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over time and additional metabolism studies would aid characterization and reduce 

uncertainty. 

 Anaerobic Aquatic Metabolism (OCSPP guideline # 835.4400) Further data are needed 

to support risk assessment. Concentrations in submitted soil metabolism studies 

fluctuated over time and additional metabolism studies would aid characterization and 

reduce uncertainty. In the absence of data, it will be assumed that the anaerobic aquatic 

metabolism half life is double that of the anaerobic soil metabolism half life. 

 Environmental Chemistry Methods and Independent Laboratory Validation (OCSPP 

guideline # 850.6100) This study is required to corroborate methods used in 

environmental fate studies and to assist in the enforcement of pesticide tolerances. 

The available effects data are considered incomplete for the purposes of risk assessment. The 

following data gap(s) exist: 

 OCSPP 850.2100 – Avian Acute Oral. An avian acute oral toxicity study with a 

passerine is necessary for risk assessment. Additionally the available avian acute 

oral study resulted in a LD50 value of 169.2 mg a.i./kg-bw, which classifies 

metribuzin as moderately toxic to Northern bobwhite quail via the oral route. 

Therefore additional data are necessary to strengthen the risk assessment for birds via the 

oral route since the subacute dietary studies via the dietary route do not indicate toxicity 

from metribuzin exposure. It is recommended that a definitive study be conducted; 

however, a limit test with a higher exposure concentration (to be determined through 

consultation with the Agency) may also be acceptable. 

 OCSPP 850.2300- Avian Reproduction. An avian reproduction toxicity study with 

the Northern bobwhite quail is necessary for risk assessment. The available avian 

reproduction studies with bobwhite quail and mallard duck indicated no doseresponse effects were observed at any concentrations including the highest test 

concentrations tested. However, in the RED, the risk assessment suggests the risk to 

birds exists based on the toxicity value selected from the reproduction studies for use in 

the avian risk assessment. But a closer look of the study indicated the test concentrations 

were not tested high enough or at/above environmentally relevant concentrations, which 

explain why the LOC exceedance exists when no effects occurred; it is uncertain if there 

is actually any risks to birds. A new study with higher test concentrations (up to 1,500 mg 

a.i./kg diet) should address the risk uncertainty. The recommended test species for the 

new reproduction study is the Northern bobwhite quail since acute effects were observed 

in this species in the avian acute oral toxicity test. 

Additional detail on the current data gaps for environmental fate and effects is provided in 

Section 8.0. 
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Problem Formulation for the Environmental Fate and 

Ecological Risk, Endangered Species, and Drinking 

Water Assessments in Support of the Registration 

Review of Metribuzin 

4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one 

CAS Registry Number: 21087-64-9 

PC Code: 101101 

Prepared by:

Stephen Carey, Biologist 

Andrew Shelby, Physical Scientist

U. S. Environmental Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division

Environmental Risk Branch IV

1200 Pennsylvania Ave., NW

Mail Code 7507P

Washington, DC 20460

Reviewed by:

Stephanie Syslo, Environmental Scientist 

Brian Kiernan, Biologist

William P. Eckel, Acting Branch Chief, ERB 6

June 27, 2012 
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1. Purpose 

The purpose of this problem formulation is to provide an understanding of what is known about 

the environmental fate and ecological effects of the registered uses of metribuzin (PC Code 

101101) in preparation for conducting environmental fate and ecological risk, endangered 

species, and drinking water assessments for its registered uses. Additionally, this problem 

formulation is intended to identify data gaps, uncertainties, and potential assumptions used to 

characterize the ecological risk associated with the registered uses of metribuzin. 

Metribuzin belongs to the Triazinone group of herbicides and it is applied as a pre-plant and preemergence herbicide with selective activity on grasses and broad leaf weeds. Its mode of action 

is inhibition of photosynthesis via photosystem II. 

2. Problem Formulation 

2.1. Nature of Regulatory Action 

Under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), all pesticides distributed 

or sold in the United States generally must be registered by the Environmental Protection 

Agency (EPA). As part of determining whether a pesticide can be registered in the U.S., the 

Agency evaluates its safety to non-target species based on a wide range of environmental and 

health effects studies. In 1996, FIFRA was amended by the Food Quality Protection Act, and the 

Agency was mandated to implement a new program for the periodic review of pesticides, i.e., 

Registration Review (http://www.epa.gov/oppsrrd1/registration_review/). The Registration 

Review program is intended to ensure that, as the ability to assess risk evolves and as policies 

and practices change, all registered pesticides continue to meet the statutory standard of no 

unreasonable adverse effects to human health and the environment. Changes in science, public 

policy, and pesticide use practices occur over time. Through the Registration Review program, 

the Agency periodically reevaluates pesticides to make sure that as change occurs, products in 

the marketplace can be used safely. 

As part of the implementation of the Registration Review program pursuant to Section 3(g) of 

the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA), the Agency is beginning its 

evaluation of metribuzin to determine whether it continues to meet the FIFRA standard for 

registration. This problem formulation for the environmental fate, ecological risk, endangered 

species, and drinking water assessment chapter in support of the Registration Review will be 

posted in the initial docket, which will open the public phase of the review process. 

2.2. Previous Risk Assessments 

Metribuzin was registered in the United States in 1973 as a pre-emergent and post-emergent 

herbicide to selectively control certain broadleaf weeds and grassy weed species in various crop 

and noncrop areas. The Registration Standard was issued in July 1985. A Reregistration 

Eligibility Decision (RED) was issued in 1998 and concluded that registered uses exceed levels 
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of concern for acute and chronic avian and mammalian effects, nontarget terrestrial and aquatic 

plant species, and endangered species. It also concluded groundwater contamination is a 

concern. In addition, concentrations of metribuzin in ground water have not exceeded the LOC 

for terrestrial plants, but over the long term, persistent pesticides that have leached to ground 

water will be discharged to surface-water bodies. Although there is not a concern at the present 

time, in areas where irrigation water is contaminated with metribuzin, residues could pose a 

threat to plants since levels of metribuzin detected in ground water have approached 

approximately 40% of the concentrations which could present a risk. 

3. Stressor Source and Distribution 

3.1. Mechanism of Action 

Metribuzin is an herbicide with selective activity on certain grasses and broad leaf weeds. 

Metribuzin is a triazinone and inhibits electron transport in photosynthesis via photosystem II. 

Triazinones are very closely related to triazines and are thought to have very similar modes of 

action. Triazinones and triazines bind to D-1 proteins forming free radicals that destroy 

chloroplasts and associated plant tissue. 

3.2. Overview of Pesticide Use and Usage 

Metribuzin is a pre-emergent and post-emergent herbicide. It is formulated as an emulsifiable 

concentrate, flowable concentrate, wettable powder, water soluble packaging, and water 

dispersible granules. Application methods for metribuzin include ground spray, aerial spray, and 

chemigation. The application rate varies with the formulation used, stage of weed growth, and 

application timing. Table 1 below provides the specific use patterns and provides the 

PRZM/EXAMS scenarios that may be used to represent each use pattern.
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Table 1. Registered uses of Metribuzin. 

Use pattern Maximum 

single 

application rate 

(lb ai/A) 

Maximum annual 

application rate 

(lb ai/A) 

Application 

interval 

Soybean 1 1 Pre-emergence 

0.625+0.5 1.125 Pre-plant then preemergence 

Potato 1 1 NA1

Alfalfa/ 

Sainfoin 

1 1 NA 

Sugarcane 6 (Hawaii) 8 NS 

3 (Louisiana 

and Texas) 

NS NS 

2 (Florida) 2 NS 

Lentils/peas 0.375 

(preemergence) 

0.25 

(postemergence) 

0.5 Pre-emergence and 

post-emergence 

Chick peas 0.375 0.375 Pre-emergence 

Carrots 0.25 0.5 21 days 

Tomatoes 1 1 NA 

Cereals 0.5 0.5 NA 

Asparagus 2 2 NA 

Corn 0.375 0.562 Pre-plant and preemergence 

Perennial 

grasses 

0.5625 0.5625 NA 

Fall seeded 

triticale 

0.085 0.085 NA 

Winter wheat 0.085 0.085 NA 

1 NA = Not applicable (single application only) 

2 NS = Not specified 
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The Biological and Economic Analysis Division’s (BEAD) Screening Level Usage Analysis 

(SLUA; USEPA 2011) shows that the largest average amount of metribuzin is applied annually 

on potatoes, followed by soybeans, sugarcane, and alfalfa (Table 2). 

Table 2. Screening Level Estimates of Agricultural Uses of Metribuzin (SLUA) Sorted 

Alphabetically from February 25, 2011. 

 Percent Crop Ttd.

 

 Crop 

Lbs. A.I. Avg. Max.

1 Alfalfa 200,000 <2.5 <2.5

2 Apples+ <500 <1 <2.5

3 Asparagus 20,000 30 50

4 Barley <500 <1 <2.5

5 Carrots 1,000 <2.5 5

6 Corn 50,000 <1 <2.5

7 Dry Beans/Peas 10,000 <2.5 5

8 Grapes+ <500 <1 <2.5

9 Pasture/Range+ <500 <1 <2.5

10 Peas, Green 1,000 5 10

11 Peppers+ 1,000 <2.5 5

12 Potatoes 400,000 75 85

13 Pumpkins+ <500 <1 <2.5

14 Soybeans 300,000 <2.5 5

15 Squash+ <500 <1 <2.5

16 Strawberries+ <500 <2.5 <2.5

17 Sugarcane 300,000 25 45

18 Sunflowers + 6,000 <1 <2.5

19 Sweet, Corn <500 <1 <2.5

20 Tomatoes 40,000 20 30

21 Wheat 50,000 <1 <2.5

All numbers rounded. 

<500 Less than 500 pounds of active ingredient 

<2.5 Less than 2.5 percent of crop treated 

<1 Less than 1 percent of crop treated 

 

+ Crops not known to be listed on active end use product registrations or as Section 

18 emergency exemptions when this report was run 

 

SLUA data sources include: 
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USDA-NASS (United States Department of Agriculture's National Agricultural Statistics 

Service) 

Private Pesticide Market Research 

California DPR (Department of Pesticide Regulation) 

Figure 1 represents the national agricultural usage pattern of metribuzin in 20021. It shows that 

usage was dispersed around the country but most concentrated in the Midwest and along the 

Mississippi Delta. Soybeans dominated the agricultural use patterns in 2002 accounting for an 

estimated 44% of metribuzin usage. Since that time, use on potatoes has become the dominant 

use pattern though use on soybeans, sugarcane, and alfalfa remain high. 

Figure 1. Metribuzin Use in Total Pounds per County*. 

 

1 The map was downloaded from a U.S. Geological Survey (USGS), National Water Quality Assessment Program (NAWQA) website. 

(http://water.usgs.gov/nawqa/pnsp/usage/maps/compound_listing.php?year=02) 
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3.3. Environmental Fate and Transport 

The Agency has reviewed or preliminarily reviewed all available fate data previously submitted 

by the registrant. All environmental fate data requirements have been fully satisfied, except for 

aerobic aquatic metabolism and anaerobic aquatic metabolism. 

Aerobic aquatic metabolism and anaerobic aquatic metabolism studies have been submitted but a 

preliminary review indicates they will not be used to support risk assessment because aerobic 

and anaerobic conditions were not controlled or demonstrated and replicate data were not 

provided (MRID 00159423). Submitted aerobic soil metabolism (MRID 40367602) and 

anaerobic soil metabolism (MRID 40367603) are sufficient to support risk assessment but 

metribuzin concentrations in both studies fluctuated such that sample concentrations early in the 

study were less than some samples later in the study. Submission of additional metabolism 

studies will generally reduce uncertainty in the estimation of metabolism half-lives and allow for 

better characterization of the degradation kinetics. 

Two terrestrial field dissipation studies have been submitted. One of the studies (MRID 

40380901) provided useful information on the dissipation of metribuzin residues in the surface 

layer, but did not account for leaching below 6 inches in the soil profile. Neither terrestrial field 

dissipation study was fully acceptable: however, a small-scale prospective ground water study 

has been performed (MRID 45737901). This study accounted for the leaching of metribuzin and 

its degradates over the seven years it was conducted. 

Table 3 summarizes the submitted environmental fate data for metribuzin. 

TABLE 3. General Chemical Properties and Environmental Fate Properties of Metribuzin 

Parameter Value Reference (MRID) 

/Comment

Physical/Chemical Parameters

CAS Registry Number 21087-64-9 Metribuzin RED 

Chemical formula C8H14N4OS Metribuzin RED 

Molecular mass 214.3 g/mol Metribuzin RED 

SMILES Codes O=C1N(N)C(SC)=NN=C1C(C)(C)C EPISuite1

Vapor pressure 9.1 x10-7 torr Foot Print Pesticide Properties 

Database2.

Henry’s Law Constant 3.5*10-11 atm-m3/mol Metribuzin RED 

Water solubility (20°C) 1165 mg/L Foot Print Pesticide Properties 

Database. 

Octanol-water partition coefficient 

(Log KOW) 

1.65 Foot Print Pesticide Properties 

Database. 

Dissociation Constant pKa = 0.99 

pKb = 13

Foot Print Pesticide Properties 

Database. 
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Parameter Value Reference (MRID) 

/Comment

Mobility

Freundlich Kads (mL/g), Koc- ads

(mL/goc)

Metribuzin 

1) 0.246 (1/n = 1.08), 46.7 (sand) 

0.0182 (1/n = 1.52), 3.14 (sandy loam) 

0.221 (1/n = 1.16), 14.5 (silt loam) 

0.196 (1/n = 1.06), 17.0 (clay loam) 

Metribuzin-DADK 

2) 0.70 (1/n = 1.07), 44.7 (sandy loam)

0.15 (1/n = 1.16), 43.1 (sand) 

0.29 (1/n = 0.96), 55.6 (clay) 

0.95 (1/n = 1.08), 49.6 (silt loam) 

Metribuzin-DK

3) 0.47 (1/n = 1.00), 30.0 (sandy loam)

0.13 (1/n = 1.06), 37.4 (sand) 

0.19 (1/n = 1.04), 36.4 (clay) 

0.51 (1/n = 1.10), 26.6 (silt loam)

1) MRID 42283001 

2) MRID 43058502 

3) MRID 43058501 

Persistence in Water

Hydrolysis half-life (25°C, pH 5, 7, & 

9) 

Stable at pH 5, 7 and 9 MRID 00157530

Aqueous photolysis half-life (26-29 

°C, unbuffered pH 6.6) 

t1/2 = 4.34 hours MRID 00157529

Persistence in Soil

Aerobic soil metabolism half-life t1/2 = 106 d (sandy loam; parent) MRID 40367602

Anaerobic soil metabolism half-life t1/2 = 112 d (sandy loam; parent) MRID 40367603 

Soil Photolysis t1/2 = 2.5 d MRID 00157531

Field Dissipation

Terrestrial field dissipation half-life 1) t1/2 = 58d silty clay loam) 

t1/2 = 107d sandy loam muck 

t1/2 = 93d clay loam 

t1/2 = 67d sandy clay loam 

muck 

t1/2 = 68d sandy clay loam 

t1/2 = 42d loamy sand 

t1/2 = 37d sandy loam 

t1/2 = 9d sandy loam 

t1/2 = 26d sandy loam 

t1/2 = 64d sandy loam 

t1/2 = 12d sand 

2) t1/2 = 40d days sandy loam 

t1/2 = 128d days sandy loam 

1) MRID 40380901 

2) MRID 42236101 

1 EPISuite v4.0 

2 http://sitem.herts.ac.uk/aeru/footprint/
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Parent metribuzin is very stable to abiotic hydrolysis and relatively stable to both aerobic and 

anaerobic soil metabolism (T1/2s of 106 and 112 days, respectively). Although metribuzin 

appears to degrade rapidly in the laboratory by direct photolysis in water and on soil (T1/2s of 4.3 

hours 1⁄2 and 2.5 days), only metribuzin that is on the surface of the soil is affected by photolysis. 

For that reason, persistence in field soil appears to be more affected by soil metabolism (aerobic 

and anaerobic) than by photolysis, with field half-lives of 40-128 days. While the rate of 

photodegradation is rapid for exposed chemicals, only approximately the top 1mm of soil is 

actually exposed to sunlight. Once in ground water, metribuzin is expected to persist due to its 

stability to hydrolysis and the lack of light penetration. Conversely, residues of metribuzin in 

surface water are not likely to persist in clear, well-mixed, shallow surface water with good light 

penetration since parent metribuzin degrades rapidly by aqueous photolysis with a calculated half 

life of 4.3 hours. However, if the surface water that receives metribuzin runoff contains 

significant sediments, metribuzin is expected to persist since it is stable to hydrolysis and since 

light penetration would be limited. 

Based on available data, the primary routes of degradation of metribuzin are microbial 

metabolism and photolytic degradation on soil and its major degradates (>10% of applied 

metribuzin detected) are deaminated metribuzin (DA), and deaminated diketo metribuzin 

(DADK). The minor degradates are diketo metribuzin (DK), pentylidene metribuzin, and 

hexylidene metribuzin. The DA and DK moieties degrade further to DADK as the respective 

keto group or amino group is removed. While formation and decline can be observed in the 

review aerobic soil metabolism study for DA and DK, only formation can be observed for 

DADK. Therefore the persistence of DADK is an uncertainty. The photolytic products were 

deaminated (DA) metribuzin in water and pentylidene and hexylidene metribuzin on soil. These 

compounds will be available for leaching to ground water and runoff to surface water in many 

use conditions. 

Metribuzin and its degradates diketo metribuzin (DK) and deaminated diketo metribuzin 

(DADK) are persistent and mobile in soil. Metribuzin has been found in ground water in 27 

states as a result of normal agricultural applications done under a wide range of hydrogeological 

and climatic conditions. The highest metribuzin groundwater detection reported through the 

National Water-Quality Assessment Program (NAWQA) is 2.92 ppb in Virginia. The 

Metribuzin RED (1998) reports a concentration of 54 ppb in groundwater in Wisconsin but the 

source of this data could not be confirmed. Metribuzin has been detected in surface water in 38 

states with the highest concentration of 6.61 ppb detected in Louisiana. These monitoring 

concentrations are derived from non-targeted studies and do not represent the highest metribuzin 

residue concentration that can occur in the environment. There is a great deal of uncertainty 

regarding the degree to which these concentrations are representative. 

3.4. Bioaccumulation 
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The Log Kow of metribuzin is 1.65 indicating the compound has a low potential to 

bioaccumulate. Because of this low bioaccumulation potential, fish bioconcentration study has 

not been requested. 

4. Receptors 

Consistent with the process described in the Overview Document (USEPA 2004), the risk 

assessment for metribuzin will rely on a surrogate species approach. Toxicological data 

generated from surrogate test species, which are intended to be representative of broad 

taxonomic groups, are used to extrapolate the potential effects on a variety of species (receptors) 

included under these taxonomic groupings. 

Acute and chronic toxicity data from studies submitted by the pesticide registrant, along with the 

available open literature, will be used to evaluate the potential direct and indirect effects of 

metribuzin to aquatic and terrestrial receptors. This includes toxicity on the technical grade 

active ingredient, degradates, and when available, formulated products (e.g., “Six-Pack” studies). 

Open literature studies may be identified through EPA’s publically available ECOTOXicology 

(ECOTOX) database, which employs a literature search engine for locating chemical toxicity 

data for aquatic and terrestrial plants and wildlife. The evaluation of both sources of data may 

also provide insight into the direct and indirect effects of metribuzin usage on biotic communities 

from loss of species that are sensitive to the chemical and from changes in structure and 

functional characteristics of the affected communities. 
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A brief summary of the aquatic and terrestrial toxicity data available for the species most 

sensitive to metribuzin is provided in Sections 4.1 and 4.2, respectively. A more complete 

summary of the available data is presented in Appendix B. In addition, a summary of ecological 

incidents associated with metribuzin and a description of ecosystems potentially at risk are 

provided in Sections 4.3 and 4.4, respectively. As noted previously, data on the formation and 

decline of metribuzin degradates are minimal, and there are practically no data on the potential 

toxicity of any degradates except for a chronic aquatic invertebrate life cycle study with 

degradate DADK. In the DADK study, similar chronic effects were observed at levels similar to 

what was observed in the parent study, making the degradate DADK equally toxic as the parent. 

Additionally, the ECOSAR model -a predictive system that estimates aquatic toxicity- indicates 

that DA, DK, and DADK are potentially similarly toxic as parent metribuzin (Appendix E) and 

because the toxicity of the degradates were estimated to be similar to of the parent’s, additional 

toxicological data on the degradates is not required for registration review. 

Toxicity to fish and aquatic invertebrates is categorized using the system shown in Table 4 (US 

EPA, 2004). Toxicity to terrestrial fauna (birds and mammals) is categorized using the system 

shown in Table 5. Toxicity categories for plants have not been defined. 

Table 4. Categories of Acute Toxicity for Aquatic Animals 

LC50 (ppm) Toxicity Category 

< 0.1 Very highly toxic 

> 0.1 - 1 Highly toxic 

> 1 - 10 Moderately toxic 

> 10 - 100 Slightly toxic 

> 100 Practically nontoxic 

Table 5. Categories of Acute Toxicity for Terrestrial Animals 

LD50 (mg/kg) LC50 (ppm) Toxicity Category 

<10 <50 Very highly toxic 

10-50 50-500 Highly toxic 

51-500 501 - 1000 Moderately toxic 

501-2000 1001 - 5000 Slightly toxic 

>2000 >5000 Practically nontoxic 

4.1. Effects to Aquatic Organisms 

The most sensitive of the available aquatic toxicity data from registrant-submitted studies are 

provided in Table 6. If open literature studies are identified through ECOTOX, they will be 

reviewed for the risk assessment. Based on the available information, metribuzin is slightly toxic 

to most aquatic animals yet is moderately toxic to freshwater invertebrates on an acute exposure 

basis. Chronic studies with freshwater fish and invertebrates indicate reproductive and growth 

effects were observed as low as 1.29 mg a.i./L; however, the NOAEC for growth in the fish 

study could not be determined due to effects observed in all treatment levels (<3.0 mg a.i./L). A 

review of the comments from the registrant suggests it is unlikely a new study with lower 

concentrations to achieve a NOAEC will impact the risk assessment; thus, a re-examine of the 

study indicates that the growth NOAEC would have to be 64 times more toxic to result in a LOC 

exceedance, the reviewer agreed that it is unlikely and the freshwater fish early life-stage study 

does not have to be repeated to achieve a growth NOAEC. In addition, the structure-activity 
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relationship (SAR) model, ECOSAR, in Appendix D indicates the estimated chronic values are 

similar to the values observed in the laboratory studies, it is unlikely a new study will obtain a 

NOAEC 64 times more toxic that would impact the risk assessment. In the absence of a NOAEC 

to assess chronic risk to freshwater fish, the LOAEC will be used as the surrogate. No chronic 

studies with estuarine/marine fish and invertebrates were available, acute-to-chronic calculations 

(ACR) would be used to estimate the LOAEC for estuarine/marine fish and the NOAEC for 

estuarine/marine invertebrates. The use of ACR calculations are for assessing whether additional 

testing are needed from the registrant in absence of available data when concerns are raised. 

Available data indicate both vascular and nonvascular aquatic plants are affected by metribuzin. 

Based on ECOSAR modeling for the degradate toxicity estimations and an acceptable DADK 

chronic study with daphnids indicate that the degradates are as toxic as the parent; thus, the 

ECOSAR values will be used to assess risk to aquatic organisms exposed to the degradates; 

however, the uncertainty on using the ECOSAR values may be reduced with the submission of 

aquatic toxicity data for the degradates. 
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Table 6. Aquatic Toxicity Profile for Metribuzin and its Degradate DADK 

Taxonomic 

Group 

Species 

(Common Name) 

Endpoint 

(mg a.i./L) 

Acute 

Classification 

Reference 

(MRID) 

Freshwater Fish 

Oncorhyncus 

mykiss 

(rainbow trout)

LC50=42 Slightly toxic 40098001 

(Acceptable) 

Oncorhyncus 

mykiss 

(rainbow trout) 

NOAEC<3.0 a

LOAEC =3.0 -- 42447801 (Acceptable) 

Freshwater 

Invertebrates 

Daphnia magna 

(daphnid shrimp) EC50=4.2 Moderately toxic 0072083 

(Acceptable) 

Daphnia magna 

(daphnid shrimp) 

NOAEC=1.29 

LOAEC=2.62 -- 42447802 (Acceptable) 

Daphnia magna 

(daphnid shrimp) 

exposed to DADK, 

a degradate

NOAEC=3.3 

LOAEC=5.6 -- 42732501 (Acceptable) 

Estuarine/marine 

Fish 

Cyprinodon 

variegatus

(Sheepshead 

minnow) 

LC50=85 Slightly toxic 42094502 

(Acceptable) 

Acute to Chronic 

ratio (ACR) 

ACR 

LOAEC= 6.1* -- Not Applicable 

Estuarine/marine 

Invertebrates 

Crassostrea 

virginica 

(Eastern oyster) 

LC50=40.7 Slightly toxic 00106197 

(Acceptable) 

Acute to Chronic 

ratio (ACR) 

ACR 

NOAEC=30 b -- Not Applicable 

Vascular plants Lemna gibba

(duckweed) 

IC50=0.13 

NOAEC=0.018 --c 43893501 (Acceptable) 

Nonvascular 

plants 

Psuedokirchneriella 

subcapitata (Green 

algae) 

IC50=0.0081 

NOAEC=0.0023 --c 43921001 (Acceptable) 

a

 Memorandum dated April 7, 1994 – A new study with lower test concentrations to determine the NOAEC for 

growth is not required since the LOAEC was still above exposure estimates. The maximum use rate was 6 lb a.i./A, 

with multiple applications allowed up to 16 lb a.i./A, resulting in a maximum EEC of 183 μg a.i./L. The EEC of 183 

μg a.i./L is 16 times less than the LOAEC determined from the study for growth (3000 μg a.i./L) and are 64 times 

less than the NOAECs determined for survival and hatchability (11,700 μg a.i./L). Since the NOAEC for growth is 

uncertain, applying a safety factor of 10 still results in a value (300 μg a.i./L) greater than the EEC. 

b Pink shrimp, an estuarine/marine invertebrate (MRID 00106197), was used in the acute-chronic ratio calculation 

since the species is more similar to daphnid shrimp than Eastern oyster. c

 Toxicity categories for plants have not been defined. 

* The LOAEC from the freshwater fish early life stage (MRID 42447801) was used in the ACR calculation to 

estimate the LOAEC for chronic toxicity to estuarine/marine fish because the NOAEC was not available for 

calculation. 

4.2. Effects to Terrestrial Organisms 
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Table 7 summarizes the most sensitive terrestrial toxicity endpoints for metribuzin, based on an 

evaluation of the submitted data. If open literature studies are identified through ECOTOX, they 

will be reviewed for the risk assessment. Metribuzin is classified as moderately toxic to upland 

game birds based on the available acute oral study with the Northern bobwhite quail; however, 

an acute oral study with a passerine species is necessary for risk assessment to strengthen the 

toxicity classification for birds via the acute oral route. Passerines tend to have greater sensitivity 

to pesticides than game birds. No effects from subacute and chronic exposure to upland game 

birds and waterfowls via the dietary route were observed; however, an uncertainty exists because 

in the RED, there were LOC exceedances for birds when no effects were observed in the studies. 

Thus, the test concentrations were not tested high enough or tested at/above environmentally 

relevant concentrations, a new reproduction study at or above the environmentally relevant 

concentrations is recommended to address the uncertainty whether there are actual risks to birds. 

The recommended species for the new reproduction study should be the bobwhite quail since 

acute effects in quails were observed in the avian acute oral study. Metribuzin is classified as 

slightly toxic to mammals on an acute oral exposure basis, although effects from chronic 

exposure include reduced body weights and body weight gain. Metribuzin is classified as 

practically nontoxic to honey bees (Apis mellifera) on an acute contact exposure basis. As 

expected with an herbicide, metribuzin has adverse effects on terrestrial plants. 

In summary, terrestrial data needed: 

OCSPP 850.2100 – Avian Acute Oral. An avian acute oral toxicity study with a passerine 

species is necessary for risk assessment. Additionally the available avian acute oral study 

with bobwhite quail resulted in a LD50 value of 169.2 mg a.i./kg-bw, which classifies 

metribuzin as moderately toxic to birds via the oral route. Therefore additional data are 

necessary to strengthen the risk assessment for birds from acute effects since the subacute dietary 

studies indicate metribuzin is not toxic to birds via the dietary route. It is recommended that a 

definitive study be conducted; however, a limit test with a higher exposure concentration (to be 

determined through consultation with the Agency) may also be acceptable. 

OCSPP 850.2300- Avian Reproduction. An avian reproduction toxicity study with the 

Northern bobwhite quail is necessary for risk assessment. The available avian reproduction 

studies with bobwhite quail and mallard duck indicated no dose-response effects were 

observed at any concentrations including the highest test concentrations tested. However, in 

the RED, the risk assessment suggests the risk to birds exists based on the toxicity value selected 

from the reproduction studies for use in the avian risk assessment. But a closer look at the study 

indicated the test concentrations were not tested high enough or at/above environmentally 

relevant concentrations, which explain why the LOC exceedance exists when no effects 

occurred; it is uncertain if there is actually any risks to birds. A new study with higher test 

concentrations (up to 1,500 mg a.i./kg diet) should address the risk uncertainty. The 

recommended test species for the new reproduction study is the Northern bobwhite quail since 

acute effects were observed in this species in the avian acute oral toxicity test. 
-17 of 61-

Table 7. Terrestrial Toxicity Profile for Metribuzin. 

Taxonomic 

Group 

Surrogate 

Species 

(Common 

Name) 

Endpoint Acute Classification/ 

Chronic effect 

Reference 

(MRID) 

Birds 

Colinus 

virginianus

(Bobwhite 

quail) 

LD50=169.2 

mg a.i./kg bw Moderately toxic 00255025 (Acceptable) 

LC50>4000 mg 

a.i./kg diet Practically nontoxic 00262228 (Acceptable) 

Passerine bird No data No data -- 

Anas 

platyrhynchos

(Mallard duck) 

NOAEC=368 

mg a.i./kg-diet 

LOAEC>368 

mg a.i./kg-diet 

No effects observed 43860501 

(Acceptable) 

Mammals

Mus musculus

(Laboratory 

mouse) 

LD50=698/711 

mg a.i./kg bw 

(Female/Male) 

Slightly toxic 

00106158 

(Health Effects 

Division) 

Laboratory rat 

NOAEC=30 

mg a.i./kg-diet 

(1.5 mg 

a.i./kg/day-bw) 

Body weight and body 

weight gain 

40838401 

(Health Effects 

Division) 

Beneficial 

Insects

Apis mellifera

(Honey bee) 

LD50=60.4 

μg/bee Practically nontoxic 28772 (Acceptable) 

Seedling 

emergence 

Monocot: 

Onion 

Dicot: Turnip 

IC25=0.02 

NOAEC=0.014

IC25=0.008 

NOAEC=0.007

--a

 42447803 

Vegetative 

vigor 

Monocot: 

Onion 

Dicot: Turnip

IC25=0.017 

NOAEC=0.011

IC25=0.005 

NOAEC=0.003

-- 42447803 

a

toxicity categories not assigned to plants 

4.3. Incident Database Review 

A review of the EIIS database for ecological incidents involving metribuzin was completed on 

May 24, 2012. 58 total incidents are reported in the database, two aquatic and 56 terrestrial 

(Appendix B). 

The terrestrial incidents reported are for adverse effects to crops, trees, and ornamentals plants. 

Effects include mortality, plant damage, and discoloration. The incidents occurred both from 

direct application and from spray drift. Two aquatic incidents reported were for fish kills, but 
-18 of 61-

involved other herbicides (alachlor, chlorimuron-methyl, and paraquat dichloride) and the 

contribution of metribuzin is unknown. The legality of the application of metribuzin is uncertain 

for these incidents. 

A search of the Avian Incident Monitoring System (AIMS; 

http://www.abcbirds.org/abcprograms/policy/toxins/aims/aims/index.cfm), a database 

administered by the American Bird Conservancy which contains publicly available data on 

reported avian incidents involving pesticides did not report any incidents for metribuzin as of 

May 24, 2012. 

Pesticide registrants report certain types of incidents to the Agency as aggregate counts of 

incidents occurring per product per quarter. Forty one aggregated incidents have been reported 

to the Agency associated with metribuzin. All aggregated incidents are for minor plant damage. 

4.4. Ecosystems Potentially at Risk 

The ecosystems at risk are often extensive in scope; therefore, it may not be possible to identify 

specific ecosystems during the development of a nation-wide ecological risk assessment. 

However, in general terms, terrestrial ecosystems potentially at risk could include the treated 

field and immediately adjacent areas that may receive drift or runoff. Areas adjacent to the 

treated field could include cultivated fields, fencerows and hedgerows, meadows, fallow fields or 

grasslands, woodlands, riparian habitats and other uncultivated areas. 

Aquatic ecosystems potentially at risk include water bodies adjacent to, or downstream from, the 

treated field and could include impounded bodies such as ponds, lakes and reservoirs, or flowing 

waterways such as streams or rivers. For uses in coastal areas, aquatic habitat also includes 

marine ecosystems, including estuaries. 

5. Assessment Endpoints 

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

is to be protected.”2

 Selection of the assessment endpoints is based on valued entities (e.g., fish, 

birds), the ecosystems potentially at risk (e.g., waterbodies, riparian vegetation, and upland 

habitats), the migration pathways of metribuzin (e.g., runoff, drift, etc.), and the routes by which 

ecological receptors are exposed to metribuzin (e.g., direct contact, etc.). 

Assessment endpoints for metribuzin include direct toxic effects on the survival, reproduction, 

and growth of the receptors, as well as indirect effects, such as reduction of the prey base or 

modification of habitat. Each assessment endpoint requires one or more “measures of ecological 

effect,” defined as changes in the attributes of an assessment endpoint or changes in a surrogate 

entity or attribute in response to exposure to a pesticide. Specific measures of ecological effect 

are generally evaluated based on acute and chronic toxicity information from registrantsubmitted guideline tests that are performed on a limited number of organisms. Additional 

ecological effects data from the open literature will also be considered. 

 

2

 From U.S. EPA (1992). Framework for Ecological Risk Assessment. EPA/630/R-92/001. 
-19 of 61-

6. Conceptual Model 

For a pesticide to pose an ecological risk, it must reach ecological receptors in biologically 

significant concentrations. An exposure pathway is the means by which a pesticide moves in the 

environment from a source to an ecological receptor. For an ecological pathway to be complete, 

it must have a source, a release mechanism, an environmental transport medium, a point of 

exposure for ecological receptors, and a feasible route of exposure. 

The conceptual model for metribuzin provides a written description (risk hypothesis) and visual 

representation (conceptual diagram) of the predicted relationships between metribuzin, potential 

routes of exposure, and the predicted effects for the assessment endpoint. A conceptual model 

consists of two major components: risk hypothesis and a conceptual diagram (USEPA 1998). 

Based on the available data (including those from studies with problems, and considering 

existing data gaps), metribuzin is stable in neutral, acidic and basic water. Metribuzin is expected 

to be resistant to biotic degradation under aerobic and anaerobic aquatic conditions; and degrade 

slowly in soil under aerobic conditions. On the other hand; metribuzin photodegrades rapidly in 

clear, shallow surface water. However, direct photolytic degradation of the parent or its 

degradates in turbid and/or deeper waters will be limited by the attenuation of sunlight. 

Metribuzin and its degradates are mobile in all soil conditions. Metribuzin and its degradates can 

be expected to leach to groundwater, where it is expected to persist in the anaerobic conditions 

Metribuzin is not expected to bioaccumulate in aquatic or terrestrial food chains based on its low 

Log Kow value. 

6.1. Risk Hypothesis 

A risk hypothesis describes the predicted relationship among the stressor, exposure, and 

assessment endpoint response along with the rationale for their selection. For metribuzin, the 

following ecological risk hypothesis will be employed for the national-level ecological risk 

assessment: 

Given the uses of metribuzin and its environmental fate properties, there is a likelihood of 

exposure to non-target terrestrial and aquatic organisms. When used in accordance with the 

label, metribuzin may result in potential adverse effects upon the survival, growth, and 

reproduction of non-target terrestrial and aquatic organisms. 

6.2. Conceptual Diagram 

The conceptual model used to depict the potential ecological risk associated with the use of 

metribuzin (the stressor) assumes that as a pesticide, metribuzin can adversely affect terrestrial 
-20 of 61-

and aquatic organisms (the receptors) if environmental concentrations exceed toxic thresholds as 

a result of application according to the label directions (Figures 2 and 3). Ecological receptors 

that may potentially be exposed to metribuzin include terrestrial and semi-aquatic wildlife (e.g., 

mammals, birds, terrestrial-phase amphibians and reptiles), terrestrial and semi-aquatic plants, 

and terrestrial invertebrates (e.g., honey bees). Additionally, given the mobility of metribuzin, 

aquatic organisms (i.e., freshwater and estuarine/marine fish and invertebrates, aquatic-phase 

amphibians, and aquatic plants) are potential receptors in adjacent water bodies through off-site 

transport of metribuzin from the application site through runoff, erosion, and spray drift. 

Figure 2. Conceptual model for metribuzin exposure to aquatic organisms. Dashed lines 

indicate route of exposure considered of lower importance for metribuzin. 

Stressor 

Source

Receptors 

Attribute 

Change 

Pesticide applied to use site 

Spray drift 

Fish/aquatic-phase 

amphibians 

Eggs 

Larvae 

Juveniles / Adults 

Individual organisms 

Reduced survival 

Reduced growth 

Reduced reproduction 

Food chain 

Reduction in algae 

Reduction in prey 

Habitat integrity 

Reduction in primary productivity

Reduced cover 

Community change 

Surface water 

Runoff 

Aquatic Animals 

Invertebrates 

Vertebrates 

Exposure 

Media 

Uptake/gills 

or integument 

Ingestion Ingestion

Atmospheric 

transport 

Wet/dry deposition 

Soil Ground water 

Uptake/gills 

or integument 

Aquatic Plants 

Non-vascular 

Vascular 

Uptake/cell, 

roots, leaves Riparian plant

terrestrial 

exposure 

pathways see 

Figure 2 
-21 of 61-

Figure 3. Conceptual model for metribuzin exposure to terrestrial organisms. Dashed lines 

indicate route of exposure considered of lower importance for metribuzin. 

 
-22 of 61-

7. Analysis Plan 

In order to address the risk hypothesis, the potential for adverse effects on the environment will 

be estimated. The use, environmental fate, and ecological effects of metribuzin will be 

characterized and integrated to assess the risks. This will be accomplished using a risk quotient 

(ratio of exposure concentration to effects concentration) approach. Although risk is often 

defined as the likelihood and magnitude of adverse ecological effects, the risk quotient-based 

approach does not provide a quantitative estimate of likelihood and/or magnitude of an adverse 

effect. However, as outlined in the Overview Document (USEPA 2004), the likelihood of effects 

to individual organisms from particular uses of metribuzin may be estimated using the probit 

dose-response slope and either the level of concern (discussed below) or the actual calculated 

risk quotient value. 

This analysis plan will be revisited and may be revised depending upon the information 

submitted by the public in response to the opening of the Registration Review docket for 

metribuzin. 

7.1. Stressors of Concern 

The stressors of concern in the assessment will be metribuzin and its major degradates DAmetribuzin and DADK-metribuzin. Based on a chronic freshwater invertebrate study with 

DADK-metribuzin and ECOSAR modeling indicate the degradates to be of similar toxicity to 

the parent; therefore a total toxic residue approach will be applied in the risk assessment. The 

uncertainty in exposure due to the use of TTR may be reduced with the submission of aquatic 

toxicity data for the degradates if the data indicates the degradates are not of toxicological 

concern. 

In its ecological risk assessments, the Agency does not routinely include an evaluation of 

mixtures of active ingredients, either those mixtures of multiple active ingredients in product 

formulations or those in the applicator’s tank. In the case of the product formulations of multiple 

active ingredients (that is, a registered product containing more than one active ingredient), each 

active ingredient is subject to an individual risk assessment for regulatory decision regarding the 

active ingredient on a particular use site. If effects data are available for a formulated product 

containing more than one active ingredient, the data may be used qualitatively or quantitatively 

in accordance with the Agency’s Overview Document and the Services’ Evaluation 

Memorandum (USEPA, 2004; Williams and Hogarth, 2004). 

7.2. Measures of Exposure 

In order to estimate risks of metribuzin exposures in aquatic and terrestrial environments, 

exposure modeling will be based on application rates and methods for products registered at the 

time of Registration Review. Measures of exposure are based on aquatic and terrestrial models 

that predict estimated environmental concentrations (EECs) of metribuzin and degradates of 
-23 of 61-

concern. EFED will use standard available models to evaluate potential exposures to aquatic and 

terrestrial organisms as described at http://www.epa.gov/pesticides/science/models_pg.htm

7.3. Measures of Effect 

Ecological effects data are used as measures of direct and indirect effects to biological receptors. 

Effects data are obtained from registrant-submitted studies or from literature studies identified by 

the ECOTOX database. 

The acute measures of effect used for animals in this assessment are the LD50, LC50 and EC50. 

LD stands for "Lethal Dose", and LD50 is the amount of a material, given all at once, that is 

estimated to cause the death of 50% of the test organisms. LC stands for “Lethal Concentration” 

and LC50 is the concentration of a chemical that is estimated to kill 50% of the test organisms. 

EC stands for “Effective Concentration” and the EC50 is the concentration of a chemical that is 

estimated to produce a specific effect in 50% of the test organisms. Endpoints for chronic 

measures of exposure for listed and non-listed animals are the NOAEL/NOAEC. NOAEL stands 

for “No Observed-Adverse-Effect-Level” and refers to the highest tested dose of a substance that 

has been reported to have no harmful (adverse) effects on test organisms. The NOAEC (i.e., 

“No-Observed-Adverse-Effect-Concentration”) is the highest test concentration at which none of 

the observed effects were statistically different from the control. For non-listed plants, only acute 

exposures are assessed (i.e., IC25 for terrestrial plants and IC50 for aquatic plants); for listed 

plants either the NOAEC or IC05 is used. 

Where available, sublethal effects observed in both registrant-submitted and open literature 

studies will be evaluated qualitatively. Such effects may include behavioral changes (e.g., 

lethargy and changes in coloration). However, quantitative assessments of risks are limited to 

those endpoints that can be directly linked to the Agency’s assessment endpoints of impaired 

survival, growth and reproduction. 

In the absence of taxon-specific data, the assessment of risk for direct effects to non-target 

organisms makes the assumption that toxicity of metribuzin to birds is similar to terrestrial-phase 

amphibians and reptiles. The same assumption is made for fish and aquatic-phase amphibians. In 

the absence of data for either acute or chronic effects, the conservative assumption will be to 

presume that metribuzin is toxic. 

7.4. Integration of Exposure and Effects 

Risk characterization is the integration of exposure and ecological effects characterization to 

determine the potential ecological risk from the registered uses of metribuzin and the likelihood 

of direct and indirect effects to non-target organisms in aquatic and terrestrial habitats. The 

exposure and toxicity effects data are integrated in order to evaluate the risks of adverse 

ecological effects on non-target species. For the assessment of metribuzin risks, the risk quotient 

(RQ) method is used to compare exposure and measured toxicity values. EECs are divided by 

acute and chronic toxicity values. The resulting RQs are then compared to the Agency’s Levels 

of Concern (LOCs) (USEPA 2004). These criteria are used to indicate when metribuzin use, as 

directed on the labels, has the potential to cause adverse direct or indirect effects to non-target 
-24 of 61-

organisms. In addition, incident data from the EIIS will be considered as part of the risk 

characterization. 

7.5. Deterministic and Probabilistic Assessment Methods 

The quantitative assessment of risk will primarily depend on the deterministic point-estimate 

(RQ) based approach described in the risk assessment. An effort will be made to further 

qualitatively describe risk using probabilistic tools that the Agency has developed. These tools 

have been reviewed by FIFRA Scientific Advisory Panels 

(http://www.epa.gov/scipoly/sap/index.htm) and have been deemed as appropriate means of 

refining assessments where deterministic approaches have identified risks. Newer tools may be 

available to assess the routes of exposure (e.g. TIM and SWAMP models) and will be applied as 

appropriate in Registration Review. 

7.6. Endocrine Disruptor Screening Program 

As required under FFDCA section 408(p), EPA has developed the Endocrine Disruptor 

Screening Program (EDSP) to determine whether certain substances (including pesticide active 

and other ingredients) may have an effect in humans or wildlife similar to an effect produced by 

a “naturally occurring estrogen, or other such endocrine effects as the Administrator may 

designate.” The EDSP employs a two-tiered approach to making the statutorily required 

determinations. Tier 1 consists of a battery of 11 screening assays to identify the potential of a 

chemical substance to interact with the estrogen, androgen, and or thyroid (E, A, or T) hormonal 

systems. Chemicals that go through Tier 1 screening and are found to have the potential to 

interact with E, A, or T hormonal systems will proceed to the next stage of the EDSP where EPA 

will determine which, if any, of the Tier 2 tests are necessary based on the available data. Tier 2 

testing is designed to identify any adverse endocrine related effects caused by the substance, and 

establish a dose-response relationship between the dose and the E, A, or T effect. 

 

Between October 2009 and February 2010, EPA issued test orders/data call-ins for the first 

group of 67 chemicals, which contains 58 pesticide active ingredients and 9 inert ingredients. 

This list of chemicals was selected based on the potential for human exposure through pathways 

such as food and water, residential activity, and certain post-application agricultural scenarios. 

This list should not be construed as a list of known or likely endocrine disruptors. 

Metribuzin is among the group of 58 pesticide active ingredients on the initial list to be screened 

under the EDSP. The Agency will review the EDSP Tier 1 data and any “other scientifically 

relevant information” submitted in response to test orders. Based on this review the Agency will 

determine the need for additional testing. For further information on the status of the EDSP, the 

policies and procedures, the list of 67 chemicals, the test guidelines and the Tier 1 screening 

battery, please visit our website: http://www.epa.gov/endo/. 

7.7. Endangered Species Assessment 
-25 of 61-

Consistent with the Agency’s responsibility under the Endangered Species Act (ESA), the 

Agency will evaluate risks to Federally-listed threatened and/or endangered (listed) species from 

registered uses of metribuzin. This assessment will be conducted in accordance with the 

Overview Document (USEPA 2004), provisions of the ESA, and the Services’ Endangered 

Species Consultation Handbook (USFWS/NMFS, 1998). 

The assessment of effects associated with the registration of metribuzin is based on an action 

area. The action area is considered to be the area directly or indirectly affected by the federal 

action, as indicated by the exceedance of Agency Levels of Concern (LOCs) used to evaluate 

direct or indirect effects. The Agency’s approach to defining the action area under the provisions 

of the Overview Document (USEPA 2004) considers the results of the risk assessment process to 

establish boundaries for that action area with the understanding that exposures below the 

Agency’s defined LOCs constitute a no-effect threshold. For the purposes of this assessment, 

attention will be focused on the footprint of the action (i.e., the area where metribuzin application 

occurs), plus all areas where offsite transport (i.e., spray drift, runoff, etc.) may result in potential 

exposure that exceeds the Agency’s LOCs. Specific measures of ecological effect that define the 

action area for listed species include any direct and indirect effects and/or potential modification 

of its critical habitat, including reduction in survival, growth, and reproduction as well as the full 

suite of sublethal effects available in the effects literature. Therefore, the action area extends to a 

point where environmental exposures are below any measured lethal or sublethal effect threshold 

for any biological entity at the whole organism, organ, tissue, and cellular level of organization. 

In situations where it is not possible to determine the threshold for an observed effect, the action 

area will initially be assumed to include all counties where metribuzin is used in the United 

States. 

7.8. Drinking Water Assessment 

In order to bring the drinking water exposure assessment up to date with current data, models 

and simulation model guidance, a new drinking water assessment will be conducted to support 

future human health dietary risk assessments of metribuzin. The drinking water exposure 

assessment will be conducted for the maximum labeled use in support of the human health 

dietary risk assessment. The drinking water assessment will incorporate model estimates of 

metribuzin in surface water and ground water including any residues that are determined to be of 

toxicological concern. Concentrations of metribuzin and degradate in surface waters (Estimated 

Drinking Water Concentrations, or EDWCs) will be estimated using PRZM (Version 3.12.2, 

May 12, 2005), EXAMS (Version 2.98.04.06, Apr. 25, 2005), but using the Index Reservoir 

scenario rather than the standard pond used for assessment of exposure in drinking water. 

An EDWC of metribuzin in ground water will be estimated using EFED’s Tier I aquatic model 

SCIGROW (Screening Concentration in Ground Water, version 2.3; 8/8/2003). SCIGROW is a 

regression model used as a screening tool to estimate pesticide concentrations found in ground 

water used as drinking water. The output of SCIGROW represents the concentrations that might 

be expected in shallow unconfined aquifers under sandy soils, which is representative of the 

ground water most vulnerable to pesticide contamination likely to serve as a drinking water 

source. If a refined drinking water assessment is requested or if considered appropriate at the 
-26 of 61-

time of assessment, an appropriate Tier II groundwater model may be used to determine 

EDWCs. 

7.9. Environmental Monitoring Data 

Surface water:

Surface water monitoring data from the United States Geological Survey (USGS) NAWQA 

program were obtained on May 23, 2012. Of the 14,779 tests for metribuzin, 3,702 (25%) 

resulted in detections of metribuzin. Detection concentrations ranged from the lowest LOD of 

0.004 μg/L up to 6.61 μg/L detected in the Tensas River in Louisiana. 

 

Limited surface water monitoring data is available for metribuzin degradates. Only 2 tests were 

made in one state (Iowa). The only degradate that was analyzed for was DADK metribuzin; no 

detections were reported. The extent and magnitude of metribuzin degradates in surface waters 

is an uncertainty. 

Groundwater:

Ground water monitoring data from the United States Geological Survey (USGS) NAWQA 

program were obtained on May 23, 2012. A total of 13,397 water samples across various sites 

throughout the US were analyzed for metribuzin. There were 195 detections of metribuzin 

which ranged from 0.0016 to 3.69 μg/L. 

 

A Prospective Groundwater (PGW) study was conducted near Minden, Nebraska (MRID 

45737901). Metribuzin was applied at a rate of 0.816 lb a.i./ac. Total metribuzin residues were 

measured in multiple wells at the site throughout the duration of the study, with a peak 

concentrations of 1.64 μg/L occurring 2,210 days after treatment. The study was terminated 

shortly after these peak detections were recorded because bromide tracer concentrations were no 

longer increasing. 

8.0. Preliminary Identification of Data Gaps 

 8.1. Environmental Fate 

The available environmental fate and transport data are considered incomplete for the purposes 

of risk assessment. The following data gaps exist: 

 Aerobic Aquatic Metabolism (OCSPP guideline # 835.4200) Further data are needed to 

support risk assessment. Concentrations in submitted soil metabolism studies fluctuated 

over time and additional metabolism studies would aid characterization and reduce 

uncertainty. 
-27 of 61-

 Anaerobic Aquatic Metabolism (OCSPP guideline # 835.4400) Further data are needed 

to support risk assessment. Concentrations in submitted soil metabolism studies 

fluctuated over time and additional metabolism studies would aid characterization and 

reduce uncertainty. 

 Environmental Chemistry Methods and Independent Laboratory Validation (OCSPP 

guideline # 850.6100) This study is required to corroborate methods used in 

environmental fate studies and to assist in the enforcement of pesticide tolerances. 

Table 8 identifies studies by MRID that offer data for each guideline requirement, as well as 

study classifications and whether or not further data are needed in order to support risk 

assessment (i.e., whether there is a data gap). 

Table 8. Summary of available environmental fate data. 

Guideline Study Title MRID / 

ACC No. Classification Data Gap? Comments 

835.2120 Hydrolysis 00157530 Acceptable No 

835.2240 Photodegradation 

in Water 00157529 Acceptable No 

835.2410 Photodegradation 

on Soil 00157531 Acceptable No 

835.4100 Aerobic Soil 

Metabolism 40367602 Acceptable No 

835.4200 Anaerobic Soil 

Metabolism 

40367603 

41555105 

Acceptable 

In Review No 

835.4300 Aerobic Aquatic 

Metabolism 

159423 In Review Yes 

Further data are needed to support risk 

assessment. Concentrations in submitted soil 

metabolism studies fluctuated over time and 

additional metabolism studies would aid 

characterization and reduce uncertainty. In the 

absence of data, it will be assumed that the 

aerobic aquatic metabolism half life is double that 

of the aerobic soil metabolism half life. 

835.4400 

Anaerobic 

Aquatic 

Metabolism 

159423 In Review Yes 

Further data are needed to support risk assessment. 

Concentrations in submitted soil metabolism studies 

fluctuated over time and additional metabolism 

studies would aid characterization and reduce 

uncertainty. In the absence of data, it will be 

assumed that the anaerobic aquatic metabolism half 

life is double that of the anaerobic soil metabolism 

half life 

835.1240, 

835.1230 

Mobility - 

Adsorption / 

Desorption 

1) 42283001 

2) 43058502 

3) 43058501 

Acceptable No 

835.6100 Terrestrial Field 

Dissipation 

40380901 

 42236101 

Supplemental 

Invalid Yes 

Terrestrial field dissipation is considered a data gap 

because leaching was not accounted but the 

prospective ground water study accounts for 

leaching. Therefore, a new terrestrial field 

dissipation study is not being requested at this time. 

835.7100 Groundwater 

monitoring 45737901 In review No 
-28 of 61-

8.2. Ecological Effects 

The available ecological effects database for metribuzin is incomplete (Tables 9-11). Acute 

avian oral toxicity data (850.2100) on a passerine species and new avian reproduction data 

(850.2300) with the bobwhite quail are necessary: 

 OCSPP 850.2100- Avian Acute Oral. An avian acute oral toxicity study with a 

passerine is necessary for risk assessment. Additionally the available avian acute 

oral study resulted in a LD50 value of 169.2 mg a.i./kg-bw which classifies 

metribuzin as moderately toxic to bobwhite quail via oral exposure. Therefore 

additional data are necessary to strengthen the risk assessment for birds from acute 

effects since the subacute dietary studies via the dietary route indicate metribuzin is not 

toxic to birds. It is recommended that a definitive study be conducted; however, a limit 

test with a higher exposure concentration (to be determined through consultation with the 

Agency) may also be acceptable. 

 OCSPP 850.2300- Avian Reproduction. An avian reproduction toxicity study with 

the Northern bobwhite quail is necessary for risk assessment. The available avian 

reproduction studies with bobwhite quail and mallard duck indicated no doseresponse effects were observed at any concentrations including the highest test 

concentrations tested. However, in the RED, the risk assessment suggests the risk to 

birds exists based on the toxicity value selected from the reproduction studies for use in 

the avian risk assessment. But a closer look of the study indicated the test concentrations 

were not tested high enough or at/above environmentally relevant concentrations, which 

explain why the LOC exceedance exists when no effects occurred; it is uncertain if there 

is actually any risks to birds. A new study with higher test concentrations (up to 1,500 mg 

a.i./kg diet) should address the risk uncertainty. The recommended test species for the 

new reproduction study is the Northern bobwhite quail since acute effects were observed 

in this species in the avian acute oral toxicity test. 
-29 of 61-

TABLE 9. Available ecological effects data for terrestrial animals exposed to metribuzin

Guideline Description MRID/ 

Accession Classification Data Gap? Comments 

850.2100 Avian oral toxicity 255025 Acceptable Yesa Avian acute oral toxicity data 

are not available for 

passerines, which are required 

under 40 CFR Part 158. 

Additionally, the available 

acute oral study classified 

metribuzin as moderately 

toxic to upland game birds via 

the oral route. A passerine 

study would strengthen the 

oral route toxicity 

classification since available 

subacute dietary data indicate 

metribuzin is not toxic via the 

dietary route. 

There were LOC exceedances 

in the previous risk assessment 

for reproductive and growth 

risk to birds at concentrations 

where no reproductive effects 

were observed. To address this 

uncertainty, the test 

concentrations in the avian 

reproduction studies need to 

be tested high enough or at 

environmentally relevant 

concentrations to better assess 

chronic risk to birds. The 

recommended species is the 

bobwhite quail. 

850.2200 Avian dietary 

toxicity 

262228 Acceptable No 

065507 Acceptable

850.2300 Avian reproduction 43926601 Acceptable Yes 

43860501 Acceptable

850.3020 Honeybee acute 

contact toxicity 028772 Acceptable No 

a

Bold indicates data necessary for risk assessment 

TABLE 10. Available ecological effects data for aquatic animals exposed to metribuzin and its degradate 

DADK.

Guideline Description MRID/ 

Accession Classification Data Gap? Comments 

850.1075 Freshwater fish – 

Acute toxicity 

40098001 

255025 

090427 

Acceptable No 

850.1075 

Estuarine/marine 

fish – 

Acute toxicity 

42094502 Acceptable No 

850.1010 

Freshwater 

invertebrates – 

Acute toxicity 

72083 

34016 Acceptable No 
-30 of 61-

Guideline Description MRID/ 

Accession Classification Data Gap? Comments 

850.1035 

850.1055 

Estuarine/marine 

invertebrates –Acute 

toxicity 

43851 

106197 

42094501 

47023411 

Supplemental 

Acceptable 

Acceptable 

Supplemental 

No 

An ACR will be applied in the 

absence of data. 

106197 Acceptable No 

850.1300 

Freshwater 

invertebrate – 

 life cycle test 

42447802 

Acceptable No 42732501 

(DADK, a 

degradate) 

850.1350 

Estuarine/marine 

invertebrates – 

life cycle test 

-- -- Noa 

850.1400 Freshwater fish – 

early life stage test 

42447801 Acceptable No 

850.1400 

Estuarine/marine 

fish – 

early life stage test 

-- -- Noa

a Conditionally required when 40 CFR 158 criterions are triggered; however, none of the criteria were triggered, data 

are not required. 

TABLE 11. Available ecological effects data for plants exposed to metribuzin.

Guideline Description MRID Classification Data 

Gap? comments 

850.4100 

Terrestrial Plant 

toxicity: Tier II 

seedling emergence 

42447803 

43208301 Acceptable No 

850.4150 

Terrestrial Plant 

toxicity: Tier II 

vegetative vigor 

42447803 Acceptable No 

850. 5400 Aquatic Plant Growth: 

algae 

43133601 

43867701 

43826101 

43893502 

Acceptable 

Acceptable 

Acceptable 

Supplemental

No

850.4400 Aquatic Plant Growth: 

vascular plants 43893501 Supplemental No 

9. Other Information Needs 

There is specific information that will assist the Agency in refining the ecological risk 

assessment, including any species-specific effects determinations. The Agency is very much 

interested in obtaining the following information: 

1. confirmation on the following label information 

a. frequency of application, application intervals, and maximum number of 

applications per season 
-31 of 61-

b. geographic limitations on use 

2. use or potential use distribution (e.g., acreage and geographical distribution of relevant 

crops) 

3. use history 

4. median and 90th percentile reported use rates (lbs. a.i./acre) from usage data – national, 

state, and county 

5. application timing (date of first application and application intervals) by crop – national, 

state, and county 

6. More specific definitions of non-agricultural use areas 

7. sub-county crop location data 

8. directly acquired county-level usage data (not derived from state level data) 

a. maximum reported use rate (lbs. a.i./acre) from usage data – county 

b. percent crop treated – county 

c. median and 90th percentile number of applications – county 

d. total pounds per year – county 

e. the year the pesticide was last used in the county/sub-county area 

f. the years in which the pesticide was applied in the county/sub-county area 

9. state or local use restrictions 

10. ecological incidents (non-target plant damage and avian, fish, reptilian, amphibian and 

mammalian mortalities) not already reported to the Agency 

11. monitoring data 

10. References 

Open Literature Citations 

Fletcher, J.S., J.E. Nellessen, and T.G. Pfleeger. 1994. Literature review and evaluation of the 

EPA food-chain (Kenaga) nomogram, an instrument for estimating pesticide residues on 

plants. Environ. Tox. Chem. 13:1383-1391. 

Hoerger, F., and E.E. Kenaga. 1972. Pesticide residues on plants: Correlation of representative 

data as a basis for estimation of their magnitude in the environment. In F. Coulston and 

F. Korte, eds., Environmental Quality and Safety: Chemistry, Toxicology, and 

Technology, Georg Thieme Publ, Stuttgart, West Germany, pp. 9-28. 

U.S. Fish and Wildlife Service (USFWS) and National Marine Fisheries Service (NMFS). 1998. 

Endangered Species Consultation Handbook: Procedures for Conducting Consultation 

and Conference Activities Under Section 7 of the Endangered Species Act. Final Draft. 

March 1998. 

USEPA. 1998. Guidelines for Ecological Risk Assessment. Risk Assessment Forum, Office of 

Research and Development, Washington, D.C. EPA/630/R-95/002F. April 1998. 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=30759
-32 of 61-

U.S. EPA. 2002. Refined Estimated Environmental Concentrations (EECs) for Metribuzin and 

its Degradate Desmethylmetribuzin in Surface Water and Groundwater. DP 282196. 

Washington, D.C. May 1, 2002. 

USEPA. 2004. Exposure Analysis Modeling System (EXAMS): User Manual and System 

Documentation. EPA/600/R-00/81-023. May 2004. Ecosystems Research Division. 

United States Environmental Protection Agency. Available at 

http://www.epa.gov/ceampubl/swater/exams/exams29804/EXAMREVG.PDF (Accessed 

September 2, 2011). 

USEPA. 2004. Overview of the Ecological Risk Assessment Process in the Office of Pesticide 

Programs. U.S. Environmental Protection Agency, Office of Prevention, Pesticides and 

Toxic Substances, Office of Pesticide Programs, Washington DC. January 23, 2004. 

USDA, National Agricultural Statistics Service. Citrus Fruits 2011 Summary (September 2011). 

Accessed on March 19, 2012 @ 

http://usda01.library.cornell.edu/usda/current/CitrFrui/CitrFrui-09-22-2011.pdf. 

USEPA. 2011. Guidance On Using Non-Definitive Endpoints In Evaluating Risk To Listed And 

Nonlisted Animal Species. Memo from Donald J. Brady, Ph.D., Director, Environmental 

Fate and Effects Division. 5/11/2011. 

Willis, G.H. and L.L. McDowell. 1987. Pesticide Persistence on Foliage in Reviews of 

Environmental Contamination and Toxicology. 100:23-73. 

Submitted Fate Studies

161-1 Hydrolysis 

MRID Citation Reference 

66193 Day, E.W.; Sullivan, W.L.; Decker, O.D. (1976) A Hydrolysis Study of the Herbicides Oryzalin 

and Metribuzin. (Unpublished study received Sep 16, 1977 under 1471-96; submitted by Elanco 

Products Co., Div. of Eli Lilly and Co., Indianapolis, Inc.; CDL:231825-D) 

106169 Mobay Chemical Corp. (1972) Sencor (BAY 94337): Metabolic, Analyt- ical, and Residue 

Information on Tomatoes. (Compilation; un- published study received Apr 25, 1972 under 

4F1432; CDL: 093821-A) 

145117 Mobay Chemical Corp. (1985) Product Chemistry of Tycor Technical. Unpublished 

compilation. 71 p. 

157530 Morgan, J. (1986) The Hydrolysis of Sencor-5-[carbon 14]: Mobay Report No. 91764. 

Unpublished study prepared by Mobay Chemical Corp. 11 p. 

161-2 Photodegradation-water 

MRID Citation Reference 
-33 of 61-

153722 Timme, G.; Scholz, K. (1980) Behaviour in Water: Sencor. Unpublished Mobay report no. 

69692 prepared by Bayer AG. 4 p. 

153725 Timme, G.; Scholz, K. (1980) Photodegradation [of Sencor in Water and Air]. Unpublished 

study prepared by Bayer AG. 9 p. 

156339 Rosen, J.; Siewierski, M. (1971) Photolysis of 4-Amino-3-methyl- thio-6-phenyl-1,2,4-triazin-5-

one (Aglpt). Bulletin of Environ- mental Contamination 6(5):406-408. 

157529 Morgan, J. (1986) The Aqueous Photolysis of Sencor-5-[carbon 14]: Mobay Report No. 91763. 

Unpublished study prepared by Mobay Chemical Corp. 21 p. 

161-3 Photodegradation-soil 

MRID Citation Reference 

30169 Pinney, P.; Miaullis, J.B. (1979) Project 148237 R-33865: Soil Metabolism Studies--Interim 

Report No. 3. (Unpublished study received Mar 21, 1980 under 476-EX-96; submitted by 

Stauffer Chemical Co., Richmond, Calif.; CDL:242046-C) 

45272 Houseworth, L.D.; Tweedy, B.G. (1974) Photodecomposition of Sencor: Report No. 39414. 

(Unpublished study received Mar 7, 1975 under 5F1559; prepared by Univ. of Missouri, Dept. 

of Plant Pathology, submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL: 094871-S) 

106220 Mobay Chemical Corp. (1976) Supplement No. 2 to Brochure Entitled: Sencor, the Effects on 

the Environment: Document No. AS77-335. (Compilation; unpublished study received Feb 28, 

1977 under 3125-277; CDL:238058-A) 

153666 Murphy, J.; Thornton, J.; Olson, T. (19??) Metabolism and Persis- tence of SENCOR in Soils: 

Mobay Report No. 44078. Unpublished study prepared by Chemagro. 8 p. 

153667 Hyzak, D.; Zimdahl, R. (1974) Rate of degradation of metribuzin and two analogs in soils. Weed 

Science 22(1):75-79. 

153668 Bouchard, D.; Lavy, T.; Marx, D. (1982) Fate of metribuzin, meto- lachlor, and fluometuron in 

soil. Weed Science 30:629-632. 

153693 Webster, G.; Sama, L.; Macdonald (1978) Nonbiological degradation of the herbicide 

metribuzin in Manitoba soils. Bull. Environm. Contam. Toxicol. 20:401-408. 

156338 Stegh, R.; Wilmes, R. (1982) Light-Induced Degradation of Agro- chemical Parent Compounds 

on Silica Gel as a Model for Photode- gradation on Soil: Report No. 1666. Unpublished Mobay 

report no. 82363 prepared by Bayer AG. 40 p. 

157531 Morgan, J. (1986) The Photodecomposition of Sencor-5-[Carbon 14] on Soil: Mobay Report No. 

91765. Unpublished study prepared by Mobay Chemical Corp. 26 p. 

41555103 Hanlon, C. (1989) Photodegradation of Metribuzin on Soil: Lab Pro- ject Number: 99608. 

Unpublished study prepared by Mobay Corp., Ag. Chem. Div. 30 p. 

161-4 Photodegradation-air 

MRID Citation Reference 

153725 Timme, G.; Scholz, K. (1980) Photodegradation [of Sencor in Water and Air]. Unpublished 

study prepared by Bayer AG. 9 p. 
-34 of 61-

162-1 Aerobic soil metabolism 

MRID Citation Reference 

54367 Sharom, M.; Stephenson, ? (1976) Behavior and fate of Metribuzin. Weed Science 24(2):153-

160. (Submitter report no. 49127; also in unpublished submission received Aug 14, 1980 under 

3125-314; submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL: 243067-T) 

153677 Ladlie, J.; Meggitt, W.; Penner, D. (1976) Effect of soil pH on microbial degradation, 

adsorption, and mobility of metribuzin. Weed Science 24(5):477-481. 

161501 Lane, J. (1986) Sencor Aged Soil Residue Leaching: Report No. 93110. Unpublished report 

prepared by Mobay Corporation. 24 p. 

40367602 Detra, R.; Lane, J.; Thyssen, M. (1987) Degradation of Sencor in Aerobic Sandy Loam: Rept. 

No. 94880. Unpublished study prepared by Mobay Corp. 44 p. 

41555104 Lenz, M. ; Parker, G. (1989) Degradation of Metribuzin in Aerobic Sandy Loam: Lab Project 

Number: 99775. Unpublished study pre- pared by Mobay Corp., Ag. Chem. Div. 47 p. 

162-2 Anaerobic soil metabolism 

MRID Citation Reference 

40367603 Detra, R.; Thyssen, M. (1987) Degradation of Sencor in Anaerobic Sandy Loam: Rept. No. 

94881. Unpublished study prepared by Mobay Corp. 43 p. 

41555105 Lenz, M. ; Parker, G. (1989) Degradation of Metribuzin in Anaerobic Sandy Loam: Lab Project 

Number: 99774. Unpublished study pre- pared by Mobay Corp., Ag. Chem. Div. 45 p. 

162-3 Anaerobic aquatic metab. 

MRID Citation Reference 

159423 McNamara, F. (1985) The Degradation of Sencor in Aerobic and An- aerobic Aquatic Systems: 

90384. Unpublished study prepared by Mobay Chemical Corp. 27 p. 

162-4 Aerobic aquatic metab. 

MRID Citation Reference 

159423 McNamara, F. (1985) The Degradation of Sencor in Aerobic and An- aerobic Aquatic Systems: 

90384. Unpublished study prepared by Mobay Chemical Corp. 27 p. 

163-1 Leach/adsorp/desorption 

MRID Citation Reference 

25729 Obrist, J.J.; Thornton, J.S. (1979) Soil Thin-Layer Mobility of Baycor(TM), (R)Baytan, 

(R)Drydene and Peropal(TM). (Unpublished study received Dec 21, 1979 under 3125-EX-168; 

prepared in cooperation with Agricultural Consultants, Inc.; submitted by Mobay Chemical 

Corp., Kansas City, Mo.; CDL:099185-M) 

29887 Thornton, J.S.; Hurley, J.B.; Obrist, J.J. (1976) Soil Thin-Layer Mobility of Twenty Four 
-35 of 61-

Pesticides (sic) Chemicals: Report No. 51016. (Unpublished study received Jan. 28, 1980 under 

5F1547; submitted by Mobay Chemical Corp., Pittsburgh, Pa.; CDL: 099216-I) 

30169 Pinney, P.; Miaullis, J.B. (1979) Project 148237 R-33865: Soil Metabolism Studies--Interim 

Report No. 3. (Unpublished study received Mar 21, 1980 under 476-EX-96; submitted by 

Stauffer Chemical Co., Richmond, Calif.; CDL:242046-C) 

30654 Monsanto Company (19??) The Soil Dissipation of Glyphosate, Alachlor and Metribuzin 

Mixtures. (Unpublished study received Jan 16, 1978 under 524-285; CDL:232680-E) 

44979 Ford, J.J. (1979) Herbicide Combination--Soil Dissipation Study In- volving Antor Herbicide 

with Three Commercial Herbicides: RI 47- 003-06. (Unpublished study received May 3, 1979 

under 43142-1; submitted by Hercules, Inc., Wilmington, Del.; CDL:098274-L) 

45268 Houseworth, L.D.; Tweedy, B.G. (1973) Report on Parent Leaching Studies for ^(R)ISencor: 

Report No. 37180. (Unpublished study received Mar 7, 1975 under 5F1559; prepared by Univ. 

of Mis- souri, Dept. of Plant Pathology, submitted by Mobay Chemical Corp., Kansas City, Mo.; 

CDL:094871-N) 

45276 Tweedy, B.G.; Houseworth, L.D. (1974) Leaching of Aged Residues of Sencor-3-14C in Sandy 

Loam Soil: Report No. 40567. (Unpub- lished study received Mar 7, 1975 under 5F1559; 

prepared by Univ. of Missouri, Dept. of Plant Pathology, submitted by Mobay Chemical Corp., 

Kansas City, Mo.; CDL:094870-D) 

45277 Flint, D.R.; Shaw, H.R., II (1974) The Mobility of Sencor Residues in Soil Runoff Water: 

Report No. 40851. (Unpublished study received Mar 7, 1975 under 5F1559; prepared in 

cooperation with Agricultural Consultants Laboratory, submitted by Mobay Chemical Corp., 

Kansas City, Mo.; CDL:094870-H) 

54234 United States Borax Research Corporation (1975) Cobex^(R)I plus Sencor^(R)I (or Lexone)--

Degradation in Soil. (Unpublished study received May 27, 1975 under 1624-104; CDL:223287-

C) 

54367 Sharom, M.; Stephenson, ? (1976) Behavior and fate of Metribuzin. Weed Science 24(2):153-

160. (Submitter report no. 49127; also in unpublished submission received Aug 14, 1980 under 

3125-314; submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL: 243067-T) 

54368 Savage, K.E. (1976) Adsorption and mobility of Metribuzin in soil. Weed Science 24(5):525-

528. (Submitter report no. 51903; also in unpublished submission received Aug 14, 1980 under 

3125-314 submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL: 243067-V) 

68125 Ciba-Geigy Corporation (1976) ?Residue Data|. (Compilation; unpublished study received Oct 

12, 1976 under 100-523; CDL:230858-A) 

106163 Mobay Chemical Corp. (1969) Bay 94337 Metabolic, Analytical, and Residue Information on 

Potatoes and Soybeans. (Compilation; unpublished study received Jan 16, 1970 under 0G0940; 

CDL: 091607-A) 

106169 Mobay Chemical Corp. (1972) Sencor (BAY 94337): Metabolic, Analyt- ical, and Residue 

Information on Tomatoes. (Compilation; un- published study received Apr 25, 1972 under 

4F1432; CDL: 093821-A) 

106216 Horton, W. (1978) Disappearance of Fluchloralin and Metribuzin in Silt Loam and Sandy Loam 

Soils. (Unpublished study received Nov 8, 1978 under 7969-46; submitted by BASF Wyandotte 

Chemical Corp., Parsippany, NJ; CDL:235698-C) 

134358 Mobay Chemical Corp. (1979) ?Efficacy: Sencor|. (Compilation; unpublished study received 

Aug 14, 1980 under 3125-314; CDL: 243066-A) 

137540 Obrist, J.; Thorton, J. (1982) Soil Thin-layer Mobility of Bay FCR 1272, Bay Sir 8514, and Bay 
-36 of 61-

SSH 0860: 82232. (Unpublished study received Feb 13, 1984 under 3125-351; submitted by 

Mobay Chemi- cal Corp., Kansas City, MO; CDL:072361-D) 

143037 Liu, L.; Cibes-Viade, H. (1972) Absorption of fluometuron, prome- tryne, sencor, and 2,4-D by 

soils. Journal of Agriculture of University of Puerto Rico p. 286-293. 

153668 Bouchard, D.; Lavy, T.; Marx, D. (1982) Fate of metribuzin, meto- lachlor, and fluometuron in 

soil. Weed Science 30:629-632. 

153673 Jarczyk, H. (1972) Migration of herbicides in different soil types. Pflanzenschutz-Nachrichten 

25(1):3-19. 

153674 Scott, H.; Phillips, R.; Paetzold, R. (19??) Diffusion of Herbi- cides in the Adsorbed Phase. 

Unpublished study prepared by Univ. of Arkansas in cooperation with Univ. of Kentucky. 21 p. 

153675 Scott, H. (1975) Movement of Pesticides in the Soil-water-fertili- zer System: Project No.: A021-ARK. Unpublished study prepared by Water Resources Research Center in cooperation 

with Univ. of Arkansas. 129 p. 

153677 Ladlie, J.; Meggitt, W.; Penner, D. (1976) Effect of soil pH on microbial degradation, 

adsorption, and mobility of metribuzin. Weed Science 24(5):477-481. 

153682 Lafleur, K. (1980) Metribuzin movement in soil columns: Observation and prediction. Soil 

Science 129(2):107-114. 

153683 Lafleur, K. (1979) Sorption of metribuzin by model soils and agro- nomic soils: Rates and 

equilibria. Soil Science 127(1):51-55. 

153685 Schmidt, R. (1973) Calculations on the Herbicidal Activity of Metribuzin in Relation to Various 

Soil Properties. Unpublished paper presented at Proc. Eur. Weed Res. Coun. Symp. HerbicidesSoil; 1973, Leverkusen, Germany. 3 p. 

153723 Timme, G.; Scholz, K. (1980) Behaviour in Soil [of Sencor]. Unpub- lished study prepared by 

Bayer AG. 26 p. 

159418 Pettygrove, D.; Naylor, D. (1985) Metribuzin degradation kinetics in organically amended soil. 

Weed Science 33(2):267-270. 

42201607 Boesten, J.; Van Der Pas, L. (1988) Modeling adsorption/ desorption kinetics of pesticides in a 

soil suspension. Soil Science 146(4):221-231. 

42283001 Daly, D. (1988) Soil Adsorption/Desorption with ?Carbon 14|-Sencor: Lab Project Number: 

#36997. Unpublished study prepared by Analytical Bio-Chemistry Laboratories, Inc. 54 p. 

43058501 Blumhorst, M.; Halarnkar, P. (1993) Soil Adsorption/Desorption of Metribuzin-DK: Lab Project 

Number: 122S17: SE182101: 106233. Unpublished study prepared by Bio-Analytical Services, 

Inc. 58 p. 

43058502 Blumhorst, M.; Halarnkar, P. (1993) Soil Adsorption/Desorption of Metribuzin-DADK: Lab 

Project Number: 122S18: SE182102: 106234. Unpublished study prepared by EPL BioAnalytical Services, Inc. 58 p. 

163-2 Volatility - lab 

MRID Citation Reference 

150930 Obrist, J.; Thornton, J. (1976) Soil Thin-layer Mobility of Various Pesticides: Report No.: 

48887. Unpublished study prepared by Mobay Chemical Corporation. 8 p. 

153672 Savage, K. (1973) Sencor Mobility [in Soil]: Mobay Report No. 37782. Unpublished study 
-37 of 61-

prepared by USDA and Chemagro. 2 p. 

153677 Ladlie, J.; Meggitt, W.; Penner, D. (1976) Effect of soil pH on microbial degradation, 

adsorption, and mobility of metribuzin. Weed Science 24(5):477-481. 

163-3 Volatility - field 

MRID Citation Reference 

153653 Chemagro (1984) SENCOR: Environmental Chemistry. Unpublished com- pilation. 12 p. 

153721 Mobay Chemical Corp. (1978) [Residue Data and Soil Persistence of SENCOR]. Unpublished 

compilation. 776 p. 

164-1 Terrestrial field dissipation 

MRID Citation Reference 

16613 Ballantine, L.G.; Herman, M.M. (1978) Metolachlor plus Propazine Tank Mix Soil Dissipation: 

Report No. ABR-78037. Summary of studies 237816-D and 237816-E. (Unpublished study 

received Mar 16, 1979 under 100-604; submitted by Ciba-Geigy Corp., Greens- boro, N.C.; 

CDL:237816-A) 

26872 Moyer, M.; Potts, C.; Devine, J.M.; et al. (1975) Prowl^(R)I(CL 92,553): Determination of CL 

92,553 ?N-(1-Ethylpropyl)-3,4-di- methyl-2,6-dinitrobenzenamine| and Sencor ?4-Amino-6-tbutyl-3- (methylthio)-1,2,4-triazin-5-one| in Soil: Report No. C-859. (Unpublished study 

received Nov 20, 1975 under 6F1704; submitted by American Cyanamid Co., Princeton, N.J.; 

CDL:094647-H) 

26873 Potts, C.R.; Laporta, M.M.; Devine, J.; et al. (1975) Prowl^(R)I (CL 92,553): Determination of 

CL 92,553 ?N-(1-Ethylpropyl)-3,4- dimethyl-2,6-dinitrobenzenamine| and Sencor ?4-Amino-6-

t-butyl- 3-(methylthio)-1,2,4-triazin-5(4H)-one| in Soil: Report No. C- 801. (Unpublished study 

received Nov 20, 1975 under 6F1704; submitted by American Cyanamid Co., Princeton, N.J.; 

CDL: 094647-I) 

27120 Muir, D.C.; Baker, B.E. (1976) Detection of Triazine herbicides and their degradation products 

in tile-drain water from fields under intensive corn (maize) production. Journal of Agricultural 

and Food Chemistry 24(1):122-125. (Also in unpublished submission received Jul 19, 1978 

under 201-403; submitted by Shell Chemical Co., Washington, D.C.; CDL:234475-Q) 

54367 Sharom, M.; Stephenson, ? (1976) Behavior and fate of Metribuzin. Weed Science 24(2):153-

160. (Submitter report no. 49127; also in unpublished submission received Aug 14, 1980 under 

3125-314; submitted by Mobay Chemical Corp., Kansas City, Mo.; CDL: 243067-T) 

61420 Waggoner, T.B. (1973) Survey of Sencor Residues in Soils from the United States: Report No. 

37660. Rev. (Unpublished study re- ceived Sep 13, 1976 under 3125-277; submitted by Mobay 

Chemical Corp., Kansas City, Mo.; CDL:226759-E) 

61423 Olson, T.J. (1974) Survey of Sencor Residues in Soils from the United States and Canada--1973: 

Report No. 41627. (Unpublished study received Sep 13, 1976 under 3125-277; submitted by 

Mobay Chemical Corp., Kansas City, Mo.; CDL:226759-J) 

68125 Ciba-Geigy Corporation (1976) ?Residue Data|. (Compilation; unpublished study received Oct 

12, 1976 under 100-523; CDL:230858-A) 

101533 Danhaus, R.; Lenox, E.; Dubelman, S.; et al. (1982) Dissipation of Alachlor in Field Soils 

following Preemergent Applications of Lasso ME Alone or in Tank Mix Combinations with 
-38 of 61-

Roundup, Atrazine, Dyanap, Metribuzin or Cyanazine: MSL-2109. (Unpub- lished study 

received May 10, 1982 under 524-344; prepared in cooperation with ABC Laboratories, Inc. and 

Craven Laboratories, Inc., submitted by Monsanto Co., Washington, DC; CDL:070841-D) 

106163 Mobay Chemical Corp. (1969) Bay 94337 Metabolic, Analytical, and Residue Information on 

Potatoes and Soybeans. (Compilation; unpublished study received Jan 16, 1970 under 0G0940; 

CDL: 091607-A) 

106169 Mobay Chemical Corp. (1972) Sencor (BAY 94337): Metabolic, Analyt- ical, and Residue 

Information on Tomatoes. (Compilation; un- published study received Apr 25, 1972 under 

4F1432; CDL: 093821-A) 

106206 Mobay Chemical Corp. (1972) Supplement No. 4 to Brochure Entitled: Sencor: The Effects on 

the Environment: Document No. AS77-1968. (Compilation; unpublished study received Dec 5, 

1977 under 3125-277; CDL:232432-A) 

106805 Devine, J.; Kust, C.; Moyer, M.; et al. (1975) Prowl (CL 92,553): Determination of CL 92,553 ... 

and Sencor ... in Soil. Indi- ana.: Report No. C-859. (Unpublished study received Nov 14, 1975 

under 6F1704; submitted by American Cyanamid Co., Prince- ton, NJ; CDL:097956-E) 

106806 Devine, J.; Kust, C.; Potts, C.; et al. (1975) Prowl (CL 92,553): Determination of CL 92,553 ... 

and Sencor ... in Soil. Kan- sas. 1974: Report No. C-801. (Unpublished study received Nov 14, 

1975 under 6F1704; submitted by American Cyanamid Co., Princeton, NJ; CDL:097956-F) 

135055 American Cyanamid Co. (1977) Combat (=CL 206,784) Herbicide Residue and Metabolism. 

(Compilation; unpublished study received Feb 23, 1978 under 241-EX-89; CDL:096835-A) 

152205 Danhaus, R. (1982) Dissipation of Glyphosate in Field Soils follow- ing Minimum Till 

Application of Roundup Alone or in Tank Mix Combinations with Lasso ME, Atrazine, Dyanap 

or Metribuzin: Re- port No. MSL-2422. Unpublished study prepared by Monsanto Co. and ABC 

Laboratories, Inc. 143 p. 

153675 Scott, H. (1975) Movement of Pesticides in the Soil-water-fertili- zer System: Project No.: A021-ARK. Unpublished study prepared by Water Resources Research Center in cooperation 

with Univ. of Arkansas. 129 p. 

153676 Chemagro (1975) [Dissipation in Soil of Sencor 50WP]. Unpublished study. 2 p. 

153679 Ladlie, J.; Meggitt, W.; Penner, D. (1976) Role of pH on metribuzin dissipation in field soils. 

Weed Science 24(5):508-511. 

153686 Webster, G. (1975) [Non-biological Degradation of Sencor in Soil]. Unpublished study prepared 

by Chemagro in cooperation with the Univ. of Manitoba. 9 p. 

153691 Mobay Chemical Corp. (1982) [Chemistry Data of Metribuzin and Its Metabolic Behaviour in 

Soil]. Unpublished compilation. 25 p. 

153723 Timme, G.; Scholz, K. (1980) Behaviour in Soil [of Sencor]. Unpub- lished study prepared by 

Bayer AG. 26 p. 

40380901 Delk, J. (1987) Dissipation of Sencor and its Oxidative Metabolites in Field Soil: Laboratory 

Project ID: SR-8300-87. Unpublished Mobay report no. 95000 prepared by Morse Laboratories 

Inc. 198 p. 

42236101 Grace, T.; Cain, K. (1990) Dissipation of Metribuzin in California Soils: Lab Project Number: 

SE830089R01; 100170; ML022101. Unpublished study prepared by Plant Sciences, Inc.; 

Siemer and Assoc's., Inc. and NET Atlantic, Inc. 3154 p. 
-39 of 61-

164-2 Aquatic field dissipation 

MRID Citation Reference 

106150 Mobay Chemical Corp. (1971) Supplement to Synopsis of Analytical and Residue Information 

on Use of Sencor (Bay 94337) with Potatoes and Soybeans: Supplement No. 1. (Compilation; 

unpub- lished study received Jul 20, 1971 under 0G0940; CDL:091605-A) 

164-5 Long term soil dissipation 

MRID Citation Reference 

153661 Prestel, D.; Weisgerber, I.; Klein, W.; et al. (1978) Accountabil- ity of the Distribution and 

Metabolism of [Carbon-14]-metribuzin (SENCOR) in Potatoes, Carrots, and Soil under Field 

Conditions. Unpublished translation of study in Chemosphere (2):137-144 (1976). 12 p. 

165-0 Accumulation Studies -- General 

MRID Citation Reference 

106172 Mobay Chemical Corp. (1972) Sencor: The Effects on the Environment. (Compilation; 

unpublished study received May 31, 1972 under 2F1274; CDL:094868-A; 094869) 

165-1 Confined rotational crop 

MRID Citation Reference 

40577701 Naidu, M. (1988) Metribuzin Confined Accumulation Study on Peanuts: Laboratory Project ID 

AMR-563-86. Unpublished study prepared by Research and Development Division, E. I. du 

Pont de Nemours and Company, Inc. 47 p. 

40838402 Christopher, R.; Lane, J.; Parker, G. (1988) Residues of Metribuzin (Sencor) in Rotational Crops 

and Soils: Report No. 98324: Project No. SE-052301. Unpublished study prepared by Mobay 

Corp. 45 p. 

41254101 Naidu, M. (1989) Metribuzin Confined Accumulation Study of Peanuts: Supplement No. 1: 

Laboratory Project ID AMR-563-86. Unpublished study prepared by E.I. du Pont de Nemours 

and Co., Inc. 16 p. 

166-1 Ground water-small prospective 

MRID Citation Reference 

44757901 Coody, P.; Fisher, J. (1999) A Geographic Groundwater Vulnerability Assessment for 

Metribuzin (SENCOR): Lab Project Number: 108839. Unpublished study prepared by Bayer 

Corporation. 40 p. 

45737901 Dyer, D. (2002) Progress Report--July, 2002 FOE 5043 (AXIOM)--Small-Scale Prospective 

Ground-Water Monitoring Study, Corn in Nebraska, 1995: Lab Project Number: F3212402: 

200257. Unpublished study prepared by Bayer Corp. 211 p. 

Submitted Effects Studies 
-40 of 61-

850.2100 Avian Single Dose Oral Toxicity 

MRID Citation Reference 

 

145911 or ACC 

255025 

 Lamb, D.; Jones, R. (1972) Acute Oral Toxicity of Sencor Technical to Bob 

White Quail and Mallard Ducks: Mobay Report No. 33172. Unpublished study 

prepared by Mobay Chemical Corp. 6 p. 

Accession No. 

255025 

 Schafer, E.W. (1971) Bird Toxicity on Inugan (Bay 79770), Sencor – Bay DIC 

1468 (Bay 94337) and Methyl EU paren (Bay 49854). Prepared by Denver 

Laboratory) Assumed US Fish and Wildlife Service) Chemagro Report No. 

29741. EPA Accession No. 255025. 

Accession No. 

255025 

 Shellenberger, T. (1969) Toxicological Evaluations of Bay 94337 with Fish and 

Wildlife Report No. 25527. Prepared by Gulf South Research Institute, New 

Iberia, Louisiana. Submitted to Chemagro Corp., Kansas City, Missouri. EPA 

Accession No. 255025. 

850.2200 Avian Dietary Toxicity 

MRID Citation Reference 

65507 Lamb, D.W.; Burke, M.A. (1977) Dietary Toxicity of ~(R)ISencor Technical to 

Bobwhite Quail and Mallard Ducks: Report No. 51593. (Unpublished study 

received Apr 13, 1977 under 3125-270; submit- ted by Mobay Chemical Corp., 

Kansas City, Mo.; CDL:229312-A) 

157528 Stubblefield, W. (1986) Subacute Dietary LC50 of Technical Metri- buzin 

(Sencor) to Bobwhite Quail: Study No. 86-175-01: Mobay Report No. 91757. 

Unpublished study prepared by Mobay Chemical Corp. 24 p. 

Accession No. 

262228 see 

157528 

 (1986?) Subacute dietary LC50 of Technical Metribuzin (Sencor®) to 

Bobwhite Quail. Report No. 91757, March 25, 1986. Study Prepared and 

Submitted by Mobay Corporation; EPA Accession No. 262228. 

850.2300 Avian Reproduction 

MRID Citation Reference 

43860501 Hancock, G. (1995) Effect of Technical SENCOR on Mallard Reproduction: 

Lab Project Number: SE740801: 106983. Unpublished study prepared by 

Bayer Corp. 107 p. 

43926601 Hancock, G. (1996) Effect of Technical SENCOR on Northern Bobwhite 

Reproduction: Lab Project Number: SE741701: 106984. Unpublished study 

prepared by Bayer Corp. 110 p. 

850.2500 Simulated or Actual Field Testing 
-41 of 61-

MRID Citation Reference 

35931 Findlay, G.M.; Baker, S.R. (1972) Assessing the Environmental Haz- ard of 

Pesticide Use: Report No. 37553. (Unpublished study re- ceived Mar 7, 1975 

under 5F1559; prepared by Univ. of Manitoba, Dept. of Entomology, submitted 

by Mobay Chemical Corp., Kansas City, Mo.; CDL:094871-O) 

850.1075 Acute Toxicity to Freshwater Fish 

MRID Citation Reference 

75541 Chambers, H.; Fabacher, D.L. (1974) Acute toxicity of pesticides to channel 

catfish fingerlings. MAFES Research Highlights 37(2):1. 

(Also~In~unpublished submission received on unknown date under 538-135; 

submitted by O.M. Scott & Sons Co., Marysville, Ohio; CDL:221963-A) 

78513 Rowehl, E.R. (1969) ?Study on Fish Toxicity Using Dasanit and Various Other 

Chemicals|: Report No. G-3045; Report No. 26929. (Unpublished study 

received Jul 31, 1972 under 3125-213; submit- ted by Mobay Chemical Corp., 

Kansas City, Mo.; CDL:120480-AJ) 

90427 McCann, J.A. (1972) ?Sencor 70% WP: Rainbow Trout (?~Salmo gaird~?- 

?~neri?~)|: Test No. 462. (U.S. Agricultural Research Service, Pesticides 

Regulation Div., Animal Biology Laboratory; unpub- lished study; 

CDL:130267-A) 

94219 Galletta, W.A. (1969) Fish Toxicity Trial No. 1/69: Report No. G- 2857; 

24992. (Unpublished study, including letter dated Jun 6, 1969 from W.M. Zeck 

to Glenn Brussels, received Mar 25, 1970 under 3125-235; prepared by Vero 

Beach Laboratories, Inc., sub- mitted by Mobay Chemical Corp., Kansas City, 

Mo.; CDL:007178-C) 

114006 Rowehl, E. (1969) Fish Toxicity Trial No. 66/69: Report No. G-3045; 26929. 

(Unpublished study received May 1, 1973 under unknown admin. no.; prepared 

by Vero Beach Laboratories, Inc., submitted by Mobay Chemical Corp., 

Kansas City, MO; CDL:120301-L) 

145910 Linn, J.; Denning, D. (1971) Report Summary: [Fish Toxicity to Bay 94337 

Technical]: Mobay Report No. 29532. Unpublished study prepared by 

California Dept. of Fish & Game in cooperation with Chemagro Corp. 3 p. 

147251 Hermann (1982) Fish Toxicity: [Metribuzin on Rainbow Trout]: Mobay 

Agchem No. 80970. Unpublished study prepared by Bayer AG. 6 p. 

Rainbow trout study – reviewed by E. Zucker in 1985- Invalid 

Accession No. 

255025 

 Chemagro Corp. Report (1970) Bay 94337 Technical Fish Toxicity. Report No. 

29532. Prepared by Chemagro Chemical Research Department. EPA Accession 

No. 255025. 

34713 Tooby, T.E., P.A. Hursey, J.S. Alabaster (1975) The Acute Toxicity of 102 

Pesticides and Miscellaneous Substances to Fish. Chemistry and Industry 

21(?/Jun): 523-526. (Published Study). 

Accession No. 

255025 

 Lamb, D.W., D.J. Roney (1972) April 13, 1972 Acute Toxicity of Sencor to 

Fish. Report No. 33124. Prepared by Chemagro Division Research and 
-42 of 61-

ZUOMET05 on 

the DERs 

Development. EPA No. 255025. Bluegill and Rainbow trout 

40098001 Mayer F.L., Ellersieck M.R. (1986) Manual of Acute toxicity: Interpretation 

and Date Base for 410 Chemicals and 66 Species of Freshwater Animals. 

United States Department of the Interior, Fish and Wildlife Services, 

Publication No. 160. Washington, DC. 

33320 Clemens, H.P., K.E. Sneed (1959) Lethal Doses of Several Commercial 

Chemicals for Fingerling Channel Catfish. Washington, DC. US Fish and 

Wildlife Service. (Special Scientific Report No. 316; Published Study). 

45273 Fabacher, D.L.; Chambers, H. (1973) Acute Toxicity of Three Pes- ticides to 

Channel Catfish Fingerlings: Report No. 40426. (Un- published study received 

Mar 7, 1975 under 5F1559; prepared by Mississippi State Univ., Agricultural 

and Forestry Experiment Station, Dept. of Entomology, submitted by Mobay 

Chemical Corp., Kansas City, Mo.; CDL:094870-A) 

850.1010 Acute Toxicity to Freshwater Invertebrates 

MRID Citation Reference 

134495 Nelson, D.; Roney, D. (1979) Acute Toxicity of Sencor Technical to Daphnia 

magna: Report No. 67262. (Unpublished study received Oct 31, 1983 under 

3125-277; submitted by Mobay Chemical Corp., Kansas City, MO; 

CDL:072083-A) 

139483 Schneider, P.W., Jr. (1976) 48-Hour LC50 to~Daphnia magna~?: Has- kell 

Laboratory Report No. 671-76. (Unpublished study received Oct 21, 1976 

under 352-382; submitted by E.I. du Pont de Nemours & Co., Wilmington, 

Del.; CDL:226357-J) 

45088223 Ort, M.; Fairchild, J.; Finger, S. (1994) Acute and Chronic Effects of Four 

Commercial Herbicides on Ceriodaphnia dubia. Archives of Environmental 

Contamination and Toxicology 27:103-106. 

34016 Goodman, N.C. (1978) 48-Hour LC50^2I to~Daphnia magna~?: Report No. 

400-78. (Unpublished study received Nov 20, 1978 under 352- 390; submitted 

by E.I. du Pont de Nemours & Co., Wilmington, Del.; CDL:236581-K) 

72083 Roney, D.J. 1979. Acute Toxicity of Sencor technical to Daphnia magna: 

Report # 67262. Prepared by Mobay Chemical for Mobay Chemical 

Corporation, Stilwell, Kansas. 

850.1025, 850.1035, 850.1075 Acute Toxicity to Estuarine/Marine Organisms 

MRID Citation Reference 

106197 Heitmuller, T. (1975) Acute Toxicity of Sencor to Eastern Oysters (Crassostrea 

virginica), Pink Shrimp (Penaeus duorarum), and Fiddler Crabs (Uca 

pugilator): (Submitter) 43851. (Unpublished study received Aug 19, 1975 

under 3125-294; prepared by Bion- omics--EG & G, Inc., submitted by Mobay 

Chemical Corp., Kansas City, MO; CDL:165011-A) 
-43 of 61-

161502 Surprenant, D. (1986) Acute Toxicity of Sencor to Eastern Oysters (Crassostrea 

virginica): Study No. 274-1285-6105-504: Report No. BW-86-02-1941. 

Unpublished study prepared by Springborn Bio- nomics, Inc. 21 p. 

161503 Nicholson, R.; Surprenant, D. (1986) Acute Toxicity of Sencor to Sheepshead 

Minnow (Cyprinodon variegatus): Study No. 274-1285-6105-500: Report No. 

BW-86-2-1942. Unpublished study prepared by Springborn Bionomics, Inc. 20 

p. 

42094501 Gagliano, G. (1991) Raw Data Supplement for Acute Toxicity of Senc- or 

Technical to Eastern Oysters (Crassostrea virginica): Lab Project Number: 274-

1285-6105-504: 91755-1. Unpublished study prepared by Springborn 

Bionomics, Inc. 11 p. 

42094502 Gagliano, G. (1991) Raw Data Supplement for Acute Toxicity of Senc- or 

Technical to Sheepshead Minnow (Cyprindon variegatus): Lab Project 

Number: 274-1285-6105-500: 91756-1. Unpublished study prepared by 

Springborn Bionomics, Inc. 13 p. 

No MRID on 

DER 

 Dionne, Emily, Donald C. Suprenant (19??) Acute Toxicity of Sencor to 

Eastern Oysters (Crassostrea virginica). Study Performed by Springborn 

Bionomics, Inc. 790 Main Street, Wareham, Massachusetts 02571 in March 

1986 for Mobay Chemical Corporation, Stillwell, Kansas. MRID No. 470234-

011? Toxicology Report No. 728. DP Barcode 166533. Study No. 91755. 

See 161503 

above 

 Nicholson, Richardson, Donald C. Suprenant (1986?) Acute Toxicity of Sencor 

to Sheepshead Minnow (Cyprinodon variegates). Study Performed by 

Springborn Bionomics, Inc. 790 Main Street, Wareham, Massachusetts 02571 

in March 1986 for Mobay Chemical Corporation, Stillwell, Kansas. MRID No. 

470234-012? Toxicology Report No. 729. DP Barcode 166533. Study No. 

91756. 

850.1400/850.1300 Fish Early Life Stage/Aquatic Invertebrate Life Cycle Study 

MRID Citation Reference 

51483 Marking, L.L. (1969) Letter sent to Thomas H. Schultz dated Sep 4, 1969 

?Toxicity of candidate chemicals to fish: Results of pre- liminary tests|: Report 

no. 26225. (U.S. Fish and Wildlife Service, Fish Control Laboratories, 

unpublished study; CDL: 222267-B) 

42447801 Gagliano, G.; Roney, D. (1992) Early Life Stage Toxicity of Sencor Technical 

to the Rainbow Trout (Oncorhynchus mykiss) under Flow-through Conditions: 

Lab Project Number: 103249: SE842201. Unpublished study prepared by 

Miles, Inc. 87 p. 

42447802 Gagliano, G.; Bowers, L. (1992) Chronic Toxicity of Sencor Technical to the 

Waterflea (Daphnia magna) under Flow-through Conditions: Lab Project 

Number: 103248: SE840701. Unpublished study prepared by Miles, Inc. 75 p. 

42731201 Gagliano, G.; Bowers, L. (1993) Chronic Toxicity of Sencor Technical to the 

Waterflea (Daphnia magna) Under Flow-Through Conditions: Lab Project 

Number: SE840702: 105023. Unpublished study prepared by Miles Inc. 60 p. 

42732501 Heimbach, F. (1993) Influence of DADK (Metabolite of Metribuzin) on the 

Reproduction Rate of Water Fleas: Lab Project Number: E 321 0676-05: 
-44 of 61-

HBF/RDM 45: 105035. Unpublished study prepared by Bayer AG Crop 

Protection Research. 31 p. 

43002001 Thornton, J. (1993) Letter Sent to E. Feris dated September 14, 1993: Sencor 

metribuzin herbicide...: Fish early life stage study. Prepared by Miles Inc. 

Agric. Div. 7 p. 

850.1950 Aquatic Field Study 

42201600 ICI Americas Inc. (1992) Submission of New Formulation Mesocosm Data To 

Support Registration of Force 1.5G. Transmittal of 11 studies. 

850.4100/850.4150 Seed germination/seedling emergence and vegetative vigor 

MRID Citation Reference 

42447803 Burge, C. (1992) Tier 2 Seed Germination, Seed Emergence, and Seedling 

Vigor Nontarget Phytotoxicity Study Using Metribuzin: Lab Project Number: 

103800: SE201601. Unpublished study prepared by Miles, Inc. 90 p. 

43208301 Johns, C. (1994) Tier 2 Seed Germination and Seed Emergence Nontarget 

Phytotoxicity Study Using Metribuzin: Lab Project Number: SE201602: 

106568. Unpublished study prepared by Miles Inc. 44 p. 

45227704 Davis, D. (1980) Effects of Herbicides on Submerged Seed Plants: Lab Project 

Number: A-067-ALA: 14-34-0001-0101. Unpublished study prepared by 

Auburn University. 10 p. 

 

850.4400/850.5400 Aquatic plant growth 

MRID Citation Reference 

42438301 Gagliano, G.; Bowers L. (1992) Acute Toxicity of Sencor Technical to the 

Green Alga (Selenastrum capricornutum): Lab Project Number: SE881401: 

103239. Unpublished study prepared by Miles Inc., Agricultural Division. 32 p. 

43133601 Gagliano, G.; Orr, W. (1994) Acute Toxicity of SENCOR Technical to the 

Green Alga (Selenastrum capricornutum): Lab Project Number: SE881402. 

Unpublished study prepared by Miles, Inc. Sponsor report number: 106417. 33 

p. 

43826101 Bowers, L. (1995) Acute Toxicity of (carbon 14)-Sencor to the Freshwater 

Diatom (Navicula pelliculosa): Lab Project Number: 106975: SE881601. 

Unpublished study prepared by Bayer Corp. 33 p. 

43867701 Bowers, L. (1995) Toxicity of (carbon 14)-Sencor to the Marine Diatom, 

Skeletonema costatum: Lab Project Number: SE881602: 106992. Unpublished 

study prepared by Bayer Corp. 34 p. 
-45 of 61-

43893501 Boeri, R.; Magazu, J.; Ward, T. (1995) Toxicity of Sencor to the Duckweed, 

Lemna gibba G3: (Amended Report): Lab Project Number: 667-MI: 106982. 

Unpublished study prepared by T. R. Wilbury Labs, Inc. 32 p. 

43893502 Boeri, R.; Magazu, J.; Ward, T. (1995) Toxicity of Sencor to the Freshwater 

Alga, Anabaena flos-aquae: (Amended Report): Lab Project Number: 666-MI: 

106974. Unpublished study prepared by T. R. Wilbury Labs, Inc. 31 p. 

43921001 Bowers, L. (1996) Toxicity of (carbon 14)-Sencor to the Green Alga 

Selenastrum capricornutum: Lab Project Number: 107142: SE883501. 

Unpublished study prepared by Bayer Corp. 32 p. 

44016901 Bowers, L. (1996) Data Analysis Correction for the Effect of Sencor Technical 

on the Growth of Selenastrum capricornutum: Lab Project Number: SE881402: 

106417-1. Unpublished study prepared by Bayer Corp. 17 p. 

45088212 Fairchild, J.; Ruessler, D.; Haverland, P. et al. (1996) Comparative Sensitivity 

of Selenastrum capricornutum and Lemna minor to Sixteen Herbicides. 

Archives of Environmental Contamination and Toxicology 32:353-357. 

45227708 Fairchild, J.; Ruessler, S.; Carlson, A. (1998) Comparative sensitivity of five 

species of macrophytes and six species of algae to atrazine, metribuzin, 

alachlor, and metolachlor. Environmental Toxicology and Chemistry 

17(9)1830-1834. 

850.3030 Honeybee Acute Toxicity 

59460 Atkins, E.L. (1975) Letter Sent to Charles A. Hanson Dated Sep. 30, 1975. 

Laboratory Tests on Pesticides Tested for Comparative Honey Bee Toxicity. 

(Unpublished Study Received Aug. 19, 1976 under 8340-EX-3; Prepared by 

Univ. of California – Riverside, Citrus Research Center and Agricultural 

Experiment Station, Dept. of Entomology, Submitted by American Hoechst 

Corp., Somerville, NJ; CDL: 095253-AJ). 

28772 Atkins, E.L., Greywood, E.A., Macdonald, R.L. (1973) Toxicity of Pesticides 

and other Agricultural Chemicals to Honey Bees; Laboratory Studies. Rev. by 

Univ. of California-Riverside, Dept. of Entomology. Riverside, Calif.; UC, 

Agricultural Extension Service. (Also In Unpublished Submission Received 

Apr. 2, 1980 under 464-556; Submitted by Dow Chemical U.S.A. Midland, 

Mich.; CDL: 242149-Z). 

Non Guideline Selections 

42317100 Monsanto Co. (1992) Submission of an incident report under FIFRA 6(a)(2) 

status involving Canopy and Freedom herbicides. Transmittal of 1 study. 

42347000 North Carolina Cooperative Extension Service (1992) Submission of FIFRA 

6(a)2 data for Metalaxyl, Alachlor, Triazines, Nitrate- Nitrogen, Metolachlor, 

and Metribuzin adverse effects. Trans- mittal of 1 study. 

42775101 Gould, T. (1993) A Review of the Plant Metabolism of Metribuzin: 

Supplemental Information: Lab Project Number: 105166. Unpublished study 

prepared by Miles Inc. Agriculture Division. 26 p. 

43544701 Fairchild, J.; Carlson, A. (1994) An Aquatic Risk Assessment of Four 

Herbicides Using Six Species of Algae and Five Species of Aquatic 

Macrophytes. Unpublished study prepared by U. S. Fish and Wildlife Service 

and U. S. EPA. 9 p. 
-46 of 61-

43571600 Miles, Inc. (1995) Submission of Effects on Microorganisms and Plants Data in 

Support of FIFRA 6(a)(2) For Metribuzin. Transmittal of 1 Study. 

43598610 Johnson, E. (1993) The Effects of Representative Groundwater Pesticides on 

Reproduction and in utero Development of Experimental Animals: Lab Project 

Number: 311U-4349-16: 356/311U--3698. Unpublished study prepared by 

Ciba-Geigy Corp. 19 p. 

43633401 Heindel, J.; Chapin, R.; Gulati, D. et al. (1993) Assessment of the reproductive 

and developmental toxicity of pesticide/fertilizer mixtures based on confirmed 

pesticide contamination in California and Iowa groundwater. Fundamental 

and Applied Toxicology 22:605-621. (Maybe human health related) 

45020003 Davies, P.; Cook, L.; Barton, J. (1994) Triazine Herbicide Contamination of 

Tasmanian Streams: Sources, Concentrations and Effects on Biota.

Freshwater Res. 45:209-226.

45020007 Krieger, K.; Baker, D.; Kramer, J. (1988) Effects of Herbicides on Stream 

Aufwuchs Productivity and Nutrient Uptake. Archives of Environmental 

Contamination and Toxicology 17:299-306. 

47543509 Krieger, K.; Baker, D.; Kramer, J. (1988) Effects of Herbicides on Stream 

Aufwuchs Productivity and Nutrient Uptake. Arch. Environ. Contam. 

Toxicol. 17: 299-306.

45255 Chemagro Corporation (1975) Supplement to Synopsis of the Effects of Sencor 

on the Environment. Summary of Studies 094871-C through 094871-F, 

094871-I through 094871-O, 094871-Q through 094871-S. (Supplement no. 1; 

Unpublished Study Received Mar. 7, 1975 under 5F1559; Submitted by Mobay 

Chemical Corp., Kansas City, MO., CDL:094871-A). 

Accession No. 

255025 

 Shellenberger, T. (1969) Toxicological Evaluations of Bay 94337 with Fish and 

Wildlife Report No. 25527. Prepared by Gulf South Research Institute, New 

Iberia, Louisiana. Submitted to Chemagro Corp., Kansas City, Missouri. EPA 

Accession No. 255025. 

106172 Mobay Chemical Corp. (1972) Sencor: The Effects on the Environment. 

(Compilation; unpublished study received May 31, 1972 under 2F1274; 

CDL:094868-A; 094869) 
-47 of 61-

Appendix A. Summary of available effects studies 

A brief summary of submitted data considered relevant to the ecological risk assessment for the 

Registration Review is presented below. 

Toxicity of Metribuzin to Aquatic Organisms 

Toxicity to Fish 

A summary of acute and chronic freshwater fish data is provided below. 

Freshwater Fish: Acute Exposure (Mortality) Studies 

Submitted guideline studies provide insight into the potential acute effects of metribuzin on 

freshwater fish. Studies (MRIDs 255025 and 40098001; Acceptable) with bluegill sunfish, 

(Lepomis macrochirus), resulted in LC50s of 75.96, 92 and 131.1 mg a.i./L. Studies (MRIDs 

40098001, 090427 and 255025; Acceptable), with rainbow trout (Onchorhynchus mykiss; 

formerly Salmo gairdneri), resulted in LC50s of 42, 76.77, 99 and 147 mg a.i./L. For a protective 

assessment, the LC50 of 42 mg a.i./L will be used for RQ calculation. Metribuzin is categorized 

as slightly toxic to practically nontoxic to freshwater fish on an acute exposure basis. 

Freshwater Fish: Chronic Exposure (Early Life Stage and Reproduction) Studies 

A guideline early life stage (ELS) study is available for evaluation of potential chronic effects of 

metribuzin on freshwater fish. The rainbow trout study (MRID 42447801) resulted in a NOAEC 

of <3.0 mg a.i./L and the LOAEC is 3.0 mg a.i./L. No NOAEC was achieved in this study due to 

effects on growth at all levels tested. Based on the highest 56-day aquatic EEC from the RED 

assessment (0.12 mg a.i./L), and using the lowest concentration tested as the LOAEC value, the 

ratio (RQ-value) is 0.04, well below the chronic LOC of 1.0. Therefore, based on the available 

information, risk to listed freshwater fish from chronic exposure to metribuzin is not expected. 

A request for new data to establish a definitive endpoint is not recommended at this time, it is 

unlikely a new study with lower concentrations to achieve a NOAEC will impact the risk 

assessment. 

Estuarine/marine Fish: Acute exposure 

One study evaluating the toxicity of metribuzin to estuarine/marine fish is available (MRID 

42094502; Acceptable). The 96h LC50 for sheepshead minnow (Cyprinodon variegatus) is 85 

mg a.i./L; therefore metribuzin is classified as slightly toxic to estuarine/marine fish. 

Estuarine/marine Fish: Chronic Exposure (Early Life Stage and Reproduction) Studies 

There are no data on the effect of chronic exposure to metribuzin to estuarine/marine fish. In 

lieu of asking for this study, an acute to chronic ration (ACR) will be used in the risk assessment. 

Toxicity to Aquatic Invertebrates 

Freshwater Invertebrates: Acute Exposure (Mortality) Studies 

Two guideline studies with Daphnia magna (MRIDs 34016 and 72083; Acceptable) the EC50s 

were 4.2 and 98.5 mg a.i./L. For a protective assessment, the EC50 of 4.2 mg a.i./L will be used 
-48 of 61-

for RQ calculation. Metribuzin is categorized as moderately to slightly toxic to freshwater 

invertebrates on an acute exposure basis. 

Freshwater Invertebrates: Chronic Exposure (Reproduction) Studies 

A guideline study evaluating potential chronic effects to aquatic invertebrates (MRID 42447802; 

Acceptable), was conducted with the waterflea Daphnia magna. The study resulted in a NOAEC 

of 1.29 mg a.i./L. The LOAEC in the study is 2.62 mg a.i./L, based on reduced length and 

number of offspring produced compared to the control. 

In addition to toxicity testing with the active ingredient, the registrant has submitted a study 

(MRID 42732501; Acceptable) with the degradate, DADK, to evaluate the potential chronic 

effects to aquatic invertebrates. Based on the results with the daphnids, the NOAEC is 3.3 mg 

a.i./L, based on reduction in length and number of offspring produced compared to the control at 

the 5.9 mg a.i./L treatment level. 

Estuarine/marine Invertebrates: Acute Exposure (Mortality) Studies 

A 96h acute toxicity study (MRID 106197; Acceptable) with pink shrimp (Penaeus duorarum) is 

available for metribuzin. The EC50 from the mean-measured concentration is 48.3 mg a.i./L. 

Several acute toxicity studies (MRIDs 106197 and 42094501; Acceptable, MRID 0043851; 

Supplemental) examining the effect of metribuzin on larval Eastern oyster (Crassostrea 

virginica) is available for metribuzin. The EC50s ranged from 40.7 – 49.8 mg a.i./L. 

A 96h shell deposition study (MRID 47023411; Supplemental) with Eastern oyster is available 

for metribuzin. The EC50, based on mean-measured concentrations, is 52 mg a.i./L . 

Results on pink shrimp and Eastern oyster larval and shell deposition studies indicate metribuzin 

is classified as slightly toxic to estuarine/marine invertebrates on an acute exposure basis. For a 

protective assessment on estuarine/marine invertebrates, the larval Eastern oyster EC50 of 40.7 

mg a.i./L will be used for RQ calculation. 

Toxicity to Aquatic Plants 

A 14-day aquatic vascular plant study with Lemna gibba (MRID 43893501; Supplemental) 

resulted in an IC50 and NOAEC, based on frond weight, of 130 and 18 μg a.i./L, respectively. 

Three 5-day aquatic nonvascular plant studies (MRIDs 43133601 and 43921001; Acceptable, 

MRID 42438301; Invalid) with the green algae, Pseudokirchneriella subcapitata (formerly 

Selenastrum capricornutum), obtained an IC50 as low as 8.09 μg a.i./L. The corresponding 

NOAEC is 2.33 μg a.i./L. 

A 5-day aquatic nonvascular plant study (MRID 43826101; Acceptable) with the freshwater 

diatom, Navicula pelliculosa, resulted in an IC50 and NOAEC of 11.9 and 8.9 μg a.i./L, 

respectively. 

A 5-day aquatic nonvascular plant study (MRID 43867701; Acceptable) with the marine diatom, 

Skeletonema costatum, resulted in an IC50 and NOAEC of 8.84 and 5.78 μg a.i./L, respectively. 
-49 of 61-

A 5-day aquatic nonvascular plant study (MRID 43893502; Supplemental) with the blue-green 

algae, Anabaena flos-aquae, resulted in an IC50 and NOAEC of 17 and 9.7 μg a.i./L, 

respectively. 

For a protective assessment on aquatic nonvascular plants, the IC50 and NOAEC of 8.09 and 2.33 

mg a.i./L, respectively, will be used for RQ calculation, while the duckweed IC50 and NOAEC of 

130 and 18 μg a.i./L, respectively will be used to calculate RQs for aquatic vascular plants. 

Toxicity of Metribuzin to Terrestrial Animals 

Toxicity to Birds 

Birds: Acute Exposure (Mortality) Studies 

An avian acute oral toxicity study with the Northern bobwhite quail (MRID 00255025; 

Acceptable) is available for metribuzin. The LD50 for avian acute effects is 169.2 mg a.i./kg-bw. 

Metribuzin is categorized as moderately nontoxic to upland game birds on an acute oral exposure 

basis. There are no acute oral data available for passerines, the toxicity to passerines exposed to 

metribuzin is not known; a study should be submitted to satisfy guideline requirements. 

Two avian subacute dietary studies are available for metribuzin, one with bobwhite quail (MRID 

262228; Acceptable) and the other with mallard duck (MRID 0065507; Acceptable). No 

mortalities were reported in either study at the highest concentration tested (4,000 mg a.i./kg diet 

for quails and 5,000 mg a.i./kg diet for ducks). The LC50 of >4,000 mg a.i./kg-diet will be used 

for RQ calculation to assess risk to avian species via the dietary route. Metribuzin is categorized 

as practically nontoxic on a subacute dietary exposure basis. 

Birds: Chronic Exposure (Growth, Reproduction) Studies 

Effects to avian species from chronic exposure are assessed with one-generation reproduction 

studies. Two studies, one with mallard duck (Anas platyrhynchos) and one with bobwhite quail 

(Colinus virginianus) are used to evaluate the chronic toxicity of metribuzin to avian species. In 

the mallard study (MRID 43860501; Acceptable) the highest dose tested was 368 mg a.i./kg-diet 

and no effects were reported. The NOAEC was the highest concentration tested. The quail 

study (MRID 43926601; Acceptable) determined an NOAEC of 385 mg a.i./kg-diet; however, 

there was a statistically significant reduction in body weight at 14-day post-hatch at all levels 

(NOAEC<62 mg a.i./kg-diet) tested with no other effects observed. This effect appears not be 

treatment related since no reproductive parameters were affected in either study. The LOAEC 

for mallard duck is 386 mg a.i./kg-diet and for bobwhite quail is >385 mg a.i./kg-diet. 

Toxicity to Mammals 

Mammals: Acute Exposure (Mortality) Studies 

Effects to mammals are assessed with the acute oral toxicity study with the laboratory rat (Rattus 

norvegicus) and laboratory mouse (Mus musculus). The lowest LD50 from both studies (MRID 

00106158) is the mouse LD50 at 711/698 mg a.i./kg-bw (male/female); therefore, metribuzin is 

categorized as slightly toxic to mammalian species on an acute exposure basis. 
-50 of 61-

Mammals: Chronic Exposure (Growth, Reproduction) Studies 

The two-generation study with the laboratory rat is used to assess potential for effects to 

mammals from chronic exposure. The study for metribuzin (MRID 40838401) determined a 

NOAEC for both parental and offspring of 30 mg a.i./kg-diet (1.5 mg a.i./kg/day). Effects at the 

LOAEC (150 mg a.i./kg-diet) include decreased body weights and body weight gains and 

hypertrophy of the hepatocytes. 

Toxicity to Terrestrial Invertebrates 

A honeybee acute contact study (MRID 028772; Acceptable) indicates metribuzin is practically 

nontoxic to adult honeybees on an acute exposure basis, with an LD50 of 60.4 μg a.i./bee when 

exposed to technical metribuzin. 

Toxicity to Terrestrial Plants 

The results of the Tier II seedling emergence (MRID 42447803 and 43208301(for cotton); 

together Acceptable) and vegetative vigor (MRID 42447803; Acceptable) tests on non-target 

plant biomass are summarized below in Tables 1 and 2. Sensitivity varied widely across 

species, indicating that some species of plants are more sensitive to metribuzin. As expected 

with a pre-emergent and post-emergent herbicide, the results are similar in both the seedling 

emergence and vegetative vigor studies. 

Table 1. Non-target Terrestrial Plant Seedling Emergence Toxicity (Tier II) Data 

Crop Type of 

Study 

Species 

EC25

(lb a.i./A) 

NOAEC 

(lb a.i./A) 

Most sensitive 

parameter 

Monocots Corn 0.059 0.056 Dry weight 

Onion 0.02 0.014 Survival 

Wheat 0.024 0.014 Emergence and dry 

weight 

Pea >0.225 0.113 Dry weight 

Dicots Turnip 0.008 0.007 Emergence 

Soybean >0.225 0.225 None 

Cotton 0.0423 0.0281 Dry weight 

Cucumber 0.029 0.014 Shoot height 

Tomato >0.225 0.113 Survival 

Sorghum 0.043 0.028 Dry weight 
-51 of 61-

Table 2. Non-target Terrestrial Plant Vegetative Vigor Toxicity (Tier II) Data 

Crop Type of 

Study 

Species 

EC25

(lb a.i./A) 

NOAEC 

(lb a.i./A) 

Most sensitive 

parameter 

Monocots Corn >0.09 0.09 None 

Onion 0.017 0.0112 Dry weight 

Wheat 0.041 0.0225 Dry weight 

Pea >0.09 0.09 None 

Dicots Turnip 0.005 0.003 Dry weight 

Soybean >0.09 0.045 Dry weight 

Cotton 0.016 0.0028 Dry weight 

Cucumber 0.024 0.0112 Dry weight 

Tomato >0.09 0.09 None 

Sorghum >0.09 0.045 Dry weight 
-52 of 61-

Appendix B Incident Reports 

EIIS Pesticide Summary Report: General Information 

Metribuzin (101101) 

Incident # Date County State Certainty Legal. Formul. Appl. Method 

AQUATIC 

Agricultural Area 

I000636-012 5/11/1985 SCOTLAND MO 2 UN 

POND 

I008768-007 6/3/1999 2 UN N/R N/R 

PLANTS 

Agricultural Area 

I005880-037 Outagamie WI 2 UN Spray 

I005880-043 WALWORTH WI 3 MA N/R 

I005880-011 LAFAYETTE WI 3 MA Spray 

I005880-018 OUTAGAMIE WI 2 UN Spray 

I005880-044 RACINE WI 3 MA Soil incorporation 

I005880-049 SAUK WI 3 MA Spray 

I005880-022 WAUSHARA WI 3 MA N/R Soil incorporated 

I005880-040 WINNEBAGO WI 3 MA N/R 

I005880-020 WAUSHARA WI 2 UN Spray 

I005880-027 WAUKESHA WI 3 MA Spray 

Wednesday, June 13, 2012 Certainty Code: 0=Unrelated, 1=Unlikely, 2=Possible, 3=Probable, 4=Highly Probable. 

Legality Code: RU=Registered Use, M=Misuse, MA=Misuse (Accidental), MI=Misuse (Intentional), U=U
-53 of 61-

Incident # Date County State Certainty Legal. Formul. Appl. Method 

I005880-034 OUTAGAMIE WI 3 MA Soil incorporation 

I005880-033 FOND DU LAC WI 3 MA Soil incorporation 

I005880-039 OUTAGAMIE WI 3 MA Soil incorporation 

I005880-017 FOND DU LAC WI 2 UN Spray 

I005880-006 DANE WI 3 MA Spray 

I005880-024 OUTAGAMIE WI 2 UN N/R 

I005880-055 5/18/1997 MN 3 UN Spray I005879-008 6/4/1997 TAMA IA 2 MA Spray 

I005879-009 6/6/1997 GRUNDY IA 2 MA Spray 

I005879-015 7/9/1997 BUCHANAN IA 2 UN Spray I016232-003 4/22/2005 ID 2 RU Broadcast I016978-011 11/7/2005 ID 2 RU NR 

ALFALFA 

I010210-007 3/6/2000 OH 3 MA F N/R 

I012982-007 4/10/2002 Shelby OH 3 M DF Broadcast I014123-019 5/14/2003 Twin Falls ID 2 M WDG Broadcast I014123-025 5/14/2003 Twin Falls ID 2 RU WDG Broadcast 

Asparagus 

I015278-001 6/20/2003 Malheur OR 2 RU 

Bean 

Wednesday, June 13, 2012 Certainty Code: 0=Unrelated, 1=Unlikely, 2=Possible, 3=Probable, 4=Highly Probable. 

Legality Code: RU=Registered Use, M=Misuse, MA=Misuse (Accidental), MI=Misuse (Intentional), U=U
-54 of 61-

Incident # Date County State Certainty Legal. Formul. Appl. Method 

B000622-001 5/15/1986 Stark OH 2 UN Spray 

Bean, garbonzo 

I011814-012 4/12/2001 SAN JOAQUIN CA 3 MA F N/R 

Beans 

I013563-006 1/24/2001 San Joaquin CA 2 UN N/R 

CORN 

I008079-002 4/30/1998 JASPER IA 2 RU N/R GROUND/BROAD

AST 

I007755-007 5/5/1998 BOONE IA 2 RU N/R Broadcast 

I008002-001 10/11/1998 CALHOUN IA 2 UN Broadcast 

I008002-002 10/12/1998 CARROLL IA 2 RU Broadcast 

I008002-003 10/13/1998 GRUNDY IA 2 RU Broadcast 

I010460-004 6/2/2000 CALHOUN MI 4 UN DF N/R 

I010460-005 6/10/2000 KANKAKEE IL 4 MA DF N/R I011814-011 5/29/2001 STEARNS MN 3 MA F N/R 

I011662-004 5/30/2001 BUCHANAN IA 3 MA F N/R I011814-013 6/4/2001 CERRO GORDO IA 3 MA F N/R 

I011814-007 6/8/2001 SAUK WI 3 MA F N/R 

I011814-009 6/11/2001 LINN IA 3 MA F N/R 

I011814-010 6/11/2001 LINN IA 3 MA F N/R 

I012177-004 7/26/2001 HALL NE 2 RU DF N/R 

Wednesday, June 13, 2012 Certainty Code: 0=Unrelated, 1=Unlikely, 2=Possible, 3=Probable, 4=Highly Probable. 

Legality Code: RU=Registered Use, M=Misuse, MA=Misuse (Accidental), MI=Misuse (Intentional), U=U
-55 of 61-

Incident # Date County State Certainty Legal. Formul. Appl. Method 

Corn, field 

I012366-034 4/14/2001 CASS IN 2 UN 

I012982-009 5/13/2002 Vernon MO 2 RU DF Broadcast 

I015291-026 6/4/2004 Ripley IN 2 RU DF 

FIELD 

I008664-004 4/13/1999 CARROLL MO 2 RU DF BROADCAST 

BOOM 

N/R 

I005880-030 DODGE WI 2 UN Spray 

B000609-001 6/5/1986 Morrow OH 2 UN Spray 

B000603-006 5/18/1997 IN 2 UN N/R N/R 

Potato 

i017245-009 Morrow OR 3 UN DF I020459-033 5/15/2000 Benton WA 2 UN I016595-028 7/8/2005 Bonneville ID 2 RU F 

Right-of-way, road 

I019442-001 7/19/2007 Brown OH 2 MI 

Ryegrass 

I016962-027 9/8/2004 Yamhill OR 2 RU Broadcast 

Soybean 

B000625-001 5/15/1986 Dakota MN 2 RU Spray 

Wednesday, June 13, 2012 Certainty Code: 0=Unrelated, 1=Unlikely, 2=Possible, 3=Probable, 4=Highly Probable. 

Legality Code: RU=Registered Use, M=Misuse, MA=Misuse (Accidental), MI=Misuse (Intentional), U=U
-56 of 61-

Incident # Date County State Certainty Legal. Formul. Appl. Method 

I001131-001 6/27/1994 PULASKI AR 2 RU N/R Broadcast I007371-008 5/20/1997 PA 4 MA N/R 

I005879-010 6/17/1997 CLINTON IA 2 MA Spray 

I012366-012 5/12/2000 MACOMB MI 2 UN Broadcast I014106-001 5/15/2003 Arkansas AR 2 RU EC I014102-001 5/15/2003 Prairie AR 2 RU EC 

I015291-035 6/24/2004 Cerro Gordo IA 2 RU DF I016595-030 7/5/2005 Cerro Gordo IA 2 RU WDG Broadcast I016903-003 10/10/2005 Huron OH 2 RU EC Broadcast 

Soybeans 

I016328-037 5/31/2005 Scott IN 2 RU WDG Broadcast 

WHEAT 

I011508-002 1/24/2001 CLOUD KS 3 MA F N/R 

Wheat, winter 

I013942-006 3/27/2003 Lancaster VA 2 RU Broadcast 

I014029-005 3/27/2003 Lancaster VA 2 RU Broadcast 

I014123-024 5/2/2003 Douglas OR 2 RU Broadcast I014123-018 5/3/2003 Morton OR 2 RU Broadcast 

I014123-012 5/16/2003 Essex VA 2 RU Broadcast 

I014676-004 11/13/2003 VA 2 M WDG Broadcast 

Wednesday, June 13, 2012 Certainty Code: 0=Unrelated, 1=Unlikely, 2=Possible, 3=Probable, 4=Highly Probable. 

Legality Code: RU=Registered Use, M=Misuse, MA=Misuse (Accidental), MI=Misuse (Intentional), U=U
-57 of 61-

Incident # Date County State Certainty Legal. Formul. Appl. Method 

I014978-001 3/24/2004 Delaware In 2 RU WDG Broadcast 

TERRESTRIAL 

N/R 

I007334-001 6/18/1998 White IL 2 UN Spray 

Wednesday, June 13, 2012 Certainty Code: 0=Unrelated, 1=Unlikely, 2=Possible, 3=Probable, 4=Highly Probable. 

Legality Code: RU=Registered Use, M=Misuse, MA=Misuse (Accidental), MI=Misuse (Intentional), U=U
-58 of 61-

Appendix D. Results from Screening Imbibition Program (SIP) and the Screening Tool for 

Inhalation Risk Assessment (STIR) 

The Screening Imbibition Program (SIP v.1.0, Released June 15, 2010) was used to calculate an 

upper bound estimate of exposure using metribuzin solubility (1200 mg/L, @ 25oC), the most 

sensitive acute and chronic avian toxicity endpoints (Bobwhite quail with acute oral LD50 of 

169.2 mg a.i./kg-bw, acute dietary LC50 of >4000 mg a.i./kg diet and mallard duck with lowest 

reproduction NOAEC of 368 mg a.i./kg-diet) and the most sensitive acute and chronic 

mammalian toxicity endpoints (Mouse LD50 =698/711 mg a.i./kg-bw (male/female) and rat 

NOAEL = 1.5 mg a.i./kg-bw. Based on the output, exposure through drinking water alone is 

potential chronic risk to small birds and mammals. Results from SIP are shown below. 

Inhalation is another potential exposure route for terrestrial vertebrates. Based on the vapor 

pressure of metribuzin (9.1 x10-7 torr), risk to birds, terrestrial-phase amphibians, reptiles, and 

mammals from inhalation is not expected. The 4-hr LC50 inhalation study using rats was greater 

than 0.709 mg/L (MRID 46105408). The STIR model (Screening Tool for Inhalation Risk) was 

used to evaluate inhalation risk to birds. The definitive acute oral values for both birds and 

mammals were used in the model. Based on STIR, risk to birds and mammals from inhalation 

alone is not expected. Results from STIR are shown below. 

 
-59 of 61-

SIP Version 1.0. August 19, 2010 

Table 1. Inputs 

Parameter Value 

Chemical name Metribuzin 

Solubility (in water at 25oC; mg/L) 1200 

 

Mammalian LD50 (mg/kg-bw) 698 

Mammalian test species other 

Body weight (g) of "other" mammalian species 20g 

 

Mammalian NOAEL (mg/kg-bw) 30 

Mammalian test species laboratory rat 

Body weight (g) of "other" mammalian species 

 

Avian LD50 (mg/kg-bw) 169.2 

Avian test species northern bobwhite quail 

Body weight (g) of "other" avian species 

Mineau scaling factor 1.15 

 

Mallard NOAEC (mg/kg-diet) 368 

Bobwhite quail NOAEC (mg/kg-diet) 385 

NOAEC (mg/kg-diet) for other bird species 

Body weight (g) of other avian species 

NOAEC (mg/kg-diet) for 2nd other bird species 

Body weight (g) of 2nd other avian species 

 

 

Table 2. Mammalian Results 

Parameter Acute Chronic 

Upper bound exposure (mg/kg-bw) 206.4000 206.4000 

Adjusted toxicity value (mg/kg-bw) 262.4901 1.1537 

Ratio of exposure to toxicity 0.7863 178.8963 

Conclusion* 

Exposure through 

drinking water alone is 

a potential concern for 

mammals 

Exposure through 

drinking water alone is a 

potential concern for 

mammals 

 

Table 3. Avian Results 

Parameter Acute Chronic 

Upper bound exposure (mg/kg-bw) 972.0000 972.0000 

Adjusted toxicity value (mg/kg-bw) 121.8967 18.2574 

Ratio of exposure to acute toxicity 7.9740 53.2386 

Conclusion* 

Exposure through 

drinking water alone is 

a potential concern for 

birds 

Exposure through 

drinking water alone is a 

potential concern for 

birds 

 

*Conclusion is for drinking water exposure alone. This does not combine all routes of exposure. Therefore, 

when aggregated with other routes (i.e., diet, inhalation, dermal), pesticide exposure through drinking water 

may contribute to a total exposure that has potential for effects to non-target animals. 
-60 of 61-

Results From STIR Version 1.0, November 23, 2010

Screening Tool for Inhalation Risk 

This tool is designed to provide the risk assessor with a rapid method for determining the potential 

significance of the inhalation exposure route to birds and mammals in a risk assessment. 

 

Input 

Application and Chemical Information 

Enter Chemical Name metribuzin 

Enter Chemical Use Registered Uses 

Is the Application a Spray? (enter y or n) y 

If Spray What Type (enter ground or air) air 

Enter Chemical Molecular Weight (g/mole) 214.28 

Enter Chemical Vapor Pressure (mmHg) 9.10E-07 

Enter Application Rate (lb a.i./acre) 6 

 

Toxicity Properties 

Bird 

Enter Lowest Bird Oral LD50 (mg/kg bw) 169.2 

Enter Mineau Scaling Factor 1.15 

Enter Tested Bird Weight (kg) 1.58 

Mammal 

Enter Lowest Rat Oral LD50 (mg/kg bw) 698 

Enter Lowest Rat Inhalation LC50 (mg/L) 0.709 

Duration of Rat Inhalation Study (hrs) 4 

Enter Rat Weight (kg) 0.02 

 

Output 

Results Avian (0.020 kg) 

Maximum Vapor Concentration in Air at Saturation (mg/m3) 1.05E-02 

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 1.32E-03 

Adjusted Inhalation LD50 1.68E+01 

Ratio of Vapor Dose to Adjusted Inhalation LD50 7.87E-05 Exposure not Likely Significant 

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 5.76E-01 

Ratio of Droplet Inhalation Dose to Adjusted Inhalation LD50 3.44E-02 Exposure not Likely Significant 

 

Results Mammalian (0.015 kg) 

Maximum Vapor Concentration in Air at Saturation (mg/m3) 1.05E-02 

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 1.66E-03 

Adjusted Inhalation LD50 3.61E+02 

Ratio of Vapor Dose to Adjusted Inhalation LD50 4.59E-06 Exposure not Likely Significant 

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 7.25E-01 

Ratio of Droplet Inhalation Dose to Adjusted Inhalation LD50 2.01E-03 Exposure not Likely Significant 
-61 of 61-

Appendix E: ECOSAR modeled estimates of degradate toxicity 

Comparison of Observed toxicity values and QSAR-generated endpoints (mg/L) for Metribuzin 

and its degradates by chemical class chosen in ECOSAR – Hydrazines ECOSAR class 

Taxon Paren

t data 

Parent 

QSA

R 

Ratio of 

parent 

data/paren

t QSAR 

DADK

* 

QSAR 

DK* 

QSA

R 

DA* 

QSA

R 

4-

methyl 

DADK

* 

QSAR 

2-

methyl 

DADK

* 

QSAR 

3-

NH3 

DA* 

QSA

R 

Daphni

d LC50 

4.2 16 0.26 16.4 53 5.0 14.2 14.2 11.4 

Daphni

d ChV 

1.29 1.21 1.07 1.29 5.1 0.31 1.07 1.07 0.836 

*=Residue to be included in TTR 

Comparison of Observed toxicity values and QSAR-generated endpoints (mg/L) for Metribuzin 

and its degradates by chemical class chosen in ECOSAR – Amides ECOSAR class 

Taxon Parent 

data 

Parent 

QSAR 

Ratio of parent 

data/parent QSAR 

DA* 

 QSAR 

3-NH3 DA* 

QSAR 

FW fish 42 149 0.28 26 99 

SW fish 85 131 0.65 23 87 

Mysid 48 7 6.9 1.7 4.8 

*=Residue to be included in TTR 

 Comparison of Observed toxicity values and QSAR-generated endpoints (mg/L) for 

the Metribuzin degradate DADK and other metribuzin degradates by chemical class 

chosen in ECOSAR – Carbonyl ureas ECOSAR class 

Taxon DADK 

data 

DADK 

 QSAR 

Ratio of 

DADK 

data/DADK 

QSAR 

DK* 

QSAR 

4-methyl 

DADK* 

QSAR 

2-methyl 

DADK* 

QSAR 

Daphnid 

ChV 

3.3 1.22 2.7 6.1 0.961 0.961 

*=Residue to be included in TTR