E X T O X N E T
Extension Toxicology Network
A Pesticide Information Project of Cooperative Extension Offices of
Cornell University, Michigan State University, Oregon State University, and
University of California at Davis. Major support and funding was provided
by the USDA/Extension Service/National Agricultural Pesticide Impact
Publication Date: 9/93
TRADE OR OTHER NAMES
Some trade names include Bay 94337, Bay DIC 1468, Lexone, Sencor,
Sencoral and Sencorex (17).
The United States Environmental Protection Agency (EPA) issued a
registration standard in 1985 which gave metribuzin a restricted use
classification because of questions regarding groundwater contamination and
chronic toxicity. However, these questions were later resolved when
manufacturers submitted necessary information. The restricted use status was
removed and it is now classified as a general use pesticide (10, 12).
Products containing metribuzin must bear the signal word "Caution" (17).
Metribuzin is a selective triazinone herbicide which inhibits
photosynthesis. It is used for control of annual grasses and numerous
broadleaf weeds in field and vegetable crops, in turfgrass, and on fallow
lands. Metribuzin is available as liquid suspension, water dispersible
granular, and dry flowable formulations (2, 17).
Metribuzin is slightly to moderately toxic to humans by oral, skin or
inhalation routes of exposure (18). Sedation and labored breathing were
observed in metribuzin poisoned rats. Deaths occurred within 24 hours, while
survivors recovered slowly without permanent effects (5). No effects were
observed in rats exposed to an aerosol concentration of 31 mg/m3 for 6
hours/day, five days/week, for 3 weeks.
No effects were observed in rats given dermal doses of 1,000 mg/kg/ day
for 3 weeks (20). Metribuzin was not irritating to the skin or eyes of
rabbits or of human volunteers. Dermal application did not cause skin
sensitization (5, 16). Acute inhalation exposure may cause irritation of the
mucous membranes of the upper respiratory tract (9).
The amount of metribuzin that is lethal to one-half (50%) of experimental
animals fed the material is referred to as its acute oral lethal dose fifty,
or LD50. The oral LD50 for technical metribuzin in rats is 1,090 to 2,300
mg/kg; in mice it is 700 mg/kg, and in guinea pigs is 245 to 274 mg/kg. The
dermal LD50 in rabbits is greater than 20,000 mg/kg. The lethal
concentration fifty, or LC50, is that concentration of a chemical in air or
water that kills half of the experimental animals exposed to it for a set time
period. The 4-hour LC50 for metribuzin in rats is greater than 885 mg/m3
(5, 12, 17, 18, 20).
Two-year feeding studies with rats and dogs resulted in no observable
effect levels (NOEL) of 5 mg/kg for rats and 2.5 mg/kg for dogs (1, 18).
Reduced weight gain, an increase in the number of deaths, blood chemistry
changes, and liver and kidney damage were observed in a two-year study in
which dogs were given 1,500 ppm or 37.5 mg/kg of metribuzin (12, 20). No ill
effects were observed in dogs fed dietary doses of 12.5 mg/kg for three months
(5). In a 3-month study with rats given dietary doses of 0, 2.5, 7.5, 25 or
75 mg/kg of metribuzin, no effects were apparent in rats receiving 2.5 mg/kg.
At doses of 25 and 75 mg/kg, enlarged livers and thyroid glands were observed
EPA has established a Lifetime Health Advisory (LHA) level of 200 ug/l
(ppb) for metribuzin. Water containing metribuzin at or below this level is
acceptable for drinking every day over the course of one's lifetime, and does
not pose any health risk. However, consuming very high levels of metribuzin
over a long period of time has caused kidney damage in test animals (18).
Doses of 15, 45 or 135 mg/kg/day of technical metribuzin were
administered by gavage to rabbits on days 6 through 18 of pregnancy. Body
weight gain decreased at 135 mg/kg. No effects on the mothers were observed
from doses at or below 45 mg/kg. No effects on the fetuses were observed at
any of the doses tested (18). A three-generation study in rats, at 35, 100,
and 300 ppm, showed no influence on reproduction (1, 16).
No evidence of teratogenic effects were observed in rabbits given 135
mg/kg/day, the highest dose tested. Metribuzin did not have embryotoxic or
teratogenic effects in tests on rats (highest dose tested: 100 mg/kg/day) (1).
Tests on live animals and on tissue cultures have shown no mutagenic
There were no indications of carcinogenic effects in rats fed 35, 100,
and 300 ppm for two years, nor in mice fed 20, 800 and 3,200 ppm for two years
(1, 16). EPA has stated that data from these and other laboratory studies are
inadequate to determine if metribuzin causes cancer. Metribuzin remains
unclassified by the EPA regarding its carcinogenicity (18).
In single high doses, metribuzin appears to depress the central nervous
system. It affects the thyroid gland and stimulates metabolic enzymes of the
liver if given in repeated doses (1). No organ changes were seen in fatally
poisoned rats that died within 24 hours, nor in survivors of excess metribuzin
Fate in Humans and Animals
The breakdown of metribuzin in animals is not fully understood (12).
Reportedly, after metribuzin is absorbed, it is rapidly distributed in the
body and excreted unchanged in the urine (1). In warm-blooded animals, 90%
elimination occurs within 96 hours, about equally distributed between the
urine and feces (6).
Effects on Birds
Data indicate that metribuzin is moderately toxic to birds on an acute
oral basis, and that it is slightly to non-toxic to upland birds when it is
given in the diet (12). The LD50 values are greater than 100 mg/kg for
bobwhite quail, mallard ducks, canaries, red-winged blackbirds, brown headed
cowbirds, common grackles, and house sparrows (14, 18). The LD50 in Japanese
quail is 168 mg/kg, in mallard ducks is greater than 460 mg/kg, and in
bobwhite quail is greater than 164 mg/kg (16, 17).
Effects on Aquatic Organisms
Metribuzin is slightly toxic to fish. The 96-hour LC50 in rainbow trout
is 64 to 76 mg/l, 80 mg/l in bluegill sunfish, and is greater than 10 mg/l in
goldfish (6, 12, 15, 17).
Effects on Other Animals (Nontarget species)
Metribuzin is moderately toxic to freshwater invertebrates; the 96-hour
LC50 for marine/estuarine shrimp is 48.3 mg/l (12). It is non-toxic to bees
(6, 8, 17).
Endangered plant species, especially those growing on or near treated
rights-of-ways, may be jeopardized by exposure to metribuzin. Before applying
metribuzin to specific areas in which endangered species are known to be
found, applicators are required to determine that no such species are located
in or adjacent to the area to be treated. Information can be obtained from
the Endangered Species Specialist at a regional office of the U.S. Fish and
Wildlife Service (12).
Breakdown of Chemical in Soil and Groundwater
Metribuzin is highly soluble in water and has a low tendency to adsorb to
most soils (18). It has a moderate ability to adsorb to soils with high clay
and/or organic matter content. In sandy soils that are low in clay and
organic matter, the herbicide is readily leached (12, 21). The EPA considers
metribuzin to be one of a group of pesticide compounds that has the greatest
potential for leaching into, and contaminating, groundwater (11). It has been
detected in Ohio rivers and Iowa wells and groundwater (13, 18). It should
not be applied where the water table is close to the surface or where soils
are very permeable, such as on well-drained soils or loamy sands (12).
The major mechanism by which metribuzin is lost from soil is microbial
degradation. Any condition which favors activity of soil microorganisms will
increase the rate of breakdown. Losses due to volatilization or
photodegradation are not significant under field conditions (21).
The persistence of metribuzin is determined by its rate of adsorption to
soil particles, which is closely related to the organic matter content of the
soil. In soils with a high organic matter content, adsorption is increased,
loss through leaching is prevented, and the half-life can be stretched to
several months. Other soil characteristics, such as low soil moisture, low
temperatures, and acidic conditions, may also increase the persistence and
adsorption of the class of herbicides in which metribuzin is included, the
triazines. Metribuzin residues have carried over into the season following
treatment, especially on muck soils (7). The half-life of metribuzin varies
according to soil type and climatic conditions. Soil half-lives of less than
one month to 6 months have been reported (18).
Breakdown of Chemical in Water
The half-life of metribuzin in pond water is approximately seven days
(6). The hydrolysis half-life of metribuzin is nine to 28 weeks (13).
Breakdown of Chemical in Vegetation
Metribuzin is absorbed through the leaves when plants are given surface
treatment, but the primary route for uptake is through the root system. From
the roots, it is translocated upward, becoming concentrated in the roots,
stems, and leaves of treated plants (12).
PHYSICAL PROPERTIES AND GUIDELINES
Metribuzin is a white, crystalline solid with a slightly sharp, sulfurous
odor. It is stable in acidic and alkaline solutions. The pure compound is
colorless, with a mild, chemical odor. Technical metribuzin is white to
yellowish with a slightly sharp, sulfurous odor. Metribuzin is nonflammable
and noncorrosive and stable under normal temperatures and pressures. It may
burn if exposed to heat or flame, but it does not readily ignite Thermal
decomposition may release toxic fumes of oxides of carbon, nitrogen and sulfur
(1, 4, 14, 15, 16, 20, 21).
Occupational Exposure Limits:
|ACGIH TWA: ||5 mg/m3
|OSHA TWA: ||5 mg/m3
|NIOSH recommended TWA: ||5 mg/m3 (20)
|CAS #: ||21087-64-9
|Specific gravity: ||1.28
|Solubility in water: ||1,200 ppm at 20 degrees C (18)
|Solubility in other solvents:
soluble in aromatic and chlorinated hydrocarbon solvents (12).
|Solvent||g/100 g solvent at
20 degrees C
|Dimethyl formamide ||17
|kerosene ||< 1 (14)
|Melting point: ||125 to 126.5 degrees C (261 degrees F) (1, 12)
|Vapor pressure: ||less than 10 to the minus 5 mm Hg at 20 degrees C (16, 18)
|Koc: ||41 g/ml (18)
|Kd: ||0.11, 0.37 (13)
|Chemical Class/Use: ||Triazole herbicide
P.O. Box 4913
Kansas City, MO 64120
Review by Basic Manufacturer - Mobay:
Comments solicited: October, 1992
Comments received: November, 1992
Du Pont Agricultural Products
Walker's Mill, Barley Mill Plaza
PO Box 80038
Wilmington, DE 19880-0038
Review by Basic Manufacturer - Du Pont Agr.:
Comments solicited: October, 1992
Comments received: November, 1992
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