United Statej office of PestiddM and Toxic Subrtance*
Environmental Protection Office of Pesticide Program* (TS-766C)
Agency Washington, DC 20460
vvEPA Pesticide
Fact Sheet
Name of Chemical: METRIBUZIN
Reason for Issuance:
Date Issued: June 3°'
Fact Sheet Number: 53
1. Description of the Chemical
Generic name: 4-amino- 6( 1, l-dimethylethyl)-3-(methylthio)-l,
2/4-triazin-5(4H)
Empirical formula:
Common name: Netribuzin
Trade name: Sencor, Lexone
Chemical Abstracts Service (CAS) Registry Number: 21097-64-9
Office of Pesticide Program's EPA Chemical Code Number: 101101
Year of Initial Registration: 1973
Pesticide Type: Herbicide
Chemical Family: S-triazine
U.S. Producer: Mobay Chemical Corporation
2. Use Patterns and Formulations
Application sites:
Metribuzin is registered for control of broadleaf weeds
and grasses in soybeans/ potatoes, barley/ winter wheat/
dormant established and sainfoin fields/ asparagus/ sugarcane/
tomatoes/ lentils/ peas/ and non-cropland.
Type of formulation:
Metribuzin is available as a 50 percent formulation
intermediate/ 94 percent technical for formulation into
end-use products/ wettable powder/ flowable concentrate and
dry flowable concentrate.
Types and methods of application:
Metribuzin may be soil incorporated/ surface applied or
applied foliarly, broadcast or band with ground equipment.
It can be applied by aerial equipment or sprinkler irrigation
(potatoes) .
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Metribuzln
fact sheet
Application Rates:
0.25 to 4.0 ai/A on crop sites,
6.0 to 8.0 ai/A on railroad rights—of—way.
Usual Carrier:
Water
3. Science Findings (Rationale for Regulatory Position)
Summary Science Statement:
Metribuzin is not acutely toxic by oral, derinal,
inhalation, or eye irritation routes of exposure. The
available data do not indicate that any of the risk cri-
teria listed in 162.11(a) of Title 40 of the U.S. Code of
Federal Regulations have been met or exceeded for the uses
of metribuzin at the present time. Data gaps include rat
chronic study, teratology study, multigeneration repro-
duction study and two categories of mutagenicity testing.
There are also extensive residue chemistry data gaps.
Metribuzin has been found in Ohio rivers and Iowa
wells. Although there are extensive data gaps in the
area of environmental fate, available data indicate that
metribuzin has a potential to contaminate ground water in
soils lower in organic matter and clay content.
Available data indicate that metribuzin is moderately
toxic to upland bird species on an acute oral basis, no
more than slightly toxic to birds in the diet, moderately
toxic to freshwater fish and invertebrates. Metribuzin
is slightly toxic to shrimp. A detailed ecological hazard
assessment cannot be made until the acute dietary study on
an upland gamebird, acute toxicity studies on a marine!
estuarine fish species and an oyster species, and appro-
priate environmental fate data are fulfilled.
Chemical Characteristics:
Metribuzin is a solid at room temperature. Its
molecular weight is 214.28. The melting point is 125.5 to
126.5 °C. Metribuzin is soluble in aromatic and chlorinated
hydrocarbon solvents, and in water (at 20 °C) to 1220 ppm.
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fact sheet
Toxicological Characteristics:
Acute toxicity effects of metribuzin are as follows:
Acute Oral Toxicity in rats: 2200 mg/kg body weight
for males; 2345 mg/kg body weight for females, Toxicity
Category III.
Acute Dermal Toxicity in rats: 20,000 mg/kg body
weight, Toxicity Category IV.
Acute Inhalation LC 50 —rat: > 20 mg/L/l hour, Toxicity
Category IV.
Skin Irritation in rabbits: PIS = 0.33/8.0, Toxicity
Category IV.
Eye Irritation in rabbits: Not an irritant, Toxicity
Category Iv.
Dermal Sensitization in guinea pig: Not a sensitizer,
Toxicity Category IV.
Subchronic and Chronic Effects:
The 2—year dog study indicated dogs dosed with 1500
ppm (37.5 mg/kg) had reduced weight gain, increased mortality,
hematological changes and liver and kidney damage. The no—
effect level is 100 ppm. The oncogenic potential of metri—
buzin is unclear at this time. The mouse oncogenicity study
is negative for oncogenic effects. The chronic rat study
indicates a statistically significant (p < 0.05) increase in
the incidence of adenoma of liver bile duct and pituitary
gland in females at the 300 ppm dose level. Additional
histopathology and historical control data on the incidence
of these tumors in this particular strain of rats are needed
before it can be determined if the increase is compound
related. A teratology study in rabbits indicated no evidence
of teratogenic effects at 135 mg/kg/day, the highest dose
tested (HDT) and a NOEL of 15 mg/kg/day for maternal and
fetal toxicity. Data gaps include rat chronic study, rat
teratology study and multigeneration reproduction study.
Mutagenic Effects:
Available data indicate that metribuzin is not mutagenic.
Data gaps exist in two categories of mutagenicity testing,
specifically gene mutation studies in mammalian cells and
tests for primary DNA damage such as sister chromatid exchange
or unscheduled DNA synthesis assay.
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N—Nitroso Contaminants:
Available data do not provide grounds for concern at
this time. The data are incomplete. The analy is for
N—nitroso contaminants is requested.
Major Routes of Human Exposure:
Primary nondietary exposure to the farmer is expected
to be derinal and to occur during mixing, loading, and
application. Exposure through ocular, inhalation and
ingestion routes are also expected.
PHYSIOLOGICAL AND BIOCHEMICAL BEHAVIOR CHARACTERISTICS:
Adsorption Characteristics:
Metribuzin is absorbed through the leaves from surface
treatment, but the major and significant route for uptake
is via the root system.
Translocation Characteristics:
Uptake through the roots is best described as osmotic
diffusion. Metribuzin is translocated upward in the xylem
and moves distally when applied at the base of the leaves.
It concentrates in the roots, stems, and leaves.
Mechanism of Pesticidal Action:
Photosynthesis inhibitor.
Metabolism in Plants:
The major routes of detoxification are the action of
oxidation and conversion to water soluble conjugated products.
ENVIRONMENTAL CHARACTERISTICS:
Adsorption and Leaching in Basic Soil Types:
Metribuzin is moderately adsorbed on soils with high
clay and/or organic matter content. Metribuzin is readily
leached in sandy soils low in organic matter content.
Microbial breakdown:
Microbial breakdown appears to be the major mechanism
by which metribuzin is lost from soils. Breakdown occurs
fastest under aerobic conditions and at comparatively high
temperatures.
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Loss from photodecomposition and/or volatilization:
Slight loss.
Average persistence at recommended rates:
Half—life varies with soil type and climatic conditions.
Half—life of metribuzin at normal use rates is 1 to 2 months.
Potential Ground Water Problem:
Metribuzin has been found in Ohio rivers and Iowa wells.
Available data show that metribuzin has a potential to
contaminate ground water in soils low in organic and clay
content. The Agency is requesting water monitoring studies
on rnetribuzin and has determined that all uses of metribuzin
should be classified for restricted use with appropriate
labeling including a ground water advisory statement.
Ecological Characteristics:
Avian Acute Oral Toxicity: 169.22 mg/kg (moderately
toxic).
Subacute Dietary Toxicity: > 4000 ppm for mallard duck
and bobwhite quail (slightly toxic).
Acute Toxicity on Freshwater Invertebrate: 4.18 ppm
(moderately toxic).
Acute Toxicity on Fish: 76.78 ppm for rainbow trout
(slightly toxic), 75.96 ppm for bluegill sunfish
(slightly toxic).
96—Hour LC 50 on a Marine/Estuarine Shrimp: 48.3 mg/l
(slightly toxic).
Potential Problem for Endangered Species:
The Agency evaluated metribuzin under the cluster/use
pattern approach for use on corn, soybeans, and small grains.
Available data indicate that metribuzin use on crops would
probably not affect Federally listed animal species.
Consultation with Office of Endangered Species (OES) on
use of sulfometuron methyl indicated several species of
endangered plants which occur on or adjacent to rights—of—
way would be jeopardized by exposure from its use. The
Agency has concluded that these plants would be jeopardized
by exposure to metribuzin. The Agency is imposing a state-
ment concerning endangered plant species on all end—use
products containing metribuzin and labeled for use on
rights-of—way.
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Metribuz In
fact sheet —6—
Tolerance Assessment:
The Acceptable Daily Intake (ADI) is based on a
no—observable effect level of 100 ppm (2.5 mg/kg) from
the 2—year dog study. Using a 100—fold safety factor the
ADI is 0.025 mg/kg/day with a Maximum Permissible Intake
(MPI) of 1.5 mg/kg for a 60 kg adult human. Theoretical
maximum residue contribution (TMRC) for metribuzin based on
established tolerances is 0.3508 mg/day for a 1.5 kg diet.
Currently, the permanent tolerances utilize 23.39 percent
of the ADI.
The Agency is unable to complete a full tolerance
reassessment because the available metribuzin toxicology and
residue data do not fully support the established tolerances
listed below. The metabolism of metribuzin in animals is
not fully understood. Therefore, the Agency is requiring
data on metabolism of metribuzin and related metabolites in
ruminants, poultry, and several crops. The additional data
will be used to assess dietary exposure to metribuzin and
may lead to revisions in the existing tolerances.
Commodities Parts Per Million
Alfalfa, green 2.0
Alfalfa, hay 7.0
Asparagus 0.05
Barley, grain 0.75
Barley, straw 1.0
Cattle, fat 0.7
Cattle, mbyp 0.7
Cattle, meat 0.7
Corn, fodder 0.1
Corn, forage 0.1
Corn, fresh (inc sweet K + CWHR) 0.05
Corn, grain (inc popcorn) 0.05
Eggs 0.01
Goats, fat 0.7
Goats, mbyp 0.7
Goats, meat 0.7
Grass 2.0
Grass, hay 7.0
Hogs, fat 0.7
Hogs, mbyp 0.7
Hogs, meat 0.7
Horses, fat 0.7
Horses, mbyp 0.7
Horses, meat 0.7
Lentils (dried) 0.05
Lentils, forage 0.5
Lentils, vine hay 0.05
Milk 0.05
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fact sheet
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Commodities
Peas
Peas (dried)
Peas, forage
Peas, vine hay
Potatoes
Poultry, fat
Poultry, mbyp
Poultry, meat
Sainfoin
Sainfoin, hay
Sheep, fat
Sheep, mbyp
Sheep, meat
Soybeans
Soybeans, forage
Soybeans, hay
Sugarcane
Tomatoes
Wheat, forage
Wheat, grain
Wheat, straw
Parts Per Million
0.1
0.05
0.5
0.05
0.6
0.7
0.7
0.7
2.0
7.0
0.7
0.7
0.7
0.1
4.0
4.0
0.1
0.1
2.0
0.75
1.0
Food
Barley, milled fractions
(except flour)
Potatoes, processed
(inc potato chips)
Sugarcane molasses
Wheat, milled fractions
(except flour)
Feed
Barley, milled fractions
(except flour)
Potato waste, processed
(dried)
Sugarcane bagasse
Sugarcane molasses
Tomato pomace, dried
Wheat, milled fractions
(except flour)
Parts
Per
Million
3.0
3.0
2.0
3.0
Parts
Per
Million
3.0
3.0
0.5
0.3
2.0
3.0
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fact sheet
International Tolerances — Canada
CommoditIes Parts Per Million
Asparagus 0.1
Barley grain 0.1
Lentils 0.1
Peas 0.1
Potatoes 0.1
Soybeans 0.1
Tomatoes 0.1
Wheat grain 0.1
Although the above Canadian tolerances differ from
those in the United States, it is inappropriate for the
Agency to harmonize these tolerances at the present time
because of extensive toxicology and residue chemistry data
gaps.
There are no tolerances for residues of metribuzin in
Mexico or Codex Alimentarius .
Problems Known to Have Occurred With Use:
The Pesticide Incident Monitoring System (PIMS) does
not indicate any incident involving agricultural uses of
met r i bu z in.
4. Summary of Regulatory Position
Based on the review and evaluation of all available data
and other relevant information on metribuzin, the Agency has
made the following determinations.
The available data do not indicate that any of the risk
criteria listed in 162.11(a) of Title 40 of the U.S. Code of
Federal Regulations have been met or exceeded for the uses
of metribuzin at the present time.
The Agency will not allow any significant new uses to be
established for metribuzin until the toxicology and residue
chemistry deficiencies identified have been satisfied.
The Agency is requesting data on presence of nitroso—
contaminants in metribuzin. Available data do not provide
grounds for concern at this time.
Based on concern for ground water contamination, the
Agency has determined that all uses of metribuzin should be
classified as restricted use and carry appropriate labeling
including a ground water advisory statement.
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Metribuzln —9—
fact sheet
The agency is concerned about the exposure of endangered/
threatened plant species occurring on or asjacent to rights-
of—way from the use of metribuzin. An Endangered Species statement
is being required on the labeling.
The Agency Is Imposing a rotatonal crop restriction. The
extent of this restriction will be reconsidered when additional
data are recieved.
Specific Label Precautionary Statements:
Hazard Information:
The Human Precautionary Statements must appear on all MP
labels as precribed in 40 CPR 162.10.
Environmental Hazards Statements:
All manufacturing—use products (MP’s) intended for
formulation into end—use products (EP’s) must bear the following
statements:
Do not discharge effluent containing this product
into lakes, streams, ponds, estuaries, oceans or
public water unless this product Is specifically
indentif led and addressed In an NPDES permit. Do
not discharge effluent containing this product to
sewer systems without previously notifying the sewage
treatment plant authority. For guidance contact your
State Water Board or Regional Office of EPA.
All end—use products with outdoor uses must bear the
following statement.
Do not apply directly to water or wetlands. Do not
contaminate water by cleaning of equipmet or disposal
of waste.
Ground Water Statement:
All end—use products (EP’s) must be classified as
“RESTRICTED USE” (Refer to 110 CPR 162.1O(j)(2)(B)) and
the label8 must bear the following ground water advisory.
Metribuzin is a chemical which can travel (seep
or leach )though soil and can contaminate ground
water which may be used a drinking water.
metribuzin has been found in ground water as
result of agricultural use. Users are advised
not to apply metribuzin where the water table
(ground water) Is close to the surface and where
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soils are very permeable; i.e., well drained
soils such as loamy sands. Your local agricultural
agencies can provide further information on .the
type of soil in your area and the location of
ground water.
Endangered Species:
Notice : The use of this product on rights—of—way may
pose a hazard to certain Federally designated endangered
plant species. They are known to be found in specific areas
within the locations noted below. Prior to making applications,
the user of this product must determine that no such species
are located in or immediately adjacent to the area to be
treated. For information on protected species contact the
Endangered Species Specialist of the appropriate Regional
Office of the U.S. Fish and Wildlife Service listed below:
Region 1—Portland, Oregon
California counties of Contra Costa, Solano, San Diego,
Santa Barbara, Ventura, Los Angeles and Orange.
Idaho, Idaho County.
Oregon, Rarney County.
Region 2—Albuquerque, New Mexico
Arizona counties of Coconino and Navajo.
New Mexico counties of San Juan, Otero, Chaves, Lincoln,
Eddy and Dona Ana.
Texas counties of El Paso, Pecos and Runnels.
Region 3—Twin Cities, Minnesota
Iowa counties of Allamakee, Clayton, and Jackson.
Region 4—Atlanta, Georgia
Florida counties of Clay, Gulf, Gadsden, Franklin and
Liberty.
Georgia counties of Wayne and Brantley.
North Carolina, Henderson County.
South Carolina, Greenville County.
Region 5—Newton Corner, Massachusetts
New York, Ulster County.
Region 6-Denver, Colorado
Utah counties of Emery, Piute, Garfield, Washington,
Utah and Wayne.
Colorado counties of Montezuma, Delta and Montrose.
Restrictions on Rotational Crops
Do not plant food and feed crops other than those
which are registered for use on metribuzin treated soils.
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