotient demonstrates probable risk.

For plants in terrestrial and semi-aquatic environments, the screening assessment endpoint is the

perpetuation of populations of non-target species (both crop and non-crop species). Endpoints

assessed include height measurement and plant weight from seedling emergence and vegetative

vigor studies. Although it is recognized that the seedling emergence and vegetative vigor

endpoints may not address all plant life cycle components, it is assumed that impacts at

emergence and during active growth may impact individual competitive ability and reproductive

success. Data on the formulated product (as opposed to the active ingredient) are used to

characterize plant exposure effects. If the maximum exposure rate results in an effect level that

is less than 25% in a Tier I study, low risk is presumed and a Tier II study is not required. 

However, if the effect level is less than 25% but greater than 5% in a Tier I study, potential risk

may still occur to listed species that cannot be quantified.

a. Terrestrial Organisms

Acute Toxicity to Birds

Technical Sethoxydim is practically non-toxic to the mallard duck on an acute oral basis, and

practically non-toxic to the mallard duck and the bobwhite quail on a subacute dietary basis. 

The surrogate chemical (Naphthalene) for the Petroleum solvent is also non-toxic to the

bobwhite quail on an acute oral basis (LD50 is 2690 mg/kg) and subacute dietary basis (LC50 is

> 5620 ppm). 

Therefore, no adverse effects to birds are expected from acute exposure to either Sethoxydim or

Petroleum solvent, and no acute risk assessment for birds will be done.
-16-

Chronic Toxicity to Birds

A reproduction study in bobwhite quail resulted in no treatment-related effects, with a NOAEC

of >1000 ppm. However, the number of normal hatchlings was affected in the mallard duck,

with a LOAEC of 100 ppm, and no NOAEC established. Both tests were performed using the

technical active ingredient (96.8%). Therefore, a chronic risk assessment based on the

reproductive risk to birds, as represented by the mallard duck, will be performed.

A single application of 0.47 lb/A assuming a foliar dissipation half-life of 1 day gives chronic

RQ of >1.13 for birds consuming short grass food items. EFED will conclude that there is a

presumption of chronic (reproductive) risk to birds.

Acute Toxicity to Mammals

The acute toxicity of technical (94-99%) Sethoxydim, and of the 18.0% formulated product

(POAST®) was tested on laboratory rats. The LD50 of the technical ingredient was 3125 mg/kg

in male rats, and 2676 mg/kg in females. The LD50 of the 18% formulation was 5000 mg/kg in

male rats, and 4385 mg/kg in female rats. These results indicate that Sethoxydim and its 18%

formulation are practically non-toxic to rats. 

Therefore, an acute risk assessment for mammals will not be done.

Chronic Toxicity to Mammals

A two-generation reproduction study in rats was performed with technical (94-99%) Sethoxydim. 

The reproductive toxicity NOAEC value (3000 ppm) indicates that Sethoxydim does not cause

reproductive effects at expected exposure levels in rats (as a surrogate for all mammals);

however, developmental effects were observed in offspring and the developmental toxicity

NOAEC was 600 ppm.

Toxicity to Plants

Phytotoxicity data indicate that Sethoxydim is practically nontoxic to dicots and a non-grass

monocot (onion). It is, however, toxic to grasses. The phytotoxicity data that can be used is

from vegetative vigor and seedling emergence studies which is used to determine risk to nontarget plants from spray drift and runoff, respectively. However, vegetative vigor study was done

with the technical grade Sethoxydim. The Agency’s guidelines call for the vegetative vigor study

to be conducted with the TEP because of enhanced penetration of the herbicide with adjuvants to

penetrate the plant epidermis. The plant risk assessment will focus on risks to non-target grassy

plants (including listed grass species) from spray drift and runoff, and possibly indirect risks to

listed animal and non-grass species that may depend on them. There is added uncertainty in the

assessment of risk to non-target terrestrial plants from spray drift (due to no phytotoxic data

using the TEP). Risk to non-target terrestrial plants from spray drift may be underestimated

from lack of TEP phytotoxic data.
-17-

b. Aquatic Organisms

Acute Toxicity to Aquatic Organisms

The acute toxicity data for technical Sethoxydim show that it is practically non-toxic to the

rainbow trout, bluegill sunfish, sheepshead minnow, eastern oyster, and mysid shrimp, and only

slightly toxic to the waterflea (Daphnia magna). 

In contrast, acute toxicity data for 18.0, 19.3 and 20.4% formulated products indicate moderate

toxicity to the rainbow trout, bluegill sunfish, waterflea, and sheepshead minnow, and high

toxicity to the eastern oyster and mysid shrimp.

EFED concludes that the observed acute toxicity of formulated Sethoxydim is due to the action

of the petroleum solvent. The acute hazard to aquatic and estuarine organisms is therefore due to

the solvent, and not the active ingredient, Sethoxydim.

Chronic Toxicity to Aquatic Organisms

No chronic studies were submitted for freshwater invertebrates or fish.

The only available chronic toxicity data for aquatic organisms are for an estuarine fish

(sheepshead minnows) and an estuarine invertebrate (mysid shrimp). Both studies were rated as

“supplemental” because they were conducted using 40 and 43% sethoxydim formulations

respectively.

No adverse effects were observed in the sheepshead minnow at the highest tested concentration

of 98 ppm (the solubility of Sethoxydim is 4700 ppm). For the mysid shrimp, both survival and

length were affected. The NOAEC (no observed effect concentration) was 6.5 ppm ai, and the

LOAEC (lowest observed effect concentration) was 13 ppm ai. 

c. Discussion

For aquatic plants, the assessment endpoint is the maintenance and growth of standing crop or

biomass. Measures of effect focus on algal and vascular plant (i.e., duckweed) growth rates and

biomass measurements. 

The ecological relevance of selecting the above-mentioned assessment endpoints is three-fold: 1) 

complete exposure pathways exist for the receptors; 2) the receptors are potentially sensitive to

pesticides in the affected media and from residues on plants, seeds, insects, and other food

sources; and 3) the receptors could inhabit areas where pesticides are applied or where runoff

and/or spray drift could impact sites containing available suitable habitat.
-18-

Each assessment endpoint requires one or more “measures of ecological effect,” which are

defined as changes in the attributes of an assessment endpoint itself or changes in a surrogate

entity or attribute in response to pesticide exposure. Ecological measures of effectfor the

screening level risk assessment are based on a suite of registrant-submitted toxicity studies

performed on a limited number of organisms in the following broad groupings:

• Birds (mallard duck and bobwhite quail) used as surrogate species for terrestrialphase amphibians and reptiles

• Mammals (laboratory rat)

• Freshwater fish (bluegill sunfish and rainbow trout) used as a surrogate for aquatic

phase amphibians

• Freshwater invertebrates (water flea - Daphnia magna)

• Estuarine/marine fish (sheepshead minnow)

• Estuarine/marine invertebrates (Eastern oyster and mysid shrimp)

• Terrestrial plants (various crop species)

• Algae and aquatic plants (algae, diatoms, and duckweed)

A discussion of toxicity data available for this risk assessment and the resulting measures of

effect selected for each taxonomic group are included in Section III C of this document. A

summary of the assessment and measures of effect selected to characterize potential ecological

risk associated with Sethoxydim exposure is provided in Table 2.

Table 2. Summary of Assessment and Measurement Endpoints

Assessment Endpoint Measurement Endpoint

1. Abundance (i.e., survival,

reproduction, and growth) of

individuals and populations of birds

1a. Mallard duck oral LD50

1b. 1b. Bobwhite quail and mallard duck subacute dietary

LC50

1c. Bobwhite quail and mallard duck chronic reproduction

NOAEC and LOAEC (one acceptable study and one

supplemental study)

2. Abundance (i.e., survival,

reproduction, and growth) of

individuals and populations of

mammals

2a. Laboratory rat acute oral LD50

2b. Laboratory rat chronic NOAEC (2-generation rat

reproductive study)

3. Survival and reproduction of

individuals and communities of

freshwater fish and invertebrates

3a. Rainbow trout acute LC50

3b. Fathead minnow or rainbow trout chronic early life stage

NOAEC (weight & length) NO STUDY SUBMITTED

3c. Water flea acute LC50

3d. Water flea life cycle chronic NOAEC (offspring) - NO

STUDY SUBMITTED
-19-

4. Survival and reproduction of

individuals and communities of

estuarine/marine fish and

invertebrates

4a. Sheepshead minnow acute LC50

4b. Estuarine/marine fish chronic NOAEC & LOAEC 

4c. Eastern oyster and mysid shrimp acute LC50

4d. Estuarine/marine invertebrate (mysid shrimp) chronic

NOAEC & LOAEC 

5. Perpetuation of individuals and

populations of non-target terrestrial

plants (crops and non-crop plant

species)

5a. Monocot and dicot seedling emergence EC25 

5b. Monocot and dicot vegetative vigor EC25 -STUDY

NEEDED WITH TEP

6. Survival of beneficial insect

populations

6a. Honeybee acute contact LD50

7. Maintenance and growth of

individuals and populations of

aquatic plants from standing crop or

biomass

7a. Algae and duckweed acute EC50

LD50 = Lethal dose to 50% of the test population

NOAEC = No observed adverse effect concentration

LOAEC - Lowest observed adverse effect concentration

LC50 = Lethal concentration to 50% of the test population

EC50/EC25 = Effect concentration to 50%/25% of the test population

2. Measures of Exposure

Methods used to determine exposure concentrations of a pesticide are dependent on its

environmental fate and physicochemical properties, the application method, and the presence of

reliable monitoring data that are considered representative of the proposed use sites. Initial

factors considered for the exposure assessment are provided in Table 3 below. 

Table 3. Factors that may affect the exposure to Sethoxydim

Consideration Sethoxydim-specific data Conclusion

Monitoring data No appropriate or targeted

monitoring data are currently

available to EFED

EECs will be determined using

EFED’s exposure models

Degradation degradation: Sethoxydim is very

rapidly metabolized in soil to

sulfoxide and sulfone degradates as

well as hydrolysis products (M1

and M2 series) 

EECs will be calculated for Total

Residues including M-SO, M-SO2,

M1-S, M1-SO, M1-SO2, M2-S,

M2-SO, and M2-SO2

Mobility Sethoxydim and its degradates are

highly mobile in soil-water systems

Runoff will need to be considered

Bioconcentration BCF values for Sethoxydim are

low: 21x in whole fish

Bioconcentration will not be

modeled

Application method Ground or air spray Spray drift and runoff need to be

considered in this assessment
-20-

Formulation Naphthalene-containing solvent is

demonstrably more toxic to aquatic

organisms than Sethoxydim

Toxicity of solvent will be

considered when exposure to active

ingredient and solvent is

simultaneous (spray drift)

Aquatic Systems

For aquatic organisms, such as plants, fish, aquatic-phase amphibians, and invertebrates, OPP

generally uses computer simulation models to estimate exposure to a pesticide active ingredient. 

These models calculate estimated environmental concentrations (EECs)in surface water using

laboratory data that describe the rate at which the pesticide breaks down and how it moves into

the environment. Monitoring data, if available, may also be used to determine EECs or to

support the model’s calculations. EECs for aquatic organisms were generated with the Tier 1

model GENEEC2.0 for this assessment.

No EECs are generated in instances where no toxicity was observed at concentrations above the

active ingredient’s water solubility or at or above the recommended limit concentration for a

particular type of study. Due to the presence of multiple Sethoxydim transformation products, it

is necessary to examine exposure to both the parent and the products. Although fate data indicate

that parent Sethoxydim degrades rapidly in soil (< 1 day) the application intervals (14 days)

indicate that herbicidal activity lasts longer than one day. Thus, some of the degradates seen in

soil metabolism studies (especially M-SO and M-SO2) must retain some herbicidal activity. In

addition, the sulfoxidation process is in principle reversible, so the parent could be regenerated

from the M-SO degradate under reducing conditions. Because we have no separate data on the

activity of the degradates, we will assume that the activity of the degradates is equal to that of the

parent. 

It is assumed that the active ingredient and the Naphthalene-containing solvent will become

separated by fate processes before run-off into the pond. Therefore, run-off exposure to aquatic

organisms will consider only the active ingredient, and toxicity data on the technical-grade active

ingredient will be used to calculate RQs. A separate screening-level assessment of possible

exposure to the Naphthalene-containing solvent will also be conducted, to indicate if further

study is warranted.

In the case of spray drift, parent Sethoxydim and its Naphthalene-containing solvent should reach

water bodies before any degradation occurs. Thus, exposure will be to the Typical End Use

Product (TEP). Therefore, toxicity data on the TEP will be used.
-21-

Terrestrial Systems

Terrestrial wildlife exposure estimates are typically calculated for birds and mammals,

emphasizing a dietary exposure route for uptake of pesticide active ingredients. These exposures

are considered as surrogates for terrestrial-phase amphibians and reptiles. For terrestrial

organism exposure, OPP primarily looks at the residues of pesticides on food items and assumes

that organisms are exposed to a single pesticide residue in a given exposure scenario. For spray

applications, estimation of pesticide concentrations in wildlife food items focuses on quantifying

possible dietary ingestion of residues on vegetative matter and insects. The residue estimates are

based on a nomogram that relates food item residues to pesticide application rates (Fletcher et al.,

1994). The first tier of the nomogram uses the maximum predicted residues. Subsequent

refinements may consider mean residues. However, maximum residue concentration is

converted to daily oral doses based on fractions of body weight consumed daily as estimated

from mammalian allometric relationships in EPA’s Wildlife Exposure Factors Handbook. In all

screening-level assessments, the organisms are assumed to consume 100% of their diet as one

food type. 

C. Conceptual Model

1. Risk Hypotheses

Ecological receptors that may potentially be exposed to sethoxydim, its degradates, and the

various products that make up the Typical End Use product (TEP) include terrestrial and

semi-aquatic wildlife (i.e., mammals, birds, and reptiles), plants, and soil invertebrates. In

addition to terrestrial ecological receptors, aquatic receptors (e.g., freshwater and

estuarine/marine fish and invertebrates, amphibians, and obligate aquatic plants) may also be

exposed to potential migration of pesticides from the site of application to various watersheds

and other aquatic environments via runoff and spray drift.
-22-

Stressor Sethoxydim Total Toxic ResiduesAND/OR PetroleumSolvent

Source/

Transport

Pathways

Direct

Deposition

Runoff/

Erosion

Leaching

(Infiltration/

Percolation

Source/

Exposure

Media

Food Items Upland

Soil

Riparian/

Wetland Soil

Receiving 

Water Body/

Sediment

Groundwater

Exposure

Route

Ingestion Direct contact/

Root Uptake

Direct contact/

Root Uptake

Gill/

Integument

Uptake

Receptors

Attribute

Changes

Terrestrial Vertebrates

Birds, Mammals,

Reptiles, Terrestrial

Phase Amphibians

Terrestrial

Upland

Plants

Reduced survival

Reduced growth

Reduced

reproduction

Aquatic

Invertebrates

Aquatic

Vertebrates

Wetland/

Riparian

Plants

Seedling emergence

Vegetative vigor

Reduced

survival

Reduced

growth

Reduced

reproduction

Plant

population

Reduced

population

growth

Aquatic

Plants

Uptake/

Adsorption

Figure 2: Conceptual Modelfor Sethoxydim And PetroleumSolvent

Stressor Sethoxydim Total Toxic ResiduesAND/OR PetroleumSolvent

Source/

Transport

Pathways

Direct

Deposition

Runoff/

Erosion

Leaching

(Infiltration/

Percolation

Source/

Exposure

Media

Food Items Upland

Soil

Riparian/

Wetland Soil

Receiving 

Water Body/

Sediment

Groundwater

Exposure

Route

Ingestion Direct contact/

Root Uptake

Direct contact/

Root Uptake

Gill/

Integument

Uptake

Receptors

Attribute

Changes

Terrestrial Vertebrates

Birds, Mammals,

Reptiles, Terrestrial

Phase Amphibians

Terrestrial

Upland

Plants

Reduced survival

Reduced growth

Reduced

reproduction

Aquatic

Invertebrates

Aquatic

Vertebrates

Wetland/

Riparian

Plants

Seedling emergence

Vegetative vigor

Reduced

survival

Reduced

growth

Reduced

reproduction

Plant

population

Reduced

population

growth

Aquatic

Plants

Uptake/

Adsorption

Figure 2: Conceptual Modelfor Sethoxydim And PetroleumSolvent

Spray Drift

2. Diagram
-23-

a. Aquatic Exposure 

For aquatic exposure, ecological receptors that may potentially be exposed to Sethoxydim

include wildlife living in semiaquatic areas (mammals, birds, and reptiles), plants in semiaquatic and aquatic areas, and other aquatic receptors (freshwater and estuarine/marine fish and

invertebrates, amphibians). Aquatic receptors (e.g., freshwater and estuarine/marine fish and

invertebrates, amphibians) may also be exposed to potential migration of pesticides from the site

of application to various watersheds and other aquatic environments via runoff and spray drift. 

(a) Potentially Complete and Quantitatively Evaluated Pathways

As shown in Figure 2, quantifiable Sethoxydim or petroleum solvent exposure pathways and

risks to aquatic ecological receptors include:

• Aquatic life (fish, invertebrates, amphibians) exposed through contact and uptake

of Naphthalene-containing solvent in surface water from spray drift or runoff.

(b) Potentially Complete But Not Quantitatively Evaluated

Pathways

The following pathways were not quantitatively evaluated in the screening level risk assessment

because exposure was identified as negligible and would not contribute measurably to risk or

exposure estimates:

• Aquatic life exposed through contact and uptake of Sethoxydim in groundwater

• Aquatic life exposed through contact, uptake, and ingestion of Sethoxydim due to

sediment partitioning, as well as through off-target surface water and sediment

• Terrestrial animals (birds, mammals, reptiles) that live in aquatic environments

exposed through contact, uptake, and ingestion of Sethoxydim from target surface

water, sediment, and off-target water and sediment

b. Terrestrial Exposure

Pesticides simultaneously reach several environmental compartments when applied. Sethoxydim

is applied to terrestrial fields as an herbicide. As a result, much of the applied compound may be

intercepted by foliage instead of reaching soil directly. For terrestrial exposure, ecological

receptors that may potentially be exposed to Sethoxydim include wildlife living in terrestrial and

semiaquatic areas (i.e., mammals, birds, land-phase amphibians and reptiles), plants in terrestrial

and semi-aquatic areas, and soil invertebrates. The source and mechanism of release of

Sethoxydim are ground or aerial application to agricultural crops (alfalfa, cotton, soybeans, dry

beans, corn, potato, peanut, sunflower ) and ground only for turf, trees, fruits (including citrus),

vegetables, berries, grapes, nuts and other registered uses. Surface water runoff from the areas of

application is assumed to follow topography. Additional release mechanisms include spray drift,

and wind erosion, which may potentially transport site-related contaminants to the surrounding

air and later to off-site terrestrial or aquatic sites. Potential emission of volatile compounds is not

considered as a viable release mechanism for Sethoxydim, since volatilization is not expected to
-24-

be a significant route of dissipation for this chemical. 

(a) Potentially Complete and Quantitatively Evaluated Pathways

As shown in Figure 2, quantifiable Sethoxydim exposure pathways and risks to ecological

receptors from terrestrial application include:

1. On-site exposure of animals (mammals, birds, reptiles) exposed by contact with or

ingestion of vegetation or insects contaminated by direct spraying.

2. On-site exposure of beneficial insects exposed by contact with residues on plants

contaminated by direct spraying.

3. Off-site exposure of non-target terrestrial plants (wetland or upland grasses only) by

spray drift or runoff. 

(b) Potentially Complete But Not Quantitatively Evaluated Pathways

The following pathways were not quantitatively evaluated in the screening level risk assessment

because exposure was identified as negligible and would not contribute measurably to risk or

exposure estimates:

• All off-site exposure of terrestrial animals, since on-site exposure is greater, and

screening level analysis indicates only chronic risk to birds and reptiles. The

chronic risk cannot be quantitatively evaluated because a NOAEC was not found

for the mallard duck.

• On-site exposure of plants, since on-site grasses are target organisms.

• On-site exposure of animals, including insects, directly to residues in the soil, or

in rainwater puddles, or to residues taken up from the soil by plants, since direct

spraying exposures are expected to be greater.

• On-site exposure of terrestrial invertebrates, since we have no toxicity data for

such organisms
-25-

D. Key Uncertainties and Information Gaps

The following uncertainties and information gaps were identified as part of the problem

formulation:

• The avian chronic (reproductive) toxicity study used in this assessment did not find a noobserved-adverse-effect-concentration (NOAEC). Adverse effects on the number of

normal mallard duck hatchlings were found the lowest dietary level tested (100 ppm in

feed). Thus the risk quotients for chronic risks to birds (and as a surrogate, to reptiles) are

minimums (“greater than” values). The chronic level of concern (RQ = 1) was exceeded

even if a foliar dissipation half-life of one day, rather than the default of 35 days, was

used in the T-REX model. In order to come to a firm conclusion on the possible chronic

risk to birds and reptiles, a NOAEC in the mallard duck needs to be established.

• It is uncertain what the foliar dissipation half-lives are on terrestrial food items. 

Magnitude-of-the-residue studies (guideline 171-4C) need to be examined to establish

Sethoxydim-specific foliar dissipation half-lives. Due to time constraints, and the fact

that a refined assumption of a one-day foliar dissipation half-life results in chronic risk to

birds above the LOC, this has not been done.

• The eight degradates of Sethoxydim are assumed to be as toxic as the parent in the

assessment of risk to aquatic freshwater and estuarine organisms. Thus, a total toxic

residue approach is used in the water exposure modeling. Data on the acute and chronic

toxicity of the degradates would help to refine this assessment.

• Submitted aquatic vascular plant phytotoxicity data were on a dicot, Lemna gibba. There

are no current guidelines for the testing of aquatic grasses. Sethoxydim only adversely

affects grasses. There is much uncertainty of the effects of POAST® to non-target

aquatic grasses.

• The mode of herbicidal action to plants indicates adverse effects to grasses. Vegetative

vigor studies (Tier II) and seedling emergence (Tier I) also confirms this. Available

vegetative vigor phytotoxicity data were only done for Sethoxydim technical grade and

not POAST®. Therefore, toxicity to grasses from spray drift may be underestimated. 
-26-

E. Analysis Plan

Ecological risk assessment is a process that evaluates the likelihood that adverse ecological

effects may occur or are occurring as a result of exposure to one or more stressors (US EPA,

1998). This risk assessment examines the ecological risk of Sethoxydim herbicide use, and

attempts to determine at what level Sethoxydim herbicides can be used to minimize deleterious

effects on the environment. These negative effects on include structural and/or functional

characteristics or components of ecosystems. In order to estimate the ecological risk associated

with Sethoxydim use, use information, chemical and physical properties, fate/transport data, and

toxicity data were examined for Sethoxydim, application methods, and chemical formulations.

1. Assessment

(a) Fate and Exposure

Parent Sethoxydim degrades rapidly (< 1 day) in soil to produce its sulfoxide (M-SO) and

sulfone (M-SO2). It is possible, under reducing conditions, that these could revert to the parent

compound. The hydrolysis products M1-S and M2-S are also formed in major amounts. Because

these degradates are believed to retain their toxicity, a Total Toxic Residue approach will be used

for the assessment of risks to aquatic freshwater and estuarine organisms.

Sethoxydim and its degradates have very low Kd values, and will therefore be mobile in soilwater systems. Transport to surface water bodies is expected to be in the dissolved phase, rather

than by sorption onto soil particles. The sulfoxide (M-SO) and sulfone (M-SO2) may pose a

ground water contamination problem, given their high mobility and longer persistence than the

parent.

Because of the low vapor pressure and high solubility of Sethoxydim, it is not expected to

partition from water bodies to the atmosphere. Off-site transport via air will be due to spray drift

only.

Photolysis data indicate that Sethoxydim total residues decline with half-lives of 20 hours on soil

and 19.8 days in water. Sethoxydim’s high solubility (4700 ppm) and low Kd values indicate

high mobility, and also suggest that it may be removed easily from plant surfaces by rainfall. 

Thus, an assumption of a quicker dissipation from leaf surfaces than the default value (35-day

half-life) is justified.

Sethoxydim’s low Kd values indicate little tendency to partition to bottom sediments in water

bodies. Thus, exposure of bottom-dwelling organisms is expected to be via degradates dissolved

in the water column. This exposure is adequately described by GENEEC2 modeling of total

toxic residues.

(b) Toxicity
-27-

Aquatic and terrestrial non-target toxicity endpoints (animals and plants) are provided by the

acute and chronic toxicity data. These toxicity endpoints are compared with the estimated

environmental concentrations of Sethoxydim, which are based on fate properties, chemical type,

exposure method, etc. For this assessment, the most sensitive toxicity endpoints for each

surrogate taxa (ie. freshwater fish and invertebrates, estuarine/marine fish and invertebrates,

aquatic plants, terrestrial plants, birds, and mammals) will be used in Risk Quotient (RQ)

calculation with various exposure values (see above). 

Listed species analysis will be performed using the EFED LOCATES Listed Species Database to

determine presence of listed or threatened species in counties where specific crops are grown. A

species profile analysis will be performed to determine whether there are any potential risk

scenarios to listed species in those identified areas.

In addition to the data submitted in support of registration and the information compiled through

the Agency pesticide review process the ECOTOX database was used to identify additional

toxicity data from the open literature. The ECOTOX database is a user-friendly, publiclyavailable, quality-assured, comprehensive tool for locating toxicity data from the open literature

and is maintained by EPA Mid-Atlantic Ecology Division. More information on ECOTOX can

be found at: http://www.epa.gov/ecotox. Research papers are thoroughly screened using

standard procedures before being accepted into ECOTOX thereby ensuring consistent, high

quality information. Appendices B lists references related to Sethoxydim toxicity that were

accepted by ECOTOX. Appendix C lists Sethoxydim references that were not accepted by

ECOTOX lists the criteria for rejection; the criteria codes are also defined in Appendix D. 

In some cases, a risk assessment may benefit from information in a reference that was rejected by

ECOTOX. Rejection by ECOTOX does not necessarily infer a lack of study quality but can be

due to a number of reasons including the lack of toxicity data or the lack of a sensitive endpoint. 

(c) Risk Quotient and Levels of Concern

Risk characterization integrates exposure and ecotoxicity data to evaluate the likelihood of

adverse effects. For ecological effects, the Agency accomplishes this integration using the risk

quotient method. Risk quotients (RQs) are calculated by dividing exposure estimates by acute

and chronic ecotoxicity values. 

RQ = EXPOSURE / TOXICITY

RQs are then compared to the Office of Pesticide Program’s levels of concern (LOCs) to assess

potential risk to non-target organisms and the need to consider regulatory action. Calculation of

an RQ that exceeds the LOC indicates that a particular pesticide use poses a presumed risk to

non-target organisms. LOCs currently address the following categories of presumed risk:

• acute - potential for acute risk is high and regulatory action beyond

restricted use classification may be warranted

• acute restricted - the potential for acute risk is high, but may be mitigated
-28-

through restricted use classification

• acute listed species - threatened and listed species may be adversely

affected

• chronic risk - the potential for chronic risk is high and regulatory action

may be warranted. 

The ecotoxicity values used in the acute and chronic risk quotients are endpoints derived from

required laboratory toxicity studies. Ecotoxicity endpoints derived from short-term laboratory

studies that assess acute effects are:

• LC50 - fish and birds

• LD50 - birds and mammals

• EC50 - aquatic plants and aquatic invertebrates

• EC25 - terrestrial plants

The NOAEC (No Observable Adverse Effect Concentration) is the endpoint used to assess

chronic effects. Table 5 gives formulas for calculating RQs and LOCs for various risk

presumptions. 

Table 5. Formulas for RQ calculations and LOC used for risk assessment of Sethoxydim

Risk Presumption RQ LOC

Terrestrial Animals

Acute Risk EEC1/LC50 0.5

Acute Restricted Use EEC/LC50 0.2

Acute Listed Species EEC/LC50 0.1

Chronic Risk EEC/NOAEC 1.0

Aquatic Animals

Acute Risk EEC/LC50 or EEC/EC50 0.5

Acute Restricted Use EEC/LC50 0.1

Acute Listed Species EEC/LC50 0.05

Chronic Risk EEC/NOAEC 1.0

Terrestrial plants and plants inhabiting semi-aquatic habitats

Acute Risk EEC/EC25 1.0

Acute Listed Species EEC/NOAEC or EC05 1.0

Aquatic Plants

Acute Risk EEC/EC50 1.0

Acute Listed Species EEC/NOAEC or EC05 1.0

1 abbreviation for estimated environmental concentration
-29-

III. Analysis

There are two chemical stressors in this risk assessment. The first is the active ingredient,

sethoxydim (PC code 121001) and its degradates, and the second is the petroleum solvent (CAS

Registry Number 64742-94-5) that is used in the end-use formulations POAST® and POAST®

Plus. The solvent is known to contain several percent by weight of Naphthalene, which is also a

registered active ingredient (PC code 055801). Sethoxydim is responsible for all presumed risks

to plants and birds, and the petroleum solvent is responsible for presumed risks to fish and

aquatic invertebrates (fresh- and salt-water).

A. Use Characterization 

Sethoxydim is a systemic herbicide which is registered for selective postemergence control of a

wide spectrum of annual and perennial grasses. It is formulated with a naphthalene-containing

petroleum solvent. Single active ingredient formulations include emulsifiable concentrates (13-

43.3% ai) which are applied by aerial or ground (banding, broadcast, spot) methods at an

application rate of 0.28 - 0.47 lbs. ai/acre/application up to four times per season.

According to the BEAD usage report (Halvorson 2004), Sethoxydim is used on at least 86

different crops. The principal uses are on Soybeans (400,000 lb annually), Sunflowers (80,000

lb), Alfalfa (60,000 lb), Dry Peas/Beans (40,000 lb), Sugar beets (30,000 lb), Peanuts (20,000 lb),

Corn (20,000 lb), Oranges (10,000 lb), Potatoes (10,000 lb), and Cotton (10,000 lb). All other

uses were estimated as 7,000 lb per year or less.

The crops with the highest estimated percent crop treated are Sunflowers and Dry Peas/Beans

(15-20%), Watermelons (10-15%), Sugar Beets (10%), Cantaloupes and Celery (5-15%), Green

Beans, Onions, Cucumbers, Cabbage, Peppers, Pumpkins, Squash, Strawberries (5-10%). 

According to USEPA (1996a), approximately 960,000 lbs of Sethoxydim a.i. are applied each

year in the U.S., with about 97% of this applied to agricultural sites. The largest agricultural use

of Sethoxydim (45-60%) is on soybeans. Approximately 3.1 million acres are treated annually,

representing about 5% of the soybeans acreage in the U.S. Cotton, sugar beets, peanuts and

alfalfa each account for 4-8% of total Sethoxydim use. Approximately 2% of the total

Sethoxydim use is used on turf, which accounts for 13-22% of the annual turf crop. Nurseries,

lawn care and cemetery use account for the remaining non-agricultural uses.
-30-
-31-

Characterization of Production Areas and Soils

Soybeans

The largest agricultural use of Sethoxydim (45-60%) is on soybeans. Soybean production is

widespread in the U.S., ranging from the western Plains States through the eastern and southern

U.S. (Kellogg, et al., 1992, USDA, 1997, USGS, ND). 

Cotton and Peanuts

Cotton production occurs principally in the southeastern U.S., along the Mississippi River in

eastern Arkansas, western Tennessee, and Mississippi, Texas, southern Arizona, southern

California and in the Central Valley of California. (Kellogg, et al., 1992, USDA, 1997, USGS,

ND). 

Peanut production occurs in the southeastern U.S. from northern Florida to Virginia and west to

northern Texas and southern Oklahoma. (Kellogg, et al., 1992, USDA, 1997, USGS, ND). 

Sethoxydim is used on cotton and peanuts in the southeastern U.S. The southeastern U.S. is

characterized by two major land resource areas: 1) Atlantic and Gulf Coast Lowland Forest and

Truck Crop Region and 2) South Atlantic and Gulf Slope Cash Crop, Forest and Livestock

Region (Austin, 1972). 

The Atlantic and Gulf Coast Lowland area consists of nearly level areas of the Atlantic and Gulf

Coastal Plains crossed by many broad valleys. Precipitation ranges from 40-50 inches annually

and is highest in the summer. Most of the soils are too wet to be used for crops without drainage;

however, peanuts are grown on the better drained soils. The predominant soils are Aquults

(Austin, 1972 and 1981). 

The South Atlantic and Gulf Slope region consists of gently sloping to rolling Piedmont and

upper Coastal Plain. Average annual precipitation ranges from 40-60 inches. Cotton is the main

crop in this area with some peanuts. (Austin, 1972 and 1981). 

Other Cotton Areas

The Mississippi Delta region also has significant cotton production. This area is principally

flood plain and terraces of the Mississippi River with annual precipitation ranging from 45-65

inches. The soils are poorly drained but can be highly productive if drained. (Austin, 1972 and

1981, USGS, ND). 

Cotton production in Texas is focused in the north central and "Panhandle" areas. The area is

characterized by smooth high plains with gentle slopes, ranging in elevation from 1,500-5,000

feet. Precipitation fluctuates widely from year to year with an annual average rainfall ranges

from 15-30 inches. The highest precipitation occurs in the late spring through autumn. There are
-32-

few perennial streams in these areas, and cotton is extensively grown under irrigation from wells. 

Water tables are declining in these areas from withdrawals for irrigation. (Austin, 1972 and

1981, USGS, ND). 

Irrigated cotton is grown in southern Arizona and southeastern California. This area consists of

broad basins, valleys and old lake beds, ranging in elevation from -280 to 4,000 feet. This area is

semi-desert to desert with 2-20 inches of precipitation annually, which is evenly distributed

throughout the year. Water is scarce and irrigation comes principally from the major rivers, such

as the Colorado, Gila and Salt Rivers. Deep irrigation wells are also used in south-central and

southeast Arizona. (Austin, 1972 and 1981, USGS, ND). 

The southern part of the Central Valley of California also produces cotton. The southern Central

Valley is broad, smooth and nearly level. It ranges in elevation from sea level to 500 feet. 

Precipitation ranges from 5-25 inches annually. The region has a long growing season with a

long dry period in the summer. Low rainfall and small streams result in a shortage of water. 

Irrigation is from the mountains and from wells. There is a diminishing supply of well water due

to dropping water tables. (Austin, 1972 and 1981, USGS, ND). 

Other Crops

Sugar beet production is focused in southern Idaho, the Red River Valley (MN-ND), and along

the Wyoming-Nebraska-Kansas-Colorado borders. Lesser production occurs along the MontanaWyoming borders, in southern Michigan, northern Texas, southeast Oregon, eastern Washington,

and the Yuma and Imperial Valleys of Arizona and California. 

Alfalfa production is widespread in the U.S. Areas of high production include southern

California, eastern Texas and Oklahoma, southern Illinois, central Wisconsin, western Dakota,

western Nebraska, and numerous individual counties scattered throughout the U.S. Alfalfa

production in the western U.S. is often irrigated. (Kellogg, et al., 1992, USGS, ND). 

The use of Sethoxydim on citrus is very limited in the U.S. Significant citrus production occurs

on the vulnerable, sandy soils of central Florida where Sethoxydim could be used. Rainfall and

ground water are abundant in this area and annual precipitation ranges from 50-57 inches. 

Precipitation is highest in the summer and early autumn. The elevation of the region ranges from

50-150 feet. (Austin, 1972 and 1981, Kellogg, et al., 1992, USGS, ND). 

B. Exposure Characterization

1. Environmental Fate and Transport Characterization

In terrestrial environments, photodegradation, aerobic metabolism, and acid-catalyzed hydrolysis

appear to be the primary routes of dissipation for Sethoxydim and for Sethoxydim total residues. 

The calculated soil photolytic half-lifes are 1 hour for Sethoxydim and 20 hours for Sethoxydim
-33-

residues (parent + transformation products), indicating rapid transformation of Sethoxydim and

its residues after application. Other potential routes of dissipation are microbial-mediated

degradation and surface water runoff. Under aerobic conditions, Sethoxydim degrades in less

than 1 day, while Sethoxydim residues have half-lives which range from 7 to 30 days. Under

anaerobic conditions, Sethoxydim and its residues are more persistent with half-lives greater than

60 days. Unacceptable field dissipation studies (two locations in California, one cotton and one

forage), indicated a moderately rapid rate of dissipation of Sethoxydim residues with half-lives

measuring 32 days in the 0- to 6-inch soil layer. 

In aquatic environments, the major route of dissipation for total Sethoxydim residues is aqueous

photolysis (t1/2=19.8 days), followed by microbial-mediated degradation. In aerobic aqueous

studies, half-lives are on the order of about one month (32.9-38.1 days), while under anaerobic

aqueous conditions, Sethoxydim and its residues are more persistent (half-lives range from 132-

187 days). Aquatic field dissipation studies (four rice paddies in different states) showed that

Sethoxydim residues degraded faster in the field than in the laboratory with half-lives on the

order of 1-9 days. In the field, many processes may occur simultaneously and many variables

may effect these processes. For example, the presence of significant microbial populations in the

floodwater, the presence of natural photosensitizers that may accelerate photolysis, intensity of

sunlight, cloud cover, and temperature can affect the dissipation rate in the field.

Sethoxydim is a highly soluble compound (4700 mg/L in pH 7 water) with a low octanol/water

partition coefficient (Kow for Sethoxydim is 45.1). Despite its high solubility and mobility, parent

Sethoxydim is unlikely to pose a threat to surface and ground waters because it does not persist

under most conditions. Sethoxydim transformation products, however, may be persistent and

mobile enough to pose a potential threat to water resources. The extent of this impact on water

resources, though, cannot be determined until EFED receives additional data on the fate of these

transformation products.

Acceptable and supplemental environmental fate data indicate that Sethoxydim is labile and

mobile in the environment. The transformation of parent Sethoxydim, primarily by

photodegradation, aerobic metabolism in water and soil, and acid-catalyzed hydrolysis, is rapid

(half-lives on the order of hours to days). Sethoxydim transformation products, though, result

from modifications to functional groups rather than the main structure and are of potential

biological concern. Sethoxydim total residues (Sethoxydim plus Sethoxydim transformation

products) are determined using a common moiety method that cannot distinguish parent

Sethoxydim from its transformation products. These rearrangement products are more persistent

in the environment than Sethoxydim (half-lives on the order of weeks to months) and potentially

mobile.

Laboratory studies show that Sethoxydim is a highly soluble compound (4700 mg/L in water)

and that it hydrolyzes at moderately rapid rates at low pH's, but is more stable at high pH's. The

calculated half-lives are 8.7, 155, and 284 days in pH 5, 7, and 9 solutions, respectively. The

major observed hydrolysis transformation product is M2-S or 6-(2-(ethylthio)propyl)-4-oxo-2-

propyl-4,5,6,7-tetrahydrobenzoxazole. In contrast, evaluation of the data of Sethoxydim total
-34-

residues shows that they remain stable at all three pHs tested.

Sethoxydim and Sethoxydim total residues degrade photolytically in both water and soil. In pH

buffered water, the calculated half-life of Sethoxydim is 5.23 days, and the major transformation

product is M1-S or 2-(1-aminobutylidene)-5-(2-(ethylthio)-propyl)-cyclohex-1,3-dione. In sandy

loam soil irradiated with a xenon light source, the half-life of Sethoxydim is approximately 1

hour, and the major transformation product is M-SO or 2-(1-ethoxyiminobutyl)-5-(2-

(ethylsulfinyl)propyl)-3-hydroxycyclohex-2-enone. Sethoxydim total residues photodegrade

slower than parent Sethoxydim in soil and water. Using a linear regression analysis, EFED

calculated a half-life of 19.8 days for the photolysis in water of Sethoxydim total residues and a

half-life of 20 hours in soil.

Under aerobic conditions, parent Sethoxydim transformed with short half-lives (<1 day) both in

soil and aquatic environments. It degraded with a half life of less than one day in sandy loam and

sandy clay loam soils. The major transformation product at 2 months was M-SO, and after 12

months the major product was CO2. Using aerobic clay loam soil:water and aerobic clay

soil:water systems, it was determined that, under aerobic aquatic conditions, Sethoxydim

transformed with a half-life of <1 day. After 28 days, the major transformation products were

CO2, M-SO, M2-S, and M-SO2 or 2-(1-ethoxyiminobutyl)-5(2-(ethylsulfonyl)propyl)-3-

hydroxycyclohex-2-enone. In contrast to parent Sethoxydim, Sethoxydim total residues were

more persistent. The observed half-life was 1 month for Sethoxydim total residues in the aerobic

sandy loam study, and 7 days in the aerobic sandy clay loam study. In an aerobic clay loam

soil:water system, the calculated half-life for Sethoxydim total residues was 38.1 days, while in

an aerobic clay soil:water system the half-life was 32.9 days.

Under anaerobic conditions, parent Sethoxydim is even more persistent than under aerobic

conditions. It transformed with half-lives of 11 to >60 days under anaerobic soil conditions and

25-39 days in anaerobic aquatic metabolism studies. M-SO was the major transformation

product in both studies. It was observed that M2-S, which was a major transformation product in

the hydrolysis study, was only a minor transformation product in the anaerobic aquatic

metabolism study. Anaerobic studies with Sethoxydim total residues showed that they were

more persistent than parent Sethoxydim. A half-life of 91.6 days was observed in the anaerobic

soil metabolism study, while half-lives of 132-187 days were observed in the anaerobic aquatic

metabolism study.

Based on batch equilibrium experiments, Sethoxydim and its transformation products, M2-SO2,

M-SO, M-SO2, and M2-SO, were determined to be mobile to very mobile in sterile (autoclaved)

sand, sandy loam, sandy clay loam, silt loam, and clay loam soils. Freundlich Kads values were

<1.00 for Sethoxydim and its transformation products M-SO and M-SO2. The Freundlich Kads

values for M2-SO and M2-SO2 ranged from 0.06 to 9.12. 

Information from two unacceptable terrestrial field dissipation studies indicated that total

(uncharacterized) Sethoxydim residues dissipated with half-lives of about 32 days in two

California soils with cotton and forage crops. This study showed two detections of Sethoxydim
-35-

near the quantitation level below the 0- to 6-inch depth.

In aquatic field studies, total (uncharacterized) Sethoxydim residues dissipated fairly rapidly,

with half-lives of 1-9 days in the floodwater of rice paddies in California, Mississippi, and

Louisiana, and 10-13 days in the soils of nonflooded rice paddies in Mississippi and Louisiana

that were subsequently flooded 4-5 days after application. Residues were detected in the 0- to 6-

inch soil layer in both paddies. When Sethoxydim was applied to flooded paddies, most of the

residues remained in the floodwater. When applied to the soil prior to flooding, there were no

detectable residues in the water samples.

The low octanol/water partition coefficient (Kow = 45.1) for Sethoxydim suggests that

Sethoxydim has a low tendency to bioaccumulate or bioconcentrate. The calculated BCF's for

Sethoxydim and total residues were 7X, 25X, and 21X for edible, nonedible, and whole fish,

respectively. Depuration was fast, with a half-life of 3.6 days. Based on the information

provided, bioaccumulation is not likely to occur.

Because of its low vapor pressure (1.6x10-7 mm Hg) and Henry's Law Constant (1.47x1011 atmm3/mol), Sethoxydim is not expected to be highly and the Agency is waiving the volatility

studies.

Degradation Scheme for Sethoxydim

Scheme I represents the relationship between parent Sethoxydim (M) and its eight identified

degradates. There are three groups of degradates: a group formed from the parent by oxidation of

the sulfur atom (M group), the “amines” or M1 group, and the tetrahydrobenzoxazoles, or M2

group. The first M1 group daughter (M1-S) is formed by cleavage of the ethoxyimino group to

an amino group, either by hydrolysis or aqueous photolysis. The first member of the M2 group

(M2-S) is then formed from M1-S by ring closure to produce a tetrahydrobenzoxazole ring.

Each of the three groups of degradates also has a sulfoxide (-SO) and a sulfone (-SO2), to round

out the total of eight degradates. This reaction dominates in aerobic soil metabolism and soil

photolysis experiments, especially to form the M-SO and M-SO2 degradates.

Scheme I

M 6 M1-S 6 M2-S

9 9 9

M-SO 6 M1-SO 6 M2-SO

9 9 9

M-SO2 6 M1-SO2 6 M2-SO2

The eight degradates include 2 daughter products, which are formed from the parent by a single

reaction (M1-S and M-SO), 3 granddaughter products that are two reactions removed from the

parent (M-SO2, M2-S, and M1-SO), 2 great-granddaughters that are three reactions removed
-36-

(M1-SO2 and M2-SO), and one great-great-granddaughter that is four reactions removed (M2-

SO2).

It is important to note that M1-SO, M2-SO, M1-SO2, and M2-SO2 may be formed by more than

one pathway, and that the sulfur oxidation may be reversible under reducing conditions, so that

the kinetics of the formation is very complicated, and probably cannot be elucidated with any

confidence from the current fate data.

Because parent Sethoxydim is very rapidly (half-life < 1 day) degraded to its sulfoxide (M-SO) in

soil, it is reasonable to conclude that some of the degradates retain some herbicidal activity. 

There are no data on the toxicity of the separate degradates. However, the Sethoxydim used in

the toxicity tests was subject to the same degradation mechanisms, so the actual exposure was to

a mixture of Sethoxydim and its degradation products. For this reason, we decided to use a “total

toxic residues” approach to modeling the decay. In other words, the calculated half-lives will

include the concentrations of the parent plus the major degradates.

For purposes of modeling the decay of total Sethoxydim residues, it is therefore most important

to include the two daughter products, M-SO and M1-S. The next group to be included in the

total residues would be the 3 granddaughters (M2-S, M1-SO, and M-SO2). We will neglect

degradates that are present in only minor amounts (generally < 10% of the applied radiation of

the parent), or that are kinetically far-removed from the parent, as long as the calculated half-life

is not greatly changed.

Table 6 and 7 below describes the environmental fate properties of Sethoxydim and it’s residues

and the mobility of Sethoxydim and transformation products. 

Table 6: Environmental Fate Properties of Sethoxydim and its Residues

Sethoxydim Residues

Hydrolysis pH 5.0 t1/2=8.7 days

pH 7.0 t1/2=155 days

pH 8.6 t1/2=284 days

stable at all pH's

Photodegradation in water t1/2=5.23 days t1/2=19.8 days

Photodegradation on soil t1/2=1 hour t1/2=20 hours

Aerobic soil metabolism t1/2<1 day t1/2=7-30 days

Anaerobic soil metabolism t1/2>60 days t1/2=78-91.6 days

Aerobic aquatic metabolism t1/2=0.7-1.0 days t1/2=32.9-38.1 days

Anaerobic aquatic metabolism t1/2=39.9 days t1/2=132-187 days

Terrestrial Field Dissipation N/A 32 days in two locations in CA

(cotton and forage)
-37-

Aquatic Field Dissipation N/A 1-9 days in four rice paddies in

various states.

Terrestrial Field Dissipation:

detection limit 0.01 ppm

application rate 0.5 lb ai/A x 3, during a 3 week period

two detections near the quantification level were found below the 0-6 inch soil level during the study

Table 7: Mobility of Sethoxydim and transformation products:

Kads ml/g

Parent Sethoxydim 0.03-0.94

M-SO 0.02-0.17

M-SO2 0.02-0.13

M2-SO 0.06-9.12

M2-SO2 sterile 0.12-8.43

nonsterile 0.14-9.03

2. Measures of Aquatic Exposure 

a. Aquatic Exposure Modeling

EFED uses GENeric Expected Environmental Concentration Program (GENEEC) as described

in the surface water section on page 21 to calculate EECs that are used for assessing acute and

chronic risks to aquatic organisms. Acute risk quotients are calculated using peak EEC values

for single and multiple applications, while chronic risk quotients are calculated using the 21-day

EECs for invertebrates and 56-day EECs for fish. 

The GENEEC program uses basic environmental fate data and pesticide label application

information to estimate EECs following treatment of a 10-hectare field. The model calculates

the concentration (i.e., EEC) of a pesticide in a 1-hectare, 2-meter deep pond, taking into account

the following factors: (1) adsorption to soil or sediment (2) soil incorporation (3) degradation in

soil before washoff to a water body and (4) degradation within the water body. The model also

accounts for direct deposition of spray drift into the water body (the GENEEC v. 2.0 model

determines spray drift percentage based on the other model inputs). GENEEC v. 2.0 was used to

estimate sethoxydim total residues concentrations in the standard pond. The environmental fate

parameters used in the model for this pesticide are: soil Kd = 0.03 ml/g (MRID 41475212),

solubility = 4700 ppm, aerobic soil metabolism half-life = 30 days (MRID 41475210), water

photolysis half-life = 19.8 days (MRID 41475208), aquatic metabolism half-life = 38.1 days

(MRID 42165604). 

The calculated peak GENEEC EECs for Sethoxydim total residues were 87 and 110 ug/l. The

maximum “peak” occurred when the modeling was based on a total of four applications per

season at 0.47 lb ai/A. 
-38-
-39-

Table 8 Input Parameters For GENEEC2 Runs for Sethoxydim Total Toxic Residues

Parameter Value Source

Maximum Application Rate 0.47 lb a.i./acre at 4x @ 14

days)

Use Closure Memorandum

Aerobic Soil Half-life 54 days (90th upper % ile of 2

values)

EFGWB Environmental Fate

Chapter, March 1997

MRID 41475210

Aerobic Aquatic Half-life 44 days (90th upper % ile of 2

values)

EFGWB Environmental Fate

Chapter, March 1997

MRID 42165604

Distribution Coefficient, Kd 0.03 (minimum value for

parent)

EFGWB Environmental Fate

Chapter, March 1997

MRID 41475212

Solubility 4700 ppm (parent at pH7) EFGWB Environmental Fate

Chapter, March 1997

Aqueous Photolysis Half-life 19.8 days EFGWB Environmental Fate

Chapter, March 1997

MRID 41475208

Application type Aerial, no incorporation label

Droplet Size (Spray Drift) Fine to Medium (13%) GENEEC default

Wet-in? No label

Distance to Pond Zero feet Default assumption

Table 9

RUN No. 1 FOR Sethoxydim ON four apps * INPUT VALUES * 

--------------------------------------------------------------------

RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE NO-SPRAY INCORP

ONE(MULT) INTERVAL Kd (PPM ) (%DRIFT) ZONE(FT) (IN)

--------------------------------------------------------------------

.470( 1.465) 4 14 .0 4700.0 AERL_B( 13.0) .0 .0

FIELD AND STANDARD POND HALF-LIFE VALUES (DAYS) 

--------------------------------------------------------------------

METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

(FIELD) RAIN/RUNOFF (POND) (POND-EFF) (POND) (POND) 

--------------------------------------------------------------------

54.00 2 N/A 19.80- 2455.20 44.00 43.23

GENERIC EECs (IN MICROGRAMS/LITER (PPB)) Version 2.0 Aug 1, 2001

--------------------------------------------------------------------

PEAK MAX 4 DAY MAX 21 DAY MAX 60 DAY MAX 90 DAY

GEEC AVG GEEC AVG GEEC AVG GEEC AVG GEEC

--------------------------------------------------------------------
-40-

87.07 86.00 79.90 67.94 60.37

Spray Drift Exposure from Formulated Sethoxydim and Sethoxydim.

Formulated Sethoxydim may reach water bodies directly as a result of spray drift from aerial or

ground applications. Ecotoxicity studies done with the TEP were provided to the Agency. 

Therefore, a spray drift EEC needs to take into account the TEP concentrations. 

Ecotoxicity studies done with TEP usually are measuring the active ingredient (ai) concentration. 

That measured active ingredient concentration is back-calculated to the toxicity measured effect

in terms of ppm product using the percentage active ingredient in the TEP. There are three

different percentages of the active ingredient sethoxydim in the ecotoxicity studies. There are

19.3% ai (trout and Daphnia), 18.0% ai (sheepshead minnow and mysid shrimp), and 20.4% ai

(oyster).

A single application of POAST® was modeled using 0.47 lb ai/A of sethoxydim. For the input

of application rate into the model, the 0.47 lb ai/A was divided by each of the different

sethoxydim ai percentage used in the studies to produce the amount of POAST® in the spray

drift that would go into the 6 foot farm pond. 

The AgDrift model was used to estimate concentrations of sethoxydim (a.i.) and POAST®in the

standard 6-foot deep farm pond. Tier 1 aquatic assessments were performed, assuming either

aerial or ground application.

The ground spray assessment assumed a high boom applicator, ASAE very fine to fine spray

droplet size, and 90th percentile exposure. The aerial spray assessment assumed ASAE very fine

to fine spray droplet size.

Tables 10A and 10B give the EECs in the pond for aerial and ground spraying of sethoxydim a.i.

and the TEP, respectively.

Table 10A. Aerial and Ground Spray Drift EEC of Sethoxydim a.i. Applied at 0.47 lb/acre

Application Method EEC in Standard Pond (ppb)

ground 1.6

aerial 6.4
-41-

Table 10B. Aerial and Ground Spray Drift EEC of TEP Applied at 0.47 lb ai/acre rate.

Application

Method

Percentage of active

ingredient in

POAST

Input of Application Rate

(0.47 lb ai / % ai in

POAST)

EEC of TEP in

Standard Pond (ppb)

ground 19.3 2.4 8.3

aerial 19.3 2.4 32.6

ground 18.0 2.6 9.0

aerial 18.0 2.6 35.3

ground 20.4 2.3 7.9

aerial 20.4 2.3 31.3

b. Aquatic Exposure Monitoring and Field Data

Ground and Surface Water Monitoring Data

The Pesticides in Ground Water Database report (EPA 734-12-92-001, Sept. 1992) states that

Sethoxydim was analyzed in 65 Missouri wells in 1986, and was not detected. Sethoxydim was

not detected (LOD of <0.2 ug/L) in 40 domestic wells in agricultural areas of SE Missouri. The

agency has high confidence in the sampling of the 40 domestic wells, which appear to be

representative of rural drinking water wells in the agricultural areas of SE Missouri. However,

EFED has low confidence in the samples from the public water supply wells and the five surface

water samples because of the high limits of detection (LOD’s of 2 and 5 ug/L, respectively). 

Since Sethoxydim appears to degrade rapidly in the environment and since the public drinking

water supply wells are significantly deeper than the domestic wells, the Agency would not expect

to find parent Sethoxydim residues in the public drinking water wells in this area. The lack of

detections supports the environmental fate laboratory studies which indicate that Sethoxydim

degrades rapidly in the terrestrial environment. 

Sethoxydim was detected in both surface and ground water in the Nomini Creek Watershed in

Westmoreland County, Virginia from 1986-1990 to assess the effectiveness of Best Management

Practices (BMP’s). Three detections of 2.1, 4.07, and 41.89 ug/L were reported in monitoring

wells. However, it is not known if the monitoring wells were representative of drinking water

sources in this area. There was also one detection of 0.87 ug/L in surface water. 

There were no monitoring data in STORET for Sethoxydim.

The USGS Pesticide National Synthesis Project (http://ca.water.usgs.gov/pnsp/ ) did not include
-42-

Sethoxydim (nor any of its degradates) in its list of pesticides for laboratory analysis of ground

water and surface water samples.
-43-

3. Measures of Terrestrial Exposure 

a. Terrestrial Exposure Modeling 

Hoerger and Kenaga (1973), as modified by Fletcher et al. (1994), empirically derived residue

concentrations on avian and mammalian dietary food items immediately following application of

any pesticide at 1 pound of active ingredient per acre (lb ai/A). Due to the lack of a measured

foliage residue half-life, EFED assumes 35-day half life for the residues on the food items. 

These EECs are given in Table 11. 

Table 11. Estimated Environmental Concentrations (EECs) on Avian and Mammalian Food Items for

a 1 lb ai/A Application.

Food Item EEC (ppm)

Predicted Maximum Residue

EEC (ppm)

Predicted Mean Residue

Short grass 240 85

Tall grass 110 36

Broad-leaved/forage plants, and small insects 135 45 

Fruits, pods, seeds, and large insects 15 7

Terrestrial exposure was modeled with the T-REX v.1.1 model, which automates HoergerKenaga nomogram exposure and RQ calculations. One, 2, 3, or 4 applications at 14-day

intervals were modeled at the maximum application rate of 0.47 lb ai/A, and 3 applications were

modeled at an application rate of 0.28 lb ai/A. The default assumption for the decay rate on

foliage (35-day half-life) was used. Tables 12, 13, and 14 give the predicted maximum residues

of Sethoxydim that are expected to occur on selected avian or mammalian dietary food items. 

For chronic risk, Sethoxydim transformation products are assumed to be as toxic as the parent. 

The environmental fate data indicates that the transformation products are more persistent; how

persistent is not known, therefore predicted maximum residues will be used in the chronic risk

assessment for birds and mammals.
-44-

Residues on Avian Food Items 

Table 12 Maximum Residues on terrestrial food items for wildlife Body Weight and Size 1

(EEC in mg/kg-bw)

Site (not inclusive)

No. of Apps. 

App.Rate 

(lbs ai/A)

Food Items Maximum

EEC

(ppm)

Mean

EEC 

(ppm)

Small

20 g

Medium

100 g

Large

1000 g

Citrus, tree nuts

4

0.47 Short grass 312 111 356 203 91

Tall grass 143 47 163 93 41

Broadleaf

plants/Insects

176 59 200 114 51

Seeds 20 9 22 13 6

Alfalfa, birdfoot trefoil, sainfoin,

clover, cotton, 

3

0.47 Short grass 263 93 300 171 76

Tall grass 121 40 137 78 35

Broadleaf

plants/Insects

148 49 169 96 43

Seeds 16 8 19 11 5

Soybeans, grape, berries, peanut,

Head & 

petiole vegetable crop subgroup,

mint, pea, potato, safflower, sugar

beets, Christmas

tree farms, ornamentals, rights of

way,

roadsides, turf

2

0.47 Short grass 198 70 80 46 20

Tall grass 91 30 34 19 9

Broadleaf

plants/Insects 112

37 42 24 11

Seeds 12 6 7 4 2

Fruiting and leafy vegetable crop

group

3

0.28 Short grass 156 56

Tall grass 72 24

Broadleaf

plants/Insects

88 30

Seeds 10 5

Orchard floor middles,

strawberry, sunflower

1

0.47 Short grass 113 40 46 26 12

Tall grass 52 17 19 11 5

Broadleaf

plants/Insects

63 21 24 14 6

Seeds 7 3 4 2 1

1 20-gram body weight = 114% of body weight consumed; 100-gram body weight = 65% of

body weight consumed; 1000-gram body weight = 29% of body weight consumed.
-45-

Residues on Mammalian Food Items

Environmental Concentrations (EECs) on Mammalian Food Items for Sethoxydim Application

at the Maximum Use Rate and Frequency (T-REX v.1.1 ) using Kenaga maximum residues

(EEC equivalent dose (mg/kg-bw)).

Table 13

EEC equivalent dose 

(mg/kg-bw)

4X @ 0.47 lb ai/A; 

14 day interval

Mammalian Classes and Body

weight

Herbivores/ insectivores

15 g 35 1000 g

Short Grass 297 206 47

Tall Grass 136 94 21

Broadleaf plants/sm Insects 167 116 26

Fruits/pods/seeds/lg insects 19 13 3

Granivores

Fruits/pods/seeds/lg insects 4 3 1

Table 14

EEC equivalent dose 

(mg/kg-bw)

3X @ 0.47 lb ai/A; 

14 day interval

Mammalian Classes and Body

weight

Herbivores/ insectivores

15 g 35 1000 g

Short Grass 250 174 39

Tall Grass 115 80 18

Broadleaf plants/sm Insects 141 98 22

Fruits/pods/seeds/lg insects 16 11 2

Granivores

Fruits/pods/seeds/lg insects 3 2 0

Terrestrial Plant EEC

Spray Drift

EFED estimates risk quotients to non-target plants from runoff with seedling emergence (Tier II)

data. The TerrPlant model is used for runoff. The AgDrift model will be used to determine the

terrestrial non-target plant EEC from spray drift.

A single application of Sethoxydim was modeled using 0.47 lb ai/A of sethoxydim since there

are no phytotoxicity data using TEP. The AgDrift model was used to estimate concentrations of

sethoxydim (a.i.) residues on non-target plants over an area of one acre. Tier 1 terrestrial

assessments were performed, assuming either aerial or ground application. The ground spray

assessment assumed a low boom applicator, ASAE medium to coarse droplet size, and 90th
-46-

percentile exposure. The aerial spray assessment assumed ASAE medium to coarse droplet size.

The model runs are located in Appendix O.

The summary of EEC results from the AgDrift model are below:

Table 15a

No. of application

(0.47 lb ai/A)

distance from edge

of field

% of application rate EEC (lb

ai/A)

method

1 0 feet 50 0.235 Aerial

1 50 feet 11.1 0.052 Aerial

1 100 feet 5.6 0.026 Aerial

1 200 feet 2.5 0.012 Aerial

1 300 feet 1.5 0.007 Aerial

1 10 feet 2.8 0.013 Ground

1 50 feet 0.7 0.004 Ground

Runoff

The standard runoff assumption in the EEC are 5% since solubility is greater than 100 ppm

(Appendix P). The LOC for non-target plants is 1.

Below is Table 15b which shows the EEC for runoff to non-targeted plants as per the TerrPlant

model. Details for the TerrPlant model run (see Appendix P) are as follows:

- Application rate are 0.47 and 0.28 lb ai/A

- Runoff factor from solubility is 5%

- Toxicity endpoints used are mentioned above. 

- The aerial spray drift uses an assumption of 1% for ground spray and 5% for aerial

spray.
-47-

Table 15b Non-listed Species

Method Application Rate Total Loading to Adjacent

Areas

Total Loading to Semi-Aquatic

Areas

ground 0.47 lb ai/A 0.0282 0.2397

aerial 0.47 lb ai/A 0.0376 0.1645

ground 0.28 lb ai/A 0.0282 0.2397

aerial 0.28 lb ai/A 0.0376 0.1645

Listed Species

ground 0.47 lb ai/A 0.0282 0.2397

aerial 0.47 lb ai/A 0.0376 0.1645

ground 0.28 lb ai/A 0.0282 0.2397

aerial 0.28 lb ai/A 0.0376 0.1645

C. Measures of Ecological Effects

In screening-level ecological risk assessments, the Measures of Ecological Effects Section

describes the types of effects a pesticide can produce in an organism or plant. This

characterization is based on registrant-submitted studies that describe acute and chronic toxicity

information for various aquatic and terrestrial animals and plants. In addition, other sources of

information, including reviews of the open literature and the Ecological Incident Information

System (EIIS), are conducted to further refine the characterization of potential ecological effects. 

Appendix E summarizes the results of the registrant-submitted toxicity studies used to

characterize effects for this risk assessment. Toxicity testing reported in this section does not

represent all species of birds, mammals, or aquatic organisms. Only a few surrogate species for

both freshwater fish and birds are used to represent all freshwater fish (2000+) and bird (680+)

species in the United States. Mammalian acute studies are usually limited to Norway Rat or the

house mouse. Estuarine/marine testing is usually limited to a crustacean, a mollusk, and a fish. 

Also, neither reptiles nor amphibians are tested. The risk assessment assumes that avian and

reptilian toxicities are similar. The same assumption is used for fish and amphibians.

1. Aquatic Effects

a. Aquatic Animals

(1). Acute Effects
-48-

Freshwater

Sethoxydim technical is practically non-toxic to freshwater fish (LC50 = 170 ppm ai for rainbow

trout) on an acute basis. Sethoxydim technical is slightly toxic to freshwater invertebrate (LC50

= 78.1 ppm ai for Daphnia magna) on an acute basis. 

The formulated product is moderately toxic to freshwater fish (LC50 = 6.2 ppm product for

rainbow trout) and to freshwater invertebrates (LC50 = 13.5 ppm product for Daphnia magna) on

an acute basis. The dose response slope for the formulated product of Sethoxydim tested on

Daphnia magna is 4.73723. 

Naphthalene as part of the formulation of the TEP for Sethoxydim (POAST®) will be considered

in this risk assessment as a surrogate for solvent containing Naphthalene. Naphthalene is

moderately toxic to freshwater fish (rainbow trout LC50 = 2.0 ppm) and freshwater invertebrates

(LC50 = 1.6 ppm for Daphnia magna) on an acute basis.

Estuarine

Sethoxydim technical is practically non-toxic to estuarine fish (LC50 > 145.8 ppm a.i. for

sheepshead minnow) and estuarine invertebrate (LC50 > 141.8 ppm a.i. for eastern oyster and

141,8 ppm for mysid shrimp) on an acute basis. 

The formulated product is moderately toxic to estuarine fish (LC50 = 19.4 ppm product for

sheepshead minnow) and estuarine invertebrates (LC50 = 4.4 ppm product for mysid shrimp and

4.4 ppm product for eastern oyster (embryo-larvae)) on an acute basis. The dose response slope

for the formulated product of Sethoxydim tested on eastern oyster is 4.83123. 

(2). Chronic Effects

Currently there are no chronic toxicity data available to evaluate the chronic toxic effects of

Sethoxydim technical on freshwater fish or invertebrates. There is a high value in submitting the

data in order to determine chronic toxicity to freshwater organisms. 

The supplemental chronic toxicity study on the formulated product indicated that no significant

effect on marine/estuarine fish occurred in the study (NOAEC = 98 ppm). The formulated

product chronic toxicity study on a marine/estuarine invertebrate (mysid shrimp) indicated

NOAEC of 6.5 ppm a.i. and a LOAEC = 13 ppm a.i. for effects on length and survival. 

(3). Field Studies

Currently, there are no field studies available for Sethoxydim.

b. Aquatic Plants

Sethoxydim technical is of low toxicity to a vascular dicot, duckweed (Lemna gibba), with an

EC50 > 0.281 ppm ai. However, this herbicide is considered to be toxic to grass species, which
-49-

are monocots. There are no available phytotoxicity data on aquatic grasses. It is very uncertain

what the toxicity of Sethoxydim is to aquatic grasses without data. 

2. Terrestrial Effects

a. Terrestrial Animals

(1). Acute Effects

Sethoxydim technical is practically non-toxic to birds on a subacute dietary basis (LC50 >5620

ppm a.i.) and practically nontoxic on an acute oral basis (LD50 > 2510 mg ai/kg-bw in mallard

duck). No mortalities were observed at the highest dose. Currently, there are no avian acute

toxicity data on the formulated product of Sethoxydim. 

Sethoxydim technical is practically nontoxic to rats on an acute oral basis (LD50 = 2676 mg/kgbw for females, 3125 mg/kg-bw for males). The available mammalian acute toxicity data on the

formulated product indicate that formulated Sethoxydim is practically non-toxic to mammals on

an acute basis (LD50 = 4385 mg/kg-bw for females, 5000 mg/kg-bw for males).

(2). Chronic Effects

Avian reproductive studies on bobwhite quail showed no effects at concentrations up to 1000

ppm a.i. from Sethoxydim technical, but significant effects were noted to the mallard duck for

the number of normal hatchlings at the 100 and 500 ppm levels. The NOAEC was not

determined in this study. The LOAEC is 100 ppm a.i.. No avian reproductive study was done

with the formulated product of Sethoxydim. There is an uncertainty in the mallard duck study

since the NOAEC was not determined. Data from another mallard reproductive study using the

technical Sethoxydim would be very useful in lowering the uncertainties that exist for chronic

risk to birds.

Mammalian reproductive study (2-generation rat reproduction) showed a reproductive NOAEC

of 3000 ppm (150 mg/kg-bw/day) and a developmental NOAEC of 600 ppm (30 mg/kg-bw/day). 

The rat 2-generation reproduction study (1983, MRID 41510606, 43366401) demonstrated both

a systemic NOAEC and LOAEC of $ 150 mg/kg/day. There were no adverse effects on the

reproductive performance in either sex. In addition, the study demonstrated a developmental

toxicity NOAEC and LOAEC of 30 and 150 mg/kg/day, respectively, for the offspring based on

tail abnormalities seen in F1a and F1b offspring. Malformations were only observed in one F1b

and two F2b pups. The malformations were described as thread-like tail, no anal opening,

malformed hindlimb, malpositioned kidneys, cleft lip, cleft palate and microphthalmia. There

were no dose-related gross or microscopic lesions, or developmental variations (cannibalism

complicated the evaluation).

(3). Field Studies
-50-

No terrestrial field studies were found using Sethoxydim.

b. Terrestrial Plants

Sethoxydim is a herbicide that is part of the Cyclohexanediones class of chemicals. The mode of

action for this herbicide is Lipid Biosynthesis Inhibition. Selectivity is shown to be due to

susceptibility at acetyl-coenzyme A carboxylase (ACCase). Susceptible grass species are killed

by inhibiting the ACCase which is a key enzyme in the lipid biosynthetic pathway. This

herbicide is not expected to adversely affect dicot plant families or monocot non-grass families

of plants. A submitted vegetative vigor toxicity study confirms this expectation. None of the

dicots showed any significant adverse effects from Sethoxydim up to 0.5 lb ai/a. Also, the onion

(a non- grass monocot) did not show any adverse effects from Sethoxydim exposure.

The most sensitive grass in the vegetative vigor study was the ryegrass with an EC25 of 0.029 lb

ai/A (C.I. = 0.023 - 0.037), EC50 = 0.038 lb ai/A (C.I. =0.033-0.044) and a NOAEC of 0.025 lb

ai/A. The slope of the dose response curve is 5.75. This study is classified as supplemental since

it was tested with the technical grade Sethoxydim and not the typical end product (TEP). It is

important that terrestrial plants be tested with the TEP which also contains adjuvants that help

the active ingredient to penetrate the leaf surface.

A seedling emergence study was submitted. The seedling emergence study on three grass

species (ryegrass, corn and oat) using the TEP found that the most sensitive seedling emergence

EC25 = 0.078 lb ai/A and the NOAEC = 0.0587 lb ai/A for ryegrass. The slope is 2.21. The

study also reveals that the percentage of emergence was significantly reduced at 0.1175 lb. The

EC25 value is used to assess risk to non-target non-listed plant species. The NOAEC value will

be the toxicity endpoint used to assess risk to listed species.

3. ECOTOX Database

No toxicity endpoints or literature sources were used from the ECOTOX database. The reasons

for not using the endpoints or the literatures are found in Appendix B and Appendix C. Of the

excluded data from the ECOTOX database, non were found to be pertinent to the risk

assessment.
-51-

IV. Risk Characterization

A. Risk Estimation - Integration of Exposure and Effects Data

Results of the exposure and toxicity effects data are used to evaluate the likelihood of adverse

ecological effects on non-target species. For the assessment of Sethoxydim risks, the risk

quotient (RQ) method is used to compare exposure and measured toxicity values. Estimated

environmental concentrations (EECs) are divided by acute and chronic toxicity values. The

resulting RQs are compared to the Agency’s levels of concern (LOCs). These LOCs are the

Agency’s interpretive policy and are used to analyze potential risk to non-target organisms and

assess the need to consider regulatory action. These criteria are used to indicate when a

pesticide’s directed label use has the potential to cause adverse effects on non-target organisms. 

In section 2.E.1.c. of this document, the Levels of Concern are described and presented in tabular

form.

1. Non-target Aquatic Animals

Runoff 

The most sensitive aquatic fish species (freshwater and estuarine) for acute toxicity is rainbow

trout with an LC50 = 170 ppm ai for the technical grade Sethoxydim. The maximum EEC (0.47

lb ai/A applied 4X) generated by the GENEEC aquatic exposure model is 87 ppb for technical

sethoxydim. The RQ = 0.0005 for the technical Sethoxydim (87/170,000). The technical

Sethoxydim RQ are below all LOCs including listed species from runoff exposure. 

The most sensitive aquatic invertebrates species (freshwater and estuarine) for acute toxicity is

the daphnid (EC50 = 78.1 ppm ai) for the technical Sethoxydim. The maximum EEC (0.47 lb

ai/A applied 4X) generated by the GENEEC aquatic exposure model is 87 ppb for Sethoxydim

technical. The RQ = 0.001 for the technical Sethoxydim (87/78,100). The technical Sethoxydim

RQ is below all LOCs including listed species. 

The Sethoxydim chronic LOC for estuarine fish and invertebrates has not been exceeded for

listed species (NOAEC = 6.5 ppm a.i. for mysid shrimp and 98 ppm a.i. for sheepshead minnow)

since the EEC (0.087 ppm) is less than the toxicity value.

Spray Drift

The maximum Sethoxydim TEP EEC modeled from AgDrift is 35.3 ppb of TEP in 6 feet farm

pond model. The TEP LC50 for freshwater fish is 6.2 ppm product and the EC50 for freshwater

invertebrates (Daphnia magna) is 13.5 ppm product. The RQs are 0.006 for freshwater fish and

0.003 for freshwater invertebrates, respectively. These are below all LOCs.

The estuarine fish LC50 is 19.4 ppm product and the estuarine invertebrate EC50 is 4.4 ppm

product. The RQs are 0.002 for the estuarine fish and 0.008 for the estuarine invertebrates,
-52-

respectively. These are below all LOCs.

The most sensitive aquatic plant’s EC50 is 250 ppb ai which is well below all LOCs.

2. Non-target Terrestrial Animal

There are no avian acute levels of concern since the data provided indicate that sethoxydim is

practically non-toxic to birds and no mortalities were observed at the highest dose tested.

All of the uses of Sethoxydim potentially exceeds the chronic level of concern since the avian

chronic NOAEL was not determined. The lowest dose tested was used as the toxic measure of

effect until additional avian reproduction studies provide additional data. 

Below are the Chronic RQ tables for birds exposed to Sethoxydim at different applications.

Table 16 Avian Organism Chronic Risk Quotient (Chronic Level of Concern = 1) with NOAEC <100ppm

Site (not inclusive)

No. of Apps. 

App.Rate 

(lbs ai/A) Food Items

Avian Chronic Risk Quotient

based on Max. EEC and

NOAEC < 100 ppm

Avian Chronic Risk Quotient

based on Mean EEC and

NOAEC < 100 ppm

Citrus, tree nuts

4

0.47

Short grass >3.12 >1.11

Tall grass >1.43 >0.47

Broadleaf

plants/Insect

>1.76 >0.59

Seeds >0.20 >0.09

Alfalfa, birdfoot trefoil, sainfoin, clover,

cotton, 

3

0.47

Short grass >2.63 >0.93

Tall grass >1.21 >0.39

Broadleaf

plants/Insect

>1.48 >0.49

Seeds >0.16 >0.08

Fruiting and leafy vegetable crop group

3

0.28

Short grass >1.57 >0.56

Tall grass >0.72 >0.24

Broadleaf

plants/Insect

>0.88 >0.29

Seeds >0.10 >0.05

Soybeans, grape, berries, peanut, Head

& petiole vegetable crop subgroup,

mint, pea, potato, safflower, sugar beets,

Christmas tree farms, ornamentals,

rights of way, roadsides, turf

2

0.47

Short grass >1.98 >0.70

Tall grass >0.91 >0.30

Broadleaf

plants/Insect

>1.12 >0.37

Seeds >0.12 >0.06
Table 16 Avian Organism Chronic Risk Quotient (Chronic Level of Concern = 1) with NOAEC <100ppm

Site (not inclusive)

No. of Apps. 

App.Rate 

(lbs ai/A) Food Items

Avian Chronic Risk Quotient

based on Max. EEC and

NOAEC < 100 ppm

Avian Chronic Risk Quotient

based on Mean EEC and

NOAEC < 100 ppm

-53-

Orchard floor middles, strawberry, sunflower

1

0.47

Short grass >1.13 >0.40

Tall grass >0.52 >0.17

Broadleaf

plants/Insect

>0.63 >0.21

Seeds >0.07 >0.03

There are no avian or mammalian acute LOC exceedances from the use of Sethoxydim. Avian

and acute toxicity studies show Sethoxydim to be practically non-toxic with no mortalities

observed at the highest dose concentrations. The mammalian RQ < 0.1 (LOC for listed species)

at the highest potential labeled exposure.

Since no definitive chronic avian NOAEC value was established in the available chronic avian

toxicity studies, all chronic avian RQ values are “greater than” values and therefore all

potentially exceed the chronic LOC of 1.0. The chronic avian RQ values are described below.

When Sethoxydim is applied 4 or 3 times a season at the maximum application rate of 0.47 lb

ai/A with 14-day intervals, the chronic LOC is exceeded for all birds feeding on short grass, tall

grass, and broadleaf plants/insects, and the chronic LOC is potentially exceeded (RQ>0.2 based

on maximum EEC) for birds feeding only on seeds.

When Sethoxydim is applied 3 times a season at a maximum application rate of 0.28 lb ai/A 

with 14-day intervals, the chronic LOC is exceeded for birds that feed exclusively on short grass,

and is potentially exceeded for birds feeding on tall grass, broadleaf plants/insects, and seeds.

When sethoxydim is applied 2 times a season at the maximum application rate of 0.47 lbs a.i./A

with 14-day intervals, the chronic LOC is exceeded for birds feeding on short grass and broadleaf

plants/insects, and the chronic LOC is potentially exceeded for birds feeding on tall grass and

seeds.

When sethoxydim is applied once at the maximum application rate of 0.47 lbs a.i./A, the chronic

LOC is exceeded for birds feeding on short grass, and potentially exceeded for birds feeding on

tall grass, broadleaf plants/insects, and seeds.

There are no chronic LOC exceedances for mammals because the chronic RQ is <1 for mammals

at the highest exposure (297 ppm/600 ppm = 0.5 RQ). 
-54-

3. Non-target Plants

Terrestrial

The vegetative vigor (Tier II) EC25 = 0.029 lb ai/A. The NOAEC value will be the toxicity

endpoint used to assess risk to listed species and it is 0.025 lb ai/A for ryegrass. The RQs may

be underestimated because the phytotoxicity endpoint was determined using the technical grade

Sethoxydim instead of the TEP. 

A seedling emergence study was submitted. The seedling emergence study on three grass

species (ryegrass, corn and oat) using the TEP found that the most sensitive seedling emergence

EC25 = 0.078 lb ai/A and the NOAEC = 0.0587 lb ai/A for ryegrass. The slope is 2.21. The

study also reveals that the percentage of emergence was significantly reduced at 0.1175 lb ai/A.

The seedling emergence EC25 is available to use in this risk assessment for non-target plants

exposed to sethoxydim from runoff. The EC25 value is used to assess risk to non-target nonlisted plant species. The NOAEC value will be the toxicity endpoint used to assess risk to listed

species.

Spray Drift

A single application of Sethoxydim was modeled using 0.47 lb ai/A of sethoxydim since there

are no phytotoxicity data using TEP. The AgDrift model was used to estimate concentrations of

sethoxydim (a.i.) residues on non-target plants over an area of one acre. AgDrift Tier 1 terrestrial

assessments were performed, assuming either aerial or ground application. The ground spray

assessment assumed a high boom applicator, ASAE medium to coarse spray droplet size, and 90th

percentile exposure. The aerial spray assessment assumed ASAE medium to coarse spray droplet

size. The toxicity measurement of effect (EC25) is 0.029 lb ai/A for non-listed species and 0.025

lb ai/A (NOAEC) for listed species. The model runs are located in Appendix O.

The RQ results based on the AgDrift model are below:

Table 17a Listed and Non-listed Plant Species

No. of application

(0.47 lb ai/A)

distance from edge

of field

% of application

rate

EEC (lb ai/A) Non-listed

RQ

Listed

RQ

method

1 0 feet 50 0.235 8.1 9.4 Aerial

1 50 feet 11.1 0.052 1.8 2.1 Aerial

1 100 feet 5.6 0.026 0.9 1.0 Aerial

1 200 feet 2.5 0.012 0.4 0.5 Aerial

1 300 feet 1.5 0.007 0.2 0.3 Aerial

1 10 feet 2.8 0.013 0.4 0.5 Ground

1 50 feet 0.7 0.004 0.1 0.2 Ground

Non-listed and listed plant species LOCs (RQ = 1) are exceeded from edge of field up to 50 feet

for nonlisted species and up to 100 feet for listed species with aerial application. There are no
-55-

LOC exceedances from ground applied spray drift.

Runoff

Below is Table 17b which shows the EEC and Risk Quotients for runoff to non-targeted plants. 

The inputs for the TerrPlant model (Appendix P). The model uses spray drift assumption of 1%

for ground spray and 5% for aerial spray.

Table 17b Non-listed Species

Method Application

Rate

Total Loading to

Adjacent Areas

Total Loading to

Semi-Aquatic Areas

RQ for

Adjacent Areas

RQ for Semiaquatic AreasSpray

Drift RQ 1

ground 0.47 lb ai/A 0.0282 0.2397 <1 3.1 <1

aerial 0.47 lb ai/A 0.0376 0.1645 <1 2.1 <1

ground 0.28 lb ai/A 0.0282 0.2397 <1 1.8 <1

aerial 0.28 lb ai/A 0.0376 0.1645 <1 1.3 <1

Listed Species

ground 0.47 lb ai/A 0.0282 0.2397 <1 4.1 <1

aerial 0.47 lb ai/A 0.0376 0.1645 <1 2.8 <1

ground 0.28 lb ai/A 0.0282 0.2397 <1 2.4 <1

aerial 0.28 lb ai/A 0.0376 0.1645 <1 1.7 <1

1 Spray drift assumptions of 1% for ground application and 5% for aerial application were used.

Non-listed and listed plant species LOCs (RQ = 1) are exceeded for runoff to low semi-aquatic

areas only with RQs ranging from 1.3 to 4.1.

Aquatic Plants

Sethoxydim technical is of low toxicity to a vascular dicot, duckweed (Lemna gibba) where EC50

> 0.281 ppm ai (highest dose tested). The maximum EEC (based on 4 applications of 0.47 lb

ai/A) generated from the GENEEC model is 111 ppb. The aquatic plant LOC (including listed

species) is not exceeded. However, this herbicide is considered to be toxic to grass species, a

monocot. There are no available phytotoxic data on aquatic grasses (obligate aquatic Poaceae

family). It is very uncertain what the toxicity of Sethoxydim is to aquatic grasses without data. 

B. Risk Description

1. Risks to Aquatic Animals and Plants

Summary of Major Conclusions
-56-

• There is no exceeded acute risk LOC for aquatic animals from the Sethoxydim per se.

Sethoxydim itself (the technical grade active ingredient) is not acutely toxic to aquatic

animals at the expected exposures. There is a possibility of acute risk to aquatic animals

from exposure to the petroleum solvent (CAS Reg. No. 64742-94-5) used in POAST®,

which contains Naphthalene.

• No chronic risk is expected for aquatic estuarine animals from runoff to the standard farm

pond. 

• The risk to aquatic plants is limited to aquatic grasses, due to the mode of action of

Sethoxydim. The risk cannot be quantified, because there are no toxicity testing data for

aquatic grasses. These data are being requested.

• The LOC is not exceeded for algae or diatoms.

Discussion

Aquatic animals.

Toxicity. Technical sethoxydim is not toxic to aquatic animals at estimated concentrations. 

However, the TEP is more toxic than the technical at two orders of magnitude. The greater

toxicity of the formulated product to aquatic organisms may be attributed to the petroleum

solvent (CAS Registry No. 64742-94-5). This solvent is known to contain seven percent of

Naphthalene by weight. It is likely that the other constituents of the petroleum solvent have

toxicity similar to that of Naphthalene, especially if the toxic effect is simple narcosis. 

A comparison of Subdivision N Guideline acute toxicity data for Naphthalene (PC code 055801)

to the data for formulated (18.0 - 20.4%) and technical Sethoxydim shows that the median lethal

concentrations for Naphthalene and formulated sethoxydim are similar for the trout, bluegill, and

waterflea (see table 4). Also, the toxicity of formulated Sethoxydim to estuarine organisms is in

the same low ppm range as the toxicity of Naphthalene to coho salmon.

Formulated product (POAST®) was tested on aquatic freshwater animals (see Appendix E). See

below showing the LC50/EC50 (in ppb) comparison among the Sethoxydim, POAST®, and

Naphthalene (MRID 45030801, 44302702, 44302701, ACC249909):
-57-

Table 18

organism Sethoxydim POAST® 1 Naphthalene

Rainbow trout 170,000 6,200 2,000

bluegill 265,000 8,300 3,200

Daphnia magna 78,100 13,500 1,600

Coho salmon na na 2,100

Sheepshead minnow 145,800 19,400 na

Eastern oyster >109,000 4,400 na

Mysid shrimp 141,800 4,400 na

1) POAST® values are in ppb product.

na = not available

Toxicity of Sethoxydim is under estimated and thereby the risk is also underestimated if the

petroleum solvents that also contain Naphthalene are not taken into consideration. 

Runoff Uncertainty for Petroleum Solvent. There is much uncertainty as to the amounts of the

petroleum solvent to which aquatic organisms are exposed, since the exposure modeling was

done only on the Sethoxydim technical and not on the Naphthalene component nor on POAST®. 

It is difficult to model POAST® without chemical, physical and fate data on POAST® as a

whole. The fate properties of Naphthalene are very different from the properties of Sethoxydim. 

These differences in properties will cause the two chemicals to separate from each other during a

runoff scenario, thereby creating different load or exposure time lines to aquatic organisms. 

Below is a table of some of the Fate properties of Naphthalene: 

Table 19: Fate Data on Naphthalene

organic carbon partitioning

coefficient (Koc)

1837 mL/g EPISuite estimate

Soil metabolism half-life 48 days Howard et al., 1991

Water metabolism Half-life 20 days Howard et al., 1991

Aqueous Photolysis half-life 71 days Howard et al., 1991

Volatility of Petroleum Solvent. According to Merck Index (tenth edition, paragraph 6220),

Naphthalene has moderate volatility which may cause it to evaporate from where it is sprayed,

and be transported in the atmosphere. Spray drift exposure could be augmented by later

volatilization and transport of petroleum solvent vapors to ponds or wetlands. The exposure of

the TEP to non-target plants and water bodies could be underestimated.

Potential for Risk from Spray Drift of TEP to Wetlands. Aerial spraying of the TEP in a 6-inch

deep wetland scenario may pose a potential risk to aquatic animal species including listed
-58-

species. Chemical concentrations in a 6-inch scenario are usually about 13X higher than in a 6

foot farm pond scenario. Aerial spray drift can transport the TEP to nearby wetlands that contain

6 inch deep shallow pools. Many aquatic fish and invertebrate species spend part of their

reproductive life cycle in these shallow water bodies. The timing of application of POAST®

may intersect with the time period that the aquatic species inhabit these water bodies. Since the

TEP is much more toxic than the technical sethoxydim, potential risk cannot be discounted from

these species. 

Chronic Risk. Estuarine chronic risk quotients were calculated for the estuarine fish and

invertebrates since chronic toxicity data are available. There are no freshwater fish or

invertebrate chronic toxicity data available. 

The chronic toxicity endpoint (NOAEC) that was used to calculate RQ’s was derived from

testing of a formulation containing 40% Sethoxydim. Because of the lack of acute toxicity of

Sethoxydim to aquatic organisms, we are presuming that the chronic toxicity of technical

Sethoxydim would be less sensitive than that of a TEP containing petroleum solvent. 

Chronic toxicity data are still needed to assess chronic risk to freshwater fish and invertebrates

using technical Sethoxydim and the TEP.

Aquatic Plants

Sethoxydim technical is of low toxicity to a vascular aquatic dicot, duckweed (Lemna gibba)

where EC50 > 281 ppb ai. An assumption is made that only aquatic grasses (obligate aquatic

Poaceae family) are potentially at risk, since available data indicate that Sethoxydim is only toxic

to terrestrial grass species. In addition, the herbicidal mode of action of sethoxydim is Lipid

Biosynthesis Inhibition. Selectivity is shown to be due to the greater susceptibility at acetylcoenzyme A carboxylase (ACCase) of grassy species. Susceptible grassy species are killed by

inhibition of the ACCase which is a key enzyme in the lipid biosynthetic pathway. The risk

cannot be quantified because we do not have toxicity data for aquatic grass. 

However, there is some uncertainty in this assumption. The exposure route for an aquatic grass

would likely be different that for a terrestrial grass (foliar rather than by foliar and root uptake). 

Another uncertainty is the differences in aquatic plant physiology as compared to terrestrial

plants such as many terrestrial plants tend to need oxygen for their root zones whereas the

obligate aquatic plant does not have much oxygen in their root zones thereby rely on other means

to get the oxygen down to the roots.

Unicellular aquatic plants (algae) appear to have no adverse effect from the exposure of

Sethoxydim.

2. Risks to Terrestrial Organisms
-59-

Summary of Major Conclusions

! Chronic LOCs are exceeded for birds.

! LOCs are exceeded for listed and non-listed grass species (Poaceae family) from spray

drift from aerial and spray-drift from irrigated applications. There are no LOC

exceedances for non-target listed grass species from ground applied spray drift.

! No avian or mammalian acute LOC are exceeded.

! No chronic LOCs are exceeded for mammals.

! There appear to be no LOC exceedances for non-target plants that are not members of the 

Poaceae family.

! LOCs are exceeded for terrestrial non-target grasses from runoff to low semi-aquatic

areas.

Discussion

Terrestrial animals

Acute risk to birds and mammals. No acute RQ calculations were made for birds because avian

acute and subacute dietary studies suggest that Sethoxydim technical is practically non-toxic to

birds. At the highest dose, no mortalities were observed. Thus, no listed species concern was

triggered.

Rat acute oral toxicity studies suggest that Sethoxydim technical and POAST® are practically

non-toxic to mammals. Since there were some mortalities observed in the studies, RQ

calculations were done at the maximum exposure to mammals and no LOC was exceeded. No

listed species LOC was triggered.

Chronic risk to birds and mammals. Avian reproduction studies were conducted with the

technical active ingredient of Sethoxydim. In these studies, the bobwhite quail showed no

treatment related effects up to 1000 ppm of Sethoxydim exposure. The mallard duck showed the

number of normal hatchlings was significantly less than the control at the lowest dose tested,

which was 100 ppm. Thereby, the NOAEC was not determined. There is uncertainty in this

study as to what the no adverse observed concentration thresholds really are. Another mallard

reproduction study is needed to lower the amount of uncertainty for chronic reproductive risk to

avian populations. 

All of the chronic avian RQs are “greater than” values because the NOAEC used to calculate the

RQs was a “less than” value. Therefore, all chronic avian RQ values potentially exceed the
-60-

chronic LOC. 

The rat 2-generation reproduction study indicates no adverse effects in the study at 3000 ppm

(150 mg/kg/day). However, the offspring/developmental toxicity NOAEC was 600 ppm (30

mg/kg/day) and the LOAEC was 150 mg/kg/day for the offspring based on tail abnormalities

seen in F1a and F1b offspring. The 600 ppm NOAEC measure of effect is determined on

standard FDA lab rat conversion of 20 X 30 mg/kg-diet = ppm-diet. Malformations were only

observed in one F1b and two F2b pups. The malformations were described as thread-like tail, no

anal opening, malformed hindlimb, malpositioned kidneys, cleft lip, cleft palate and

microphthalmia. There were no dose-related gross or microscopic lesions, or developmental

variations (cannibalism complicated the evaluation). There are no chronic LOC exceedances for

mammals because the most sensitive chronic endpoint (600 ppm) is much greater than the

highest exposure concentration (297 ppm). 

Terrestrial Plants

Risk to Non-Target Grass. Phytotoxicity data were submitted for vegetative vigor study on 10

species of crops. All of the dicot species showed no effects at the highest concentration. Onion,

a monocot, also showed no effects at the highest rate tested. Onion is a monocot specie that is

not a member of the grass family. The herbicidal mode of action for Sethoxydim is lipid

biosynthesis inhibition. Selectivity is shown to be due to susceptibility at acetyl-coenzyme A

carboxylase (ACCase). Susceptible grass species are killed by inhibiting the ACCase which is a

key enzyme in the lipid biosynthetic pathway. This herbicide is not expected to adversely affect

dicot plant families or monocot non-grass families of plants. A submitted vegetative vigor

toxicity study confirms this expectation. Therefore, it appears that only non-target grass species

are expected to be adversely impacted from off-site movement of Sethoxydim.

Vegetative vigor tests. Results from vegetative vigor studies are used to assess risk to non-target

plants from spray drift. A vegetative vigor study (MRID 41885906) with EC25 = 0.029 lb ai/A

and NOAEC = 0.038 lb ai/A showed the ryegrass to be the most sensitive species tested (four

monocots of which three are grasses and six dicots). This study was conducted with Sethoxydim

technical instead of the required POAST®. This creates uncertainty in the estimates of risk since

it is generally recognized that formulated herbicides are more potent toward susceptible plant

species than the technical active ingredients. Therefore, the risk to non-target plants is

underestimated. Non-listed and listed plant species LOCs (RQ = 1) are exceeded from edge of

field up to 50 feet for nonlisted species and up to100 feet for listed species with aerial

application. There are no non-target plant LOC exceedances from ground applied spray drift.

The estimated risk to non-target plants is underestimated due to lack of valid vegetative vigor

studies conducted with the TEP. Using the TEP in terrestrial plant studies is important because

most herbicides use adjuvants to penetrate the plant cuticle and other plant defenses in order to

get the active ingredient into the plant to adversely affect the plant. Another tier II vegetative

vigor study conducted with POAST® on at least three grass species including corn should be

conducted to assess risk to non-target grass species. There are currently no information on
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estimating definitely how much more toxic the TEP is as compared to the technical sethoxydim.

Seedling Emergence tests. Seedling emergence studies are used to assess risk to non-target plant

species from the exposure to runoff. One seedling emergence study on three grass species

(ryegrass, corn and oat) using the TEP found that the most sensitive plant EC25 = 0.078 lb ai/A

and the NOAEC = 0.0587 lb ai/A for ryegrass. The slope is 2.21. The study also reveals that the

percentage of emergence was significantly reduced at 0.1175 lb ai/A. 

Using TerrPlant model the LOC is exceeded for non-target grass species (including listed

species) only to low semi-aquatic areas from runoff. The range of RQ is 1.3 to 3.1 for non-listed

grasses and 1.7 to 4.1 for listed species. Since an assumption of 1% and 5% for spray drift was

built into the TerrPlant model and the AgDrift model has much higher percentage of spray drift

(50% at edge of field to 10 % at 300 feet from edge of field) movement off-site than TerrPlant,

EFED can assume that the risk may be underestimated. 

3. Uncertainty in Fate Data

Terrestrial Field Dissipation. The submitted Terrestrial Field Dissipation studies (164-1) were

judged to be unacceptable because total Sethoxydim residues, rather than the parent compound

and its eight degradates were analyzed. However, due to the large uncertainty that exists for the

degradates of Sethoxydim, additional data need to be generated to lower the amount of

uncertainties on the fate of Sethoxydim. If the assumption that the degradates are equally as

toxic as the parent is adequate for risk assessment, then the studies do not need to be repeated. 

Aerobic Aquatic Metabolism. Additional information is needed to determine if the submitted

Aerobic Aquatic Metabolism study (162-3) is acceptable. There is uncertainty in this study that

needs to be clarified.

4. Review of Incident Data

There have been two incidents related to Sethoxydim reported to the Environmental Incident

Information System (EIIS) database. One incident occurred in 1992 at an unrecorded location. 

POAST-Plus Herbicide®(Sethoxydim) and Lorsban 15G® Granular Insecticide (chlorpyrifos)

were applied to peanuts that were approximately 60 yards away from a farm pond. Pix®

(mepiquat chloride) was also applied to cotton, which was about 40 yards away from the pond. 

Two inches of rain fell after the application, and about 300 fish (Lepomis and Centrarchidae

spp.) were observed dead in the pond 8 - 10 days later. The EIIS report states that chlorpyrifos is

believed to be the primary cause of this kill, but that sethoxydim and mepiquat chloride could

have also contributed to the toxicity observed in this incident. Chlorpyrifos is very highly toxic

to several freshwater fish species on an acute basis, and all LOCs are exceeded for freshwater

fish for all uses of chlorpyrifos. Mepiquat chloride is practically non-toxic to freshwater fish

species on an acute basis, and the analysis for this chemical indicated that no acute LOC are

exceeded for freshwater fish (see links to chlorpyrifos IRED and mepiquat chloride RED at URL:

http://cfpub.epa.gov/oppref/rereg/status.cfm?show=rereg Analysis also concludes that, despite
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moderate acute toxicity of sethoxydim formulated product, no LOCs are exceeded for non-listed

freshwater fish in a standard pond. Therefore, it appears unlikely that sethoxydim contributed to

the toxicity leading to this fish kill. 

A second incident occurred in 2001, in which Checkmate®(Sethoxydim) and endosulfan were

misdirected during accidental misuse. Dozens of white Amur grass carp (Ctenopharyngondon

idella) were killed. The EIIS report states that endosulfan was likely the primary cause of

mortality, but that Sethoxydim was believed to contribute to the mortality of the fish. Given that

endosulfan is very highly toxic to freshwater fish on an acute basis and all acute aquatic LOCs

are exceeded for endosulfan (see endosulfan RED at URL:

http://www.epa.gov/oppsrrd1/REDs/endosulfan_red.pdf), it appears unlikely that sethoxydim

contributed to the toxicity leading to this incident.

5. Endocrine Disrupter

EPA is required under the FFDCA, as amended by FQPA, to develop a screening program to

determine whether certain substances (including all pesticide active and other ingredients) "may

have an effect in humans that is similar to an effect produced by a naturally occurring estrogen,

or other such endocrine effects as the Administrator may designate." Following the

recommendations of its Endocrine Disruptor Screening and Testing Advisory Committee

(EDSTAC), EPA determined that there was scientific bases for including, as part of the program,

the androgen and thyroid hormone systems, in addition to the estrogen hormone system. EPA

also adopted EDSTAC’s recommendation that the Program include evaluations of potential

effects in wildlife. For pesticide chemicals, EPA will use FIFRA and, to the extent that effects in

wildlife may help determine whether a substance may have an effect in humans, FFDCA

authority to require the wildlife evaluations. As the science develops and resources allow,

screening of additional hormone systems may be added to the Endocrine Disruptor Screening

Program (EDSP). When the appropriate screening and/or testing protocols being considered

under the Agency’s EDSP have been developed, Sethoxydim may be subjected to additional

screening and/or testing to better characterize effects related to endocrine disruption.

C. Threatened and Endangered Species Concerns

1. Taxonomic Groups Potentially at Risk

The Agency’s levels of concern for listed birds, terrestrial plants, and possibly aquatic grasses are

exceeded for uses of Sethoxydim. By extension, endangered and threatened amphibians and

reptiles may also be affected. Appendix L provides a list of listed species for each crop in which

Sethoxydim is used terrestrially in each state. A full list of listed species and the crops with

which they are associated is in Appendix M. 

The registrant must provide information on the proximity of Federally listed endangered species

to the Sethoxydim usage sites. This requirement may be satisfied in one of three ways: 1) having
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membership in the FIFRA Endangered Species Task Force (Pesticide Registration Notice 2000-

2); 2) citing FIFRA Endangered Species Task Force data; or 3) independently producing these

data, provided the information is of sufficient quality to meet FIFRA requirements. The

information will be used by the OPP Endangered Species Protection Program to develop

recommendations to avoid adverse effects to listed species.

The preliminary risk assessment for endangered species indicates that Sethoxydim exceeds the

listed species LOCs for the following combinations of analyzed uses and species groups:

• Birds (chronic): 

• Chronic RQs indicate that the following groups of listed birds are at risk under the

uses and application scenarios indicated:

N Birds consuming short grass, tall grass, broadleaf plants, small insects: Citrus,

tree nuts (4 applications at maximum rate of 0.47 lbs ai/A); alfalfa, legume

seeds, cotton (3 applications at maximum rate).

N Birds consuming short grass: fruiting and leafy vegetable crops (3 applications

at maximum rate); soybeans, grapes, caneberries, peanuts, potatoes (excluding

sweet potatoes), mint, turf, nursery crops, sugar beets, vegetable and flower

seed (2 applications at maximum rate); orchards, strawberries, sunflower (1

application at maximum rate).

• All other listed birds are possibly at risk on a chronic basis. RQs cannot be

estimated for birds associated with other crops and/or food types, so the

assumption of risk must be extended to all birds until sufficient data are provided.

• Plants: Alfalfa, berries, citrus, cotton, nursery crops, grapes, melons, orchards, tree nuts,

turf, strawberries, sugar beets, vegetable and flower seeds, fruiting and leafy vegetables, 

potatoes (excluding sweet potatoes), peanuts, soybeans, legume seeds, sunflower seed,

mint, vegetable and flower seed (2 to 4 aerial applications at maximum rate). 

Applications of Sethoxydim pose risk to aquatic and terrestrial listed species:

Amphibians

Nineteen listed species inhabit 12 states in which Sethoxydim is used. Of the major crops on

which Sethoxydim is used, 15 species are associated with uses on alfalfa, 13 species are

associated with uses in cotton, 6 with uses on soybeans, and 12 are associated with uses on

vegetable crops. In addition, 10 species are associated with nursery crops, 17 species are

associated with uses on orchard crops. Many of these species occur in only one state.

Birds

Fifty-seven listed species of birds live in 48 states in which Sethoxydim is used. Of the major
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crops on which Sethoxydim is used, 19 species are associated with uses on soybeans, 14 species

are associated with uses in peanuts, 25 species are associated with cotton, 14 species are

associated with uses on sugar beets, and 36 species are associated with alfalfa crops. In other

crops, 39 are associated with uses in citrus, 54 with nursery crops, 28 with turf (harvested sod),

and 55 are associated with vegetable crops.

Reptiles

Twenty-eight listed species of reptiles live in 19 states in which Sethoxydim is used. Of the

major crops on which Sethoxydim is used, 21 species are associated with uses in alfalfa, 14 with

uses in cotton, 13 with uses in soybeans, 6 with uses in peanuts. In addition, 16 species are

associated with uses in citrus, 23 with nursery crops, 26 with orchard crops, 17 with turf, 5 with

sugar beets, and 25 are associated with uses in vegetable crops.

Plants

The LOCATES database does not delineate between terrestrial and aquatic plants or between

grasses and other non-grass plants, so none are listed here. However, over 500 listed species of

plants are associated with crops on which Sethoxydim is used.

• Direct Effects

Guideline studies used in this ecological risk assessment indicate the possible effects that may

occur to listed taxa if directly exposed to Sethoxydim. 

Acute effects to birds are not expected; however, reproductive effects were observed in the avian

chronic test with the mallard and bobwhite. A reduction in the number of eggs laid was observed

in quail at the 500 ppm level; however, this effect was not considered to be treatment related. In

tests with the mallard, reduced number of normal hatchlings was observed in all concentrations

(100, 500, and 1000 ppm). Other effects that were observed were a reduction in the number of

eggs set the number of live embryos (observed at the 500 ppm concentration). 

Acute effects to fish from exposure to sethoxydim technical grade active ingredient (TGAI) is not

expected. Acute tests with TGAI in rainbow trout showed mortality (60%) occurring at only the

highest concentration tested (180 mg/l). Effects were not observed at the other four

concentrations ranging from 18 - 100 mg/l. However, higher mortality was observed when

sethoxydim TEP was tested in the same species, (100% at the two highest concentrations of 2.7

and 1.7 mg/l, 10% at the mid-range concentration of 1.0 mg/l). Mortality was not observed at the

three lower concentrations of 0.21, 0.37, and 0.59 mg/l. No other effects (e.g., behavioral

changes) were noted in these studies.

Acute effects to aquatic invertebrates from exposure to sethoxydim technical grade active

ingredient (TGAI) is not expected. Immobility was observed in Daphnia when tested with both

sethoxydim TGAI and TEP. The acute test with TGAI describes cumulative effects with TGAI
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of 0%, 15%, 85%, 95%, and 100% at concentrations of 32, 51, 100, 180, and 320 mg/l,

respectively. With TEP, 100% effects were observed in the two highest concentrations of 4.6

and 8.9 mg/l, 5% and 20% were observed at 0.57 and 2.4 mg/l respectively, and no effects were

observed at the second highest concentration (1.1 mg/l). Other direct effects among the test

organisms were not noted.

Acute effects to marine/estuarine fish from exposure to sethoxydim technical grade active

ingredient (TGAI) is not expected. Mortality was not observed in the sheepshead minnow in

acute tests with TGAI, but was observed when tested with the TEP (100% mortality at the four

highest concentrations of 4.5, 7.2, 10.8, and 18 mg/l. Behavioral effects of loss of equilibrium

and surfacing and gasping were observed at all but the 4.5 mg/l concentration, even the lowest

(2.7 mg/l). These sublethal effects could have indirect consequences leading to mortality; for

example, they may lead to increased vulnerability to predators. The sheepshead minnow early

life stage test did not result in any apparent direct effects. For this test, it is unclear whether

TGAI or TEP was used, but percentage of active ingredient was 40.7%. Concentrations tested

ranged from 6.6 to 98 mg/l.

Acute effects to marine/estuarine invertebrates from exposure to sethoxydim technical grade

active ingredient (TGAI) is not expected. Mortality and other effects were observed in acute

toxicity tests with marine/estuarine invertebrates. Mortality was observed in an acute test with

the mysid using Sethoxydim TGAI (30% at 141.8 mg/l). With the TEP, 100% mortality was

observed at the three highest concentrations tested (1.1, 1.8, and 3.0 mg/l), 10% was observed at

the second highest concentration (0.7 mg/l), and no mortality was observed at the lowest

concentration of 0.5 mg/l. Effects were observed in the mysid life cycle test. It is unclear

whether the test was performed with TGAI or TEP (43.1% a.i. is noted for the test material), but

the magnitude of effects with this test are more consistent with acute tests using TEP. At the two

highest concentrations of 59 and 120 mg/l, 100% effects were observed. At the other

concentrations of 7.6, 14, and 29 mg/l, effects were observed at 17%, 26% and 70%. 

Mortality and the presence of abnormal individuals are potential direct effects in marine/estuarine

invertebrates. Tests with the Eastern oyster demonstrated direct effects. For acute tests with

TGAI on oyster larvae, mortality was not observed. However, abnormal individuals were

observed in the control (7%) and at the concentration tested (109 mg/l). Sethoxydim was

measured in the control, but this did not affect the classification of the study. Mortality was

observed in all concentrations tested on Eastern oyster using the TEP in acute tests. 

Concentrations were 0.4, 0.6, 1.0, 1.5, and 2.5 mg/l, and effects were observed at 2%, 12%, 62%,

95% and 89%, respectively. No other direct effects were reported, but the number of abnormal

individuals was measured during the test.

Direct effects are expected for plants exposed to sethoxydim, and especially for grasses. Among

the species tested, ryegrass was the most sensitive species based on EC values for percent change

in weight compared to controls. The rates tested ranged from 0.0031 - 0.05 lb a.i./A, and percent

changes in weight ranged from 1.97 x 10-8 to 75.5. As a result, we expect that listed species of

grasses might be directly affected by exposure to sethoxydim.
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2. Probit Slope Analysis

The probit slope response relationship is evaluated to calculate the chance of an individual event

corresponding to the listed species acute LOCs. If information is unavailable to estimate a slope

for a particular study, a default slope assumption of 4.5 is used as per original Agency

assumptions of typical slope cited in Urban and Cook (1986).

It is recognized that extrapolation of very low probability events is associated with considerable

uncertainty in the resulting estimates. To explore possible bounds to such estimates, the upper

and lower values for the mean slope estimate can be used to calculate upper and lower estimates

of the effects probability associated with the listed species LOC. However, the 95% confidence

intervals for the slopes are unavailable in cases where slope is based on a default assumption of

4.5.

Our analysis using the exposure scenario of a 6-foot pond indicates that listed freshwater

invertebrate species LOCs are exceeded when EECs are calculated for formulated petroleum

solvents. RQs calculated using EECs for formulated petroleum solvents in a 6-inch wetland

exceeded listed and non-listed species LOCs for freshwater fish and invertebrates. Therefore,

mortality and other direct effects may be observed in freshwater animals inhabiting ponds or

flowing waters, as well as those inhabiting wetland environments. Since both listed and nonlisted LOCs are exceeded for the wetland scenario, and more taxa are at risk, it is possible that

effects are more likely to occur in those environments compared to pond and other non-wetland

habitats. For this reason, the 6-inch wetland scenario is included in the discussion of the

probability of individual direct effects, as well as the discussion of indirect effects below.

Freshwater Fish

Raw data are not available from the two rainbow trout studies with technical Sethoxydim (MRID

42815) and the formulated product (MRID 41885902) to calculate a slope. Therefore, the default

slope of 4.5 was used to calculate the event probability for freshwater fish LOC. Based on an

assumption of a probit dose response relationship with a mean estimated slope of 4.5, the

corresponding estimated chance of individual mortality associated with the listed species LOC of

0.05 is 1 in 417,000,000 for technical. 

Freshwater Invertebrates

A slope of 5.84 (95% CI: 3.84 - 7.86) was estimated using data from the study with technical

Sethoxydim in Daphnia magna (MRID 42816). Based on an assumption of a probit dose

response relationship using this slope, the corresponding estimated chance of individual mortality

associated with the listed species LOC of 0.05 is 1 in 6.57 x 1013. The upper and lower estimates

of the effects probability associated with the listed species LOC are, respectively, 1 in 3,410,000

and #1 in 1 x 1016 (this value is the limit of probability for the program used to estimate the

probability of mortality). Individual mortality associated with the calculated RQ value for

Daphnia magna exposed to technical Sethoxydim (0.001) is estimated to be #1 in 1 x 1016. The
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RQs calculated for freshwater invertebrates exposed to technical Sethoxydim fell below the LOC

of 0.05, resulting in estimated probability of individual mortality of #1 in 2.8 x 109.

Marine/Estuarine Fish

Raw data are not available from the sheepshead minnow studies with Sethoxydim technical to

calculate a slope. Therefore, the default slope of 4.5 was used to calculate the event probability

for marine/estuarine fish LOC. Based on an assumption of a probit dose response relationship

with a mean estimated slope of 4.5, the corresponding estimated chance of individual mortality

associated with the listed species LOC of 0.05 is 1 in 417,000,000 for technical Sethoxydim. 

The RQs calculated using EECs from the GENEEC model for marine/estuarine fish exposed to

technical Sethoxydim fell below the LOC of 0.05, resulting in estimated probability of individual

mortality of #1 in 2.8 x 109.

Marine/Estuarine Invertebrates

Raw data are not provided in the Eastern oyster study (MRID 42537401) using technical

Sethoxydim to calculate a slope, or in the study with mysid shrimp. Therefore, the default slope

of 4.5 was used to calculate the event probability for marine/estuarine invertebrate LOC. Based

on an assumption of a probit dose response relationship with a mean estimated slope of 4.5, the

corresponding estimated chance of individual mortality associated with the listed species LOC of

0.05 is 1 in 417,000,000 for technical Sethoxydim in these species.

With EECs calculated using the GENEEC model, the RQs for marine/estuarine fish exposed to

technical Sethoxydim fell below the LOC of 0.05, resulting in estimated probability of individual

mortality of #1 in 2.8 x 109. 

Birds

Because there were no demonstrable effects in acute toxicity tests with birds, a slope cannot be

calculated to use in estimating the probability of individual effects on birds.

Mammals

Because mortality occurred at higher doses in the toxicity test with mammals, a probit-slope

analysis must be performed for this taxon. Raw data are not provided in toxicity studies

involving laboratory rats and technical Sethoxydim (MRID 00045847) or its formulated product

(MRID 00046326) in order to calculate a slope. Therefore, the default slope of 4.5 was used to

calculate the event probability for mammal LOC. Based on an assumption of a probit dose

response relationship with a mean estimated slope of 4.5, the corresponding estimated probability

of mortality associated with the listed species LOC of 0.1 is 1 in 294,038. 

3. Indirect Effect Analyses
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The Agency acknowledges that pesticides have the potential to exert indirect effects upon the

listed organisms by, for example, perturbing forage or prey availability, altering the extent of

nesting habitat, creating gaps in the food chain, etc.

In conducting a screen for indirect effects, direct effect LOCs for each taxonomic group are used

to make inferences concerning the potential for indirect effects upon listed species that rely upon

non-listed organisms in these taxonomic groups as resources critical to their life cycle.

Because screening-level acute RQs have been exceeded for certain taxa, the Agency uses the

dose response relationship from the toxicity study used for calculating the RQ to estimate the

probability of acute effects on these taxa associated with an exposure equivalent to the EEC. 

This information serves as a guide to establish the need for and extent of additional analysis that

may be performed using Services-provided “species profiles” as well as evaluations of the

geographical and temporal nature of the exposure to ascertain if a “not likely to adversely affect”

determination can be made. The degree to which additional analyses are performed is

commensurate with the predicted probability of adverse effects from the comparison of the dose

response information with the EECs. The greater the probability that exposure will produce

effects on a taxon, the greater the concern for potential indirect effects for listed species

dependent on that taxon, and therefore, the more intensive the analysis on the potential listed

species of concern, their locations relative to the use site, and information regarding the use

scenario (e.g., timing, frequency, and geographical extent of pesticide application).

RQs calculated from GENEEC-generated EECs in a standard pond did not exceed listed LOC for

freshwater fish or invertebrates, so indirect effects resulting from losses of fish in these

environments are not expected. 

Chronic LOCs are potentially exceeded for all birds on all food items at all use sites until

additional avian chronic data is made available. At present, we have insufficient data to

determine whether chronic LOCs are exceeded with use of sethoxydim on other crops and/or in

other avian groups. Therefore, we must assume a direct chronic risk to all listed and non-listed

avian species where sethoxydim is used. As a consequence, there is concern for indirect effects

to generalist and obligate listed species dependent on birds, including mammals and other birds.

LOCs are exceeded for listed and non-listed terrestrial grass species (family Poaceae) from

Sethoxydim applied by aerial or irrigated systems and from runoff to low semi-aquatic areas. 

Therefore, listed species in all taxa depending on plants that are exposed in these scenarios may

be indirectly affected. Because sethoxydim is considered to be toxic to grasses, listed species

obligate on grasses may have a higher likelihood of being indirectly affected than listed species

that do not consume grass or are plant generalists. Information is lacking about effects to aquatic

grasses, but additional data may indicate effects to aquatic grasses, which will also have indirect

effects on listed species depending on these plants. Further analysis will be necessary to confirm

this possibility once these data are obtained, and will need to be conducted for all listed taxa,

since loss of plants could result in loss of food resources or other important habitat elements or

functions (e.g., cover from predators, loss of breeding and/or nesting sites, provision of shade to
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water bodies, etc.). 

Because LOCs are exceeded for birds, amphibians and reptiles may also be affected by exposure

to Sethoxydim. Therefore, listed species that are dependent on these taxa may be indirectly

affected by their loss. 

The above are only general descriptions of the listed species that may be indirectly affected by

applications of Sethoxydim. Further analysis using geographic information of use sites and

species occurrence (available in the LOCATES database), “species profiles” provided by the

Services and other sources, and information regarding the temporal nature of Sethoxydim

applications is necessary to determine the true indirect effects to listed species. 

4. Critical Habitats

In the evaluation of pesticide effects on designated critical habitat, consideration is given to the

physical and biological features (constituent elements) of a critical habitat identified by the U.S

Fish and Wildlife and National Marine Fisheries Services as essential to the conservation of a

listed species and which may require special management considerations or protection. The

evaluation of impacts for a screening level pesticide risk assessment focuses on the biological

features that are constituent elements and is accomplished using the screening-level taxonomic

analysis (risk quotients, RQs) and listed species levels of concern (LOCs) that are used to

evaluate direct and indirect effects to listed organisms.

The screening-level risk assessment has identified potential concerns for indirect effects on listed

species for those organisms dependant upon birds and terrestrial plants. In light of the potential

for indirect effects, the next step for EPA and the Service(s) is to identify which listed species

and critical habitat are potentially implicated. Analytically, the identification of such species and

critical habitat can occur in either of two ways. First, the agencies could determine whether the

action area overlaps critical habitat or the occupied range of any listed species. If so, EPA would

examine whether the pesticide's potential impacts on non-endangered species would affect the

listed species indirectly or directly affect a constituent element of the critical habitat. 

Alternatively, the agencies could determine which listed species depend on biological resources,

or have constituent elements that falls into, the taxa that may be directly or indirectly impacted by

the pesticide. Then EPA would determine whether use of the pesticide overlaps the critical

habitat or the occupied range of those listed species. At present, the information reviewed by

EPA does not permit use of either analytical approach to make a definitive identification of

species that are potentially impacted indirectly or critical habitats that is potentially impacted

directly by the use of the pesticide. EPA and the Service(s) are working together to conduct the

necessary analysis.

This screening-level risk assessment for critical habitat provides a listing of potential biological

features that, if they are constituent elements of one or more critical habitats, would be of

potential concern. These correspond to the taxa identified above as being of potential concern

for indirect effects and include the following: birds and terrestrial plants. This list should serve
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as an initial step in problem formulation for further assessment of critical habitat impacts

outlined above, should additional work be necessary. 

D. Description of Assumptions, Uncertainties, Strengths, and Limitations

1. Assumptions and Limitations Related to Exposure for all Taxa

Sethoxydim Total Toxic Residues are expected to persist in treated terrestrial and aquatic areas

with half-lives of about 30 days, based on laboratory studies. There were eight degradates

observed in the laboratory studies. Two of these (the sulfoxide and sulfone of the parent) were

observed in soil. At environmentally relevant pH values, the Sethoxydim and degradates will all

be present to some degree. 

2. Assumptions and Limitations Related to Exposure for Terrestrial Species

a. Location of Wildlife Species

For screening terrestrial risk assessments for listed species, a generic bird or mammal is assumed

to occupy either the treated field or adjacent areas receiving pesticide at a rate commensurate

with the treatment rate on the field. Spray drift model predictions suggest that this assumption

leads to an overestimation of exposure to species that do not occupy the treated filed. For

screening risk assessment purposes, the actual habitat requirements of any particular terrestrial

species are not considered, and it assumed that species occupy, exclusively and permanently, the

treated area being modeled. This assumption leads to a maximum level of exposure in the risk

characterization.

Terrestrial EEC are based on peak value of the Kenaga nomogram as modified by Fletcher. The

residues from the Kenaga nomogram are measured immediately after application. The peak

maximum value is the upper limit value and may be conservative. Sethoxydim is applied to

plants after crops are up and the grasses appear up to about 30 days or more prior to harvest. A

variety of avian populations are nested in cotton, potato, alfalfa, orchards, and clover fields. The

LOC for avian reproductive impairment is exceeded for Sethoxydim application, and based on

this timing information, it is apparent that wildlife will be present in the fields during the period

of application.

b. Routes of Exposure

Screening-level risk assessments for spray applications of pesticides consider dietary exposure

alone. Other routes of exposure, not considered in this assessment, are discussed below:

• Incidental soil ingestion exposure - This risk assessment does not consider

incidental soil ingestion. Available data suggests that up to 15% of the diet can

consist of incidentally ingested soil depending on the species and feeding strategy
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(Beyer et al., 1994). 

• Inhalation exposure - The screening risk assessment does not consider inhalation

exposure. Such exposure may occur through three potential sources: (1) spray

material in droplet form at the time of application (2) vapor phase pesticide

volatilizing from treated surfaces, and (3) airborne particulate (soil, vegetative

material, and pesticide dusts).

• Dermal Exposure - The screening assessment does not consider dermal exposure,

except as it is indirectly included in calculations of RQs based on lethal doses per

unit of pesticide treated area. Dermal exposure may occur through three potential

sources: (1) direct application of spray to terrestrial wildlife in the treated area or

within the drift footprint, (2) incidental contact with contaminated vegetation, or

(3) contact with contaminated water or soil.

• Drinking Water Exposure - Drinking water exposure to a pesticide active

ingredient may be the result of consumption of surface water or consumption of

the pesticide in dew or other water on the surfaces of treated vegetation. For

pesticide active ingredients with a potential to dissolve in runoff, puddles on the

treated field may contain the chemical. 

c. Residue Levels Selection

As discussed earlier in the exposure section of this document, the Agency relies on the work of

Fletcher et al. (1994) for setting the assumed pesticide residues in wildlife dietary items. The

Agency believes that these residue assumptions reflect a realistic upper-bound residue estimate,

although the degree to which this assumption reflects a specific percentile estimate is difficult to

quantify. It is important to note that the field measurement efforts used to develop the Fletcher

estimates of exposure involve highly varied sampling techniques. It is entirely possible that

much of these data reflect residues averaged over entire above ground plants in the case of grass

and forage sampling. Depending upon a specific wildlife species’ foraging habits, whole

aboveground plant samples may either underestimate or overestimate actual exposure.

d. Dietary Intake - Differences Between Laboratory and Field Conditions

The acute and chronic characterization of risk rely on comparisons of wildlife dietary residues

with LC50 or NOAEC values expressed in concentrations of pesticides in laboratory feed. These

comparisons assume that ingestion of food items in the field occurs at rates commensurate with

those in the laboratory. Although the screening assessment process adjusts dry-weight estimates

of food intake to reflect the increased mass in fresh-weight wildlife food intake estimates, it does

not allow for gross energy and assimilative efficiency differences between wildlife food items

and laboratory feed.

On gross energy content alone, direct comparison of a laboratory dietary concentration- based
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effects threshold to a fresh-weight pesticide residue estimate would result in an underestimation

of field exposure by food consumption by a factor of 1.25 - 2.5 for most food items. Only for

seeds would the direct comparison of dietary threshold to residue estimate lead to an

overestimate of exposure.

Differences in assimilative efficiency between laboratory and wild diets suggest that current

screening assessment methods do not account for a potentially important aspect of food

requirements. Depending upon species and dietary matrix, bird assimilation of wild diet energy

ranges from 23 - 80%, and mammal’s assimilation ranges from 41 - 85% (U.S. Environmental

Protection Agency, 1993). If it is assumed that laboratory chow is formulated to maximize

assimilative efficiency (e.g., a value of 85%), a potential for underestimation of exposure may

exist by assuming that consumption of food in the wild is comparable with consumption during

laboratory testing. In the screening process, exposure may be underestimated because metabolic

rates are not related to food consumption.

Finally, the screening procedure does not account for situations where the feeding rate may be

above or below requirements to meet free living metabolic requirements. Gorging behavior is a

possibility under some specific wildlife scenarios (e.g., bird migration) where the food intake rate

may be greatly increased. Kirkwood (1983) has suggested that an upper-bound limit to this

behavior might be the typical intake rate multiplied by a factor of 5.

e. Estimated Environmental Concentrations to Non-Target Plants

Currently the model (TerrPlant) for predicting the exposure to non-target plants adjacent to

application sites and also to low-lying wetlands are based on the application rate, method of

application (aerial or ground), and the solubility of the pesticide. Several transport factors are not

incorporated into the model. The exposure prediction may be underestimated or overestimated. 

The model also assumes 5% aerial spray drift when aerially applied. AgDrift usually predicts

about 10% and up to 15% spray drift at the edge of the field. The aerial application EEC in this

risk assessment may be underestimated if use based on 5%.

f. Data gaps

• The submitted Terrestrial Field Dissipation studies (164-1) were judged to be

unacceptable because total Sethoxydim residues, rather than the parent compound and its

eight degradates were analyzed. However, due to the large uncertainty that exists for the

degradates of Sethoxydim, additional data needs to be generated to lower the amount of

uncertainties on the fate of Sethoxydim. If the assumption that the degradates are equally

as toxic as the parent is adequate for risk assessment, then the studies do not need to be

repeated. 

• Additional information is needed to determine if the submitted Aerobic Aquatic

Metabolism study (162-3) is acceptable. There is uncertainty in this study that needs to

be clarified.
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3. Assumptions and Limitations Related to Aquatic Effects Assessment

a. Age Class and Sensitivity of Effects Thresholds

It is generally recognized that test organism age may have a significant impact on the observed

sensitivity to a toxicant. The screening risk assessment acute toxicity data for fish are collected

on juvenile fish between 0.1 and 5 grams. Aquatic invertebrate acute testing is performed on

recommended immature age classes (e.g., first instar for daphnids, second instar for amphipods,

stoneflies and mayflies, and third instar for midges). Similarly, acute dietary testing with birds is

also performed on juveniles, with mallard being 5-10 days old and quail 10-14 days old.

Testing of juveniles may overestimate toxicity at older age classes for pesticidal active

ingredients, such as Sethoxydim, that act directly because younger age classes may not have the

enzymatic systems associated with detoxifying xenobiotics. The screening risk assessment has

no current provisions for a generally applied method that accounts for this uncertainty. In so far

as the available toxicity data may provide ranges of sensitivity information with respect to age

class, the risk assessment uses the most sensitive life-stage information as the conservative

screening endpoint.

b. Use of the Most Sensitive Species Tested

Although the screening risk 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.

The Agency is not limited to a base set of surrogate toxicity information in establishing risk

assessment conclusions. The Agency also considers toxicity data on non-standard test species

when available.

c. Data Gaps

The following data gaps were identified with respect to the submitted ecotoxicity effects data:

• Risk to non-target terrestrial plants from spray drift is underestimated because the

vegetative vigor study was conducted on the technical Sethoxydim and not on the TEP

(POAST®).

• Risk to non-target aquatic grass species can not be certain or quantified due to lack of
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data.

• Avian chronic reproduction study did not achieve a NOAEC. Chronic avian toxicity is

underestimated. 

d. Aquatic Environment Effects

Aquatic toxicity data demonstrate that the Sethoxydim technical is less toxic than the TEP. 

Comparison of the TEP toxicity data to Naphthalene toxicity data strongly suggest that the

observed toxicity of the TEP is due to the petroleum solvent alone.

The petroleum solvent (Naphthalene log Kow = 3.3) is more likely to bioaccumulate than

Sethoxydim (logKow = 1.38). This property is likely to increase the petroleum solvent’s potential

adverse effects. 

e. Aquatic Environment Exposure

Exposure to Sethoxydim from terrestrial use can occur from both spray drift and runoff. Detailed

modeling to determine which of these two routes is predominant has not been done; however

Sethoxydim is prone to runoff due to its high solubility (4700 ppm), low Kd values, and low

Henry’s Law constant (i.e., it tends to remain in water rather than sorb to soil or evapoarate). 

The petroleum solvent’s fate properties (as exemplified by Naphthalene) differ from Sethoxydim

enough that they are expected to separate after application to a field. Naphthalene’s lower

solubility (31 ppm) and higher vapor pressure (0.085 mmHg) promote evaporation, while its

octanol-water partition coefficient (log Kow = 3.3) indicates a greater tendency to sorb to soil. The

higher logKow also indicates a greater tendency to bioaccumulate. Thus, we expect that runoff

will carry Sethoxydim to a greater extent than it will carry the petroleum solvent (i.e., the mixture

in the runoff will contain proportionately less solvent than the spray mixture).