metribuzin (Lexone, Sencor) Herbicide Profile 6/85
CHEMICAL FACT SHEET FOR:
FACT SHEET NUMBER: 53
DATE ISSUED: JUNE 30, 1985
1. DESCRIPTION OF THE CHEMICAL
- Generic Name: 4-amino-6(1,1-dimethyl)-3-(methylthio)-1,2,4-
- Common Name: metribuzin
- Trade Names: Sencor. Lexone
- EPA Shaughnessy Code: 101101
- Chemical Abstracts Service (CAS) Number: 21097-64-9
- 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.
- Types of formulations: Metribuzin is available as a 50% formulation
intermediate, and 94% 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).
- Application rates: 0.25 to 4.0 a.i./A on crop sites; 6.0 to 8.0 a.i./
A on railroad rights-of-way.
- Usual carrier: Water
3. SCIENCE FINDINGS
- 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.
- 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 LC50 in rats: >20 mg/l/hr. Toxicity Category
- Skin irritation in rabbits: PIS = 0.33/8.0. Toxicity Category
- Eye irritation in rabbits: Not an irritant. Toxicity Category IV
- Dermal sensitization in guinea pig: Not a sensitizer. Toxicity
- 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 metribuzin
is unclear at this time. The mouse oncogenicity study is negative
for oncogenic effects. The chronic rat study indicates a statis-
tically 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 multi-
generation 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.
- N-nitroso contaminants: Available data do not provide grounds for
concern at this time. The data are incomplete. The analysis for
N-nitroso contaminants is requested.
- Major routes of human exposure: Primary nondietary exposure to the
farmer is expected to be dermal and to occur during mixing, loading,
and application. Exposure through ocular, inhalation, and ingestion
routes is also expected.
- Physiological and Biochemical Behavioral Characteristics
- Absorption 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
- Environmental Characteristics
- Adsorption and leaching in basic soil types: Metribuzin is moder-
ately 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
- 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 one to two months.
- Potential groundwater problem: Metribuzin has been found in Ohio
rivers and Iowa wells. Available data show that metribuzin has a
potential to contaminate groundwater in soils low in organic and
clay content. The Agency is requesting water monitoring studies on
metribuzin and has determined that all uses of metribuzin should be
classified for restricted use with appropriate labeling, including
a groundwater advisory statement.
- Ecological Characteristics
- Avian acute oral toxicity: 169.22 mg/kg (moderately toxic).
- Subacute dietary toxicity: >4,000 ppm for mallard duck and bobwhite
quail (slightly toxic).
- Acute toxicity on freshwater invertebrates: 4.18 ppm (moderately
- Acute toxicity on fish: 76.78 ppm for rainbow trout (slightly
toxic), 75.96 ppm for bluegill sunfish (slightly toxic).
- 96-hour LC50 on a marine/estuarine shrimp: 48.3 mg/l (slightly
- 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 that 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 statement concerning endangered plant
species on all end-use products containing metribuzin and labeled
for use on rights-of-way.
- 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 bas-d on established tolerances is 0.3508 mg/day for a
1.5 kg diet. Currently, the permanent tolerances utilize 23.39%
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. There-
fore, the Agency is requiring data on metabolism of metribuzin and
related metabolites in ruminants, poultry, and several crops. The
additional data will be us-d 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
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
Goats, fat 0 7
Goats, mbyp 0.7
Goats, meat 0.7
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
Peas (dried) 0.05
Peas, forage 0.5
Peas, vine hay 0.05
Poultry, fat 0.7
Poultry, mbyp 0.7
Poultry, meat 0.7
Sainfoin, hay 7.0
Sheep, fat 0 7
Sheep, mbyp 0 7
Sheep, meat 0.7
Soybeans, forage 4.0
Soybeans, hay 4.0
Sugarcane 0 1
Wheat, forage 2.0
Wheat, grain 0 75
Wheat, straw 1.0
Barley, milled fractions (except flour) 3.0
Potatoes, processed (inc. potato chips) 3.0
Sugarcane molasses 2.0
Wheat, milled fractions (except flour) 3.0
Barley, milled fractions (except flour) 3.0
Potato waste, processed (dried) 3.0
Sugarcane bagasse 0 5
Sugarcane molasses 0.3
Tomato pomace. dried 2.0
Wheat, milled fractions (except flour) 3.0
- International tolerances: Canada
Commodities Parts per million
Barley grain 0.1
Lentils 0 1
Soybeans O 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
- Problems Known to have Occurred with Use
The Pesticide Incident Monitoring System (PIMS) does not indicate any
incident involving agricultural uses of metribuzin.
- Summary Science Statement
- Metribuzin is not acutely toxic by oral, dermal, inhalation, or eye
irritation routes of exposure. 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. Data gaps include rat
chronic study, teratology study, multi-generation reproduction
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 contam-
inate groundwater in soils lower in organic matter and clay
- 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, and 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 appropriate
environmental fate data are fulfilled.
4. SUMMARY OF REGULATORY POSITION AND RATIONALE
- Based on the review and evaluation of all available data and other
relevant information on metribuzin, the Agency has made the following
- 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 deficien-
cies 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
- Based on concern for groundwater 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.
- The Agency is concerned about the exposure of endangered/threatened
plant species occurring on or adjacent to rights-of-way from the use
of metribuzin. An Endangered Species Statement is being required on
- The Agency is imposing a rotational crop restriction. The extent of
this restriction will be reconsidered when additional data are
- Specific label precautionary statements:
- Hazard information: The human precautionary statements must
appear on all MP labels as prescribed in 40 CFR 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 identified 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 equipment or disposal of waste.
Groundwater statement: All end-use products (EP's) must be
classified as Restricted Use (refer to 40 CFR 162.10(j)(2)(B)),
and the labels must bear the following groundwater advisory:
Metribuzin is a chemical which can travel (seep or leach) through
soil and can contaminate groundwater which may be used as
drinking water. Metribuzin has been found in groundwater as a
result of agricultural use. Users are advised not to apply
metribuzin where the water table (groundwater) is close to the
surface and where 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 groundwater.
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 applica-
tions, 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 - Harney 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,
- 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,
- Region 5, Newton Corner, Massachusetts. New York, Ulster
- 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
5. SUMMARY OF MAJOR DATA GAPS
- The following toxicological studies are required:
- Acute inhalation toxicity (March 31, 1986).
- Rat chronic/oncogenicity study (August 31, 1989).
- Rat teratology study (September 30, 1986).
- 2-generation rat reproduction study (September 30, 1988).
- Mutagenicity testing (March to June, 1986).
- General metabolism study (June 30, 1988).
- The following environmental fate data are required:
- Hydrolysis (March 31, 1986).
- Photodegradation in water or soil (March 31, 1986).
- Metabolism studies in aerobic and anaerobic soils (September 30,
- A mobility test involving leaching and adsorption/desorption
(June 30, 1986).
- Soil dissipation study (September 30, 1986).
- Accumulation on rotational crops, confined (September 30, 1988).
- Accumulation on rotational crops, field (February 28, 1989).
- Accumulation study in fish (December, 1986).
- The following ecological effects data are required:
- Avian subacute dietary toxicity, waterfowl (March 31, 1986).
- Acute toxicity to estuarine and marine organisms, fish and mollusk
(June 30, 1986).
- Product chemistry data are required during 1986.
- The following residue chemistry data are required:
- Plant metabolism data (June 30, 1987).
- Metabolism studies utilizing ruminants and poultry (December 31,
6. CONTACT PERSON AT EPA
Robert 0. Taylor
Office of Pesticide Programs
Registration Division (TS-767C)
401 M Street S.W.
Washington, DC 20460
THE INFORMATION PRESENTED IN THIS CHEMICAL INFORMATION FACT SHEET
IS FOR INFORMATIONAL PURPOSES ONLY AND NOT TO BE USED TO FULFILL
DATA REQUIREMENTS FOR PESTICIDE REGISTRATION AND REREGISTRATION.