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: 6/96
TRADE OR OTHER NAMES
Trade names for products containing imazethapyr include Contour, Hammer,
Overtop, Passport, Pivot, Pursuit, Pursuit Plus, and Resolve (1, 2, 3).
Imazethapyr is a general use pesticide with a toxicity classification of
III (slightly toxic). Check with specific state regulations for local
restrictions which may apply. Products containing imazethapyr must bear the
signal word "Caution" on their label (2). It is currently being tested on
alfalfa, clover and other crops in the U.S. and is being sold on these
commodities outside the U.S. (3).
Imazethapyr is an imidazole compound used as a selective herbicide. It is
applied preplant incorporated, preemergence, at cracking, and postemergence.
The compound controls weeds by reducing the levels of three branched-chain
aliphatic amino acids, isoleucine, leucine and valine, through the inhibition
of aceto-hydroxyacid synthase, an enzyme common to the biosynthetic pathway
for these amino acids. This inhibition causes a disruption in protein
synthesis which, in turn, leads to an interference in DNA synthesis and cell
growth. The compound is used to control grasses and broadleaved weeds
including barnyardgrass, crabgrass, cocklebur, panicums, pigweeds, nightshade,
mustard, smartweed, velvetleaf, jimsonweed, foxtails, seedling johnsongrass,
lambsquarters, morningglory and others. Tolerant crops include soybeans,
peanuts, dry and edible beans, peas, alfalfa and imidazolinone
resistant/tolerant corn. Additional research is being conducted on other
leguminous crops (1, 2, 3).
Imazethapyr comes in aqueous concentrate formulations. It is also
formulated and may be combined with other herbicides (2, 3).
The amount of a chemical 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 acute toxicity of the technical and formulated products are as
Technical: The acute oral LD50 for technical imazethapyr was reported to
be >5000 mg/kg for rats. The acute inhalation LC50 for rats exposed to
imazethapyr is 3.27 mg/L. The dermal LD50 for rabbits exposed to technical
imazethapyr was >2000 mg/kg. It was found to be slightly irritating to the
rabbit eye, with complete recovery in 7 days. The technical material was found
to be slightly irritating to the skin of male rabbits (4, 6).
Pursuit formulation: The oral LD50 of the 2 lb/gallon liquid formulation
of imazethapyr was also >5000 mg/kg for the rat and rabbit. The LC50 was >2.67
mg/L. The dermal LD50 was >5000 mg/kg. The liquid formulation was non-
irritating to the rabbit eye and skin (1, 2, 3, 4, 6).
Pursuit DG formulation: (70% dispersable granule) The oral LD50 of the
granular formulation was >5000 mg/kg for the rat and >2000 mg/kg for the
rabbit. It resulted in moderate irritation to the rabbit eye, with complete
recovery in 14 days. It showed slight skin irritation to the rabbit with
complete recovery in 72 hours (1, 2, 6).
90-day dietary subchronic toxicity studies in rats and dogs had a No
Observable Effect Level (NOEL) of 10,000 ppm, the highest dose tested (1).
A 1-year feeding study in dogs fed diets containing of 0, 1,000, 5,000,
or 10,000 ppm reported a NOEL of 1,000 ppm (25 mg/kg/day) (5).
A two-generation reproduction study in rats fed dietary levels of 0,
1,000, 5,000, or 10,000 ppm reported a NOEL for systemic and reproductive
effects of 10,000 ppm (500 mg/kg/day, the highest dose tested) (5).
A developmental toxicity study in rats fed dosage levels of 0, 125, 375,
and 1,125 mg/kg/day, reported a maternal toxicity NOEL of 375 mg/kg/day and a
developmental toxicity NOEL of greater than 1,125 mg/kg/day, the highest dose
A developmental toxicity study in rabbits fed dosage levels of 0, 100,
300, and 1,000 mg/kg/day reported a maternal toxicity NOEL of 300 mg/kg/day
and a developmental toxicity NOEL of greater than 1,000 mg/kg/day, the highest
dose tested (5).
A mutagenic test with Salmonella typhimurium was negative; an in vitro
chromosomal aberration test in Chinese hamster ovary cells was positive
without metabolic activation but at dose levels that were toxic to the cells
and negative with metabolic activation; an in vivo chromosomal aberration test
in rat bone marrow cells was negative; and a DNA synthesis study in rat
hepatocytes was negative (1, 5).
An 18-month carcinogenicity study with mice fed diets containing 0,
1,000, 5,000, or 10,000 ppm with no carcinogenic effects observed under the
conditions of the study at levels up to and including 10,000 ppm (1,500
mg/kg/day, the highest dose tested) and a systemic NOEL of 5,000 ppm (750
A 2-year chronic toxicity/carcinogenicity study in rats fed diets
containing 0, 1,000, 5,000, or 10,000 ppm with no carcinogenic effects
observed under the conditions of the study at levels up to an including 10,000
ppm (500 mg/kg/day, the highest dose tested) and a systemic NOEL of 10,000 ppm
(500 mg/kg/day, the highest dose tested) (5).
No information was found.
Fate in Humans and Animals
No information was found.
Effects on Birds
Study results reported the acute oral LD50 for mallard ducks and bobwhite
quail to be >2150 mg/kg body weight. The 8-day dietary LC50 for the same
species was >5000 ppm (1).
Effects on Aquatic Organisms
Study results reported the 96-hour LC50 for channel catfish to be 240
mg/l, 420 mg/l for bluegill sunfish, and 340 mg/l for rainbow trout. The 48-
hour LC50 for the aquatic crustacean Daphnia magna (water flea) was >100 mg/l
Effects on Other Animals (Nontarget species)
The contact LD50 for honey bees was >100 microgram/bee (1).
Breakdown of Chemical in Soil and Groundwater
The formulated product does not leach through the soil. It is weakly to
moderately adsorbed and is not lost from the soil through volitilization. Soil
microorganisms do not appear to play a significant role in the degradation of
the formulated product (1).
Breakdown of Chemical in Surface Water
No information was found.
Breakdown of Chemical in Vegetation
The formulated product is readily absorbed through the roots and foliage.
It is translocated in both xylem and phloem tissues and accumulates in growing
points (1, 3).
PHYSICAL PROPERTIES AND GUIDELINES
Technical imazethapyr is an off-white to tan solid with a slightly
pungent odor. The formulated product is a clear dark brown liquid with a musty
odor. The formulated product is corrosive to zinc, mild steel, brass, copper,
and aluminum (1, 3).
Thermal decomposition products may include toxic oxides of carbon and
|ADI: ||0.25 mg/kg body weight/day (5)
|NOEL: ||25 mg/kg body weight/day (5)
|CAS #: ||81335-77-5
|Chemical name: ||2-[4.5-dihydro-4-methyl-4-(1 -methylethyl)-5-oxo-lH-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid (3)
|Chemical Class/Use: ||imidazole compound used as a selective, pre and postemergence herbicide (3)
|Specific gravity: ||1.10 to 1.12 g/ml at 21 degrees C (1)
|Solubility in water: ||(distilled) 0.14 g/ml at 25 degrees C (1, 4)
|Solubility in other solvents: ||Soluble in dimethyl sulfoxide, methanol, methylene chloride; moderately soluble in acetone and 2-propanol; slightly soluble in toluene; very slightly soluble in heptane (1, 4).
|Melting point: ||169 to 173 degrees C (336 to 343 F) (1)
|Boiling point: ||decomposition at 180 degrees C (356 F) (1)
|Flashpoint: ||>93 degrees C (>200 F) (1)
|Vapor pressure: ||<1 x 10 to the minus 7 mmHg at 60 degrees C (140 F) (1)
|Kow: ||11 at pH 5, 31 at pH 7, and 16 at pH 9
American Cyanamid Co.
One Cyanamid Plaza
Wayne, NJ 07470-8426
Review by Basic Manufacturer:
Comments solicited: April, 1995
Comments received: February, 1996
Herbicide Handbook of the Weed Science Society of America. 1989. Sixth
edition. Champaign, IL.
Farm Chemicals Handbook. 1995. Meister Publishing Co. Willoughby, OH.
Thomson, W.T. 1993. Agricultural Chemicals. Book II: Herbicides.
Thomson Publications, Fresno, CA.
OHS Database. MDL Information Systems, Inc. 1994 (June) MSDS for
Imazethapyr. MDL Information Systems, Inc., San Leandro, CA.
U.S. Environmental Protection Agency. 1992. Pesticide Tolerances for
Imazethapyr. Federal Register. Vol. 57, No. 102. Rules and Regulations.
Wednesday, May 27, 1992.
Review by American Cyanamid Company. February 21, 1996.