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Imazethapyr

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).

REGULATORY STATUS

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).

INTRODUCTION

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).

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

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 follows:

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).

CHRONIC TOXICITY

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).

Reproductive Effects

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).

Teratogenic Effects

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 tested (5).

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).

Mutagenic Effects

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).

Carcinogenic Effects

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 mg/kg/day) (5).

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).

Organ Toxicity

No information was found.

Fate in Humans and Animals

No information was found.

ECOLOGICAL EFFECTS

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 (1).

Effects on Other Animals (Nontarget species)

The contact LD50 for honey bees was >100 microgram/bee (1).

ENVIRONMENTAL FATE

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 nitrogen (4).

Exposure Guidelines:

ADI: 0.25 mg/kg body weight/day (5)
NOEL: 25 mg/kg body weight/day (5)

Physical Properties:

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

BASIC MANUFACTURER

American Cyanamid Co.
One Cyanamid Plaza
Wayne, NJ 07470-8426
Fax: 201-831-2570
Telephone: 201-831-2000
Emergency: 201-835-3100

Review by Basic Manufacturer:

Comments solicited: April, 1995
Comments received: February, 1996

REFERENCES

  1. Herbicide Handbook of the Weed Science Society of America. 1989. Sixth edition. Champaign, IL.
  2. Farm Chemicals Handbook. 1995. Meister Publishing Co. Willoughby, OH.
  3. Thomson, W.T. 1993. Agricultural Chemicals. Book II: Herbicides. Thomson Publications, Fresno, CA.
  4. OHS Database. MDL Information Systems, Inc. 1994 (June) MSDS for Imazethapyr. MDL Information Systems, Inc., San Leandro, CA.
  5. U.S. Environmental Protection Agency. 1992. Pesticide Tolerances for Imazethapyr. Federal Register. Vol. 57, No. 102. Rules and Regulations. Wednesday, May 27, 1992.
  6. Review by American Cyanamid Company. February 21, 1996.