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Clomazone

Publication Date: 9/93

TRADE OR OTHER NAMES

Command, Commence, Gamit, Magister and Merit.

REGULATORY STATUS

Products containing clomazone must bear the signal word "Caution" on the label (1).

INTRODUCTION

Clomazone is a broad spectrum herbicide used for control of annual grasses and broadleaf weeds in cotton, peas, pumpkins, soybeans, sweet potatoes, tobacco, winter squash and fallow wheat fields. It can be applied early preplant, preemergent or preplant-incorporated depending on the crop, geographical area and timing. Because clomazone is an inhibitor of plant pigments, users must exercise caution to avoid drift or vapors which may cause bleaching damage to non-target foliage. Clomazone is available in emulsifiable concentrate formulations.

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

Clomazone is a slightly toxic material by ingestion, inhalation and dermal exposure.

The oral LD50 for technical clomazone in female rats is 1,369 mg/kg and 2,077 mg/kg in male rats (1). The oral LD50 for Command 4EC is slightly higher, 1,406 mg/kg for female rats and 2,343 for male rats (1, 3). The dermal LD50 on rabbits is > 2,000 mg/kg (1). The inhalation LC50 for technical clomazone in female rats is 4.23 mg/l and in male rats is 6.52 mg/l (3). The inhalation LC50 for Command 4EC in rats is 4.5 to 4.7 mg/l (FMC Corp.).

CHRONIC TOXICITY

Two-year feeding studies with rats and mice and a 1-year feeding study with dogs showed no long term adverse effects from Command (3). In a 1-year feeding study with dogs given doses of 0, 2.5, 12.5, 62.5 or 125 mg/kg, increased liver weight occurred at the 2.5 mg/kg level. The NOEL was 12.5 mg/kg/day (6). In 2-year feeding studies with rats and mice given 20, 100, 500, 1,000 or 2,000 ppm, the NOEL was 100 ppm (4.3 mg/kg/day) in rats, and 100 ppm (15 mg/kg/day) in mice. At doses above the NOEL, rats exhibited elevated cholesterol levels, increased liver weights, and enlarged liver cells. Mice given doses above the NOEL had elevated white blood cell counts (6).

Reproductive Effects

In a 2-generation study with rats, each generation was fed clomazone at 0, 5, 50, 100 or 200 mg/kg/day for 11 weeks in between weaning and mating. There was no effect on reproductive performance other than a decrease in pup weights in the second generation at 200 mg/kg/day. The NOEL for this study was 100 mg/kg/day (7).

Teratogenic Effects

Clomazone is not teratogenic (4). No birth defects were seen in the offspring of rats given 600 mg/kg/day, the highest dose tested, nor in the offspring of rabbits given 700 mg/kg/day (5, 6).

Mutagenic Effects

Clomazone is not mutagenic. The results of several tests, including an unscheduled DNA synthesis test, 2 reverse mutation tests, and a chromosomal aberration test, have all been negative. One test was weakly positive (5, 6).

Carcinogenic Effects

EPA states that clomazone does not cause tumor formation (4). No tumor formation occurred in mice or rats given dietary doses as high as 100 mg/kg for 2 years (3, 5).

Fate in Humans and Animals

Metabolism studies in rats show that 90 to 99% of the product Command administered to rats was excreted within 72 hours and there was no significant retention of the herbicide in rat tissues (4).

ECOLOGICAL EFFECTS

Effects on Birds

The oral LD50 for technical clomazone in bobwhite quail and mallard ducks is > 2,510 mg/kg. The 8-day dietary LC50 in bobwhites and mallards is 5,620 ppm (3).

Effects on Aquatic Organisms

The 96-hour LC50 for technical clomazone in rainbow trout is 19 ppm, 34 ppm in bluegill sunfish, 6.26 mg/l in Atlantic silversides, 40.6 mg/l in sheepshead minnows, 0.566 mg/l in mysid shrimp, 5.3 mg/l in eastern oysters, and 5.2 mg/l in Daphnia magna (3).

Effects on Other Animals (Nontarget species)

No information was found.

ENVIRONMENTAL FATE

Clomazone is relatively stable to degradation by UV light (3). It is highly volatile and can drift during or after application, causing damage to sensitive, non-target plants such as ornamental trees and shrubs, roses, small grains, alfalfa, sunflowers, and vegetable crops (4).

Breakdown of Chemical in Soil and Groundwater

Clomazone is highly soluble in water, but it has a moderate tendency to adsorb to soil particles. It therefore has a low to moderate potential to contaminate groundwater (2, 9). The product Command has low mobility in sandy loam, silt loam and clay loam soils. It is moderately mobile in fine sand (3).

Microbial degradation of Command is promoted by high soil moisture, warm temperature, and by increasing the pH to 6.5. Degradation was faster in a sandy loam than in silt or clay loams (3). In field studies, the half-life of clomazone was 28 to 84 days, depending on soil type and the organic matter content (3).

When a 4EC formulation of clomazone was applied to saturated soil at 2 lb. of active ingredient per acre, no clomazone was found below the top 12 inches of soil for 61 days following the application. The level of clomazone detected in the soil just after application was 0.8 ppm. This concentration dissipated rapidly to 0.2 ppm after 6 days, and then remained constant until the end of the 61-day test (8).

Breakdown of Chemical in Surface Water

Under laboratory conditions, clomazone was not readily hydrolyzed in sterile water (10). In water, clomazone is subject to photodegradation with half-lives of 1.5 to 7 days reported for clomazone in solutions containing acetone, a photochemical sensitizer (11).

Breakdown of Chemical in Vegetation

Clomazone inhibits synthesis of chlorophyll and carotenoids in plants. It is absorbed by plants through the roots from the soil and by shoots. It is then translocated in the xylem and diffuses within leaves. It does not move downward in plants or from leaf-to-leaf. There is no foliar absorption of clomazone. Clomazone is metabolized by plants (3).

PHYSICAL PROPERTIES AND GUIDELINES

Clomazone is a colorless to light brown, viscous liquid above room temperature. When cooled, it forms a white crystalline solid. Clomazone is not flammable (3).

Workers handling clomazone should avoid breathing vapors; wear goggles to prevent eye contact and protective clothing to prevent prolonged skin contact (3).

Exposure Guidelines:

ADI: 0.043 mg/kg/day based on a NOEL of 4.3 mg/kg/day in a 2 year rat feeding study and a 100 fold safety margin (5).
MPI: 0.6 mg/kg/day for a 60 kg person (5).

Physical Properties:

CAS #: 81777-89-1
Chemical name: 2-(2-chlorophenyl) methyl-4,4-dimethyl-3-isoxazolidinone
Chemical Class/Use: herbicide
Specific gravity: 1.192 at 20 degrees C (1)
H20 solubility: 1100 ppm (1, 2)
Solubility in other solvents: soluble in acetone, acetonitrile, chloroform, cyclohexanone, dimethyl formamide, dioxane, heptane, hexane, methanol, methylene chloride, toluene, and xylene (3).
Melting point: 25 degrees C (FMC)
Boiling point: 275.4 degrees C at atmospheric pressure (3)
Decomposition temperature: > 200 degrees C (Fe catalyst) (3)
Flashpoint: 314 degrees F for technical clomazone; 106-109 degrees F for Command 4EC (1).
Vapor pressure: 1.92 x 10-2 Pa at 25 degrees C (1.44 x 10-4 mm Hg) (1, 2)
Koc: 274 (2)

BASIC MANUFACTURER

FMC Corp.
Agricultural Chemicals Group
2000 Market St.
Philadelphia, PA 19103
Emergency: 800-331-3148

Review by Basic Manufacturer:

Comments solicited: April, 1993
Comments received: April, 1993

REFERENCES

  1. Meister, R.T. (ed.). 1993. Farm Chemicals Handbook '93. Meister Publishing Company, Willoughby, OH.
  2. U. S. Department of Agriculture, Soil Conservation Service. 1990 (Nov.). SCS/ARS/CES Pesticide Properties Database: Version 2.0 (Summary). USDA - Soil Conservation Service, Syracuse, NY.
  3. WSSA Herbicide Handbook Committee. 1989. Herbicide Handbook of the Weed Science Society of America, 6th Ed. WSSA, Champaign, IL.
  4. U.S. Environmental Protection Agency. May 1, 1986. Pesticide Fact Sheet Number 90: Command Herbicide. US EPA, Office of Pesticide Programs, Registration Div., Washington, DC.
  5. U.S. Environmental Protection Agency. April 28, 1988. Pesticide tolerance for 2-(2-chlorophenyl) methyl-4,4-dimethyl-3-isoxazolidinone. Federal Register 53 (82): 15237-8.
  6. U.S. Environmental Protection Agency. August 28, 1991. Pesticide tolerance for 2-(2-chlorophenyl) methyl-4,4-dimethyl-3-isoxazolidinone. Federal Register 56 (167): 42574-5.
  7. Salamon, C.M. and C.K. Borders. June 12, 1984. 2-generation reproduction study in albino rats with FMC 57020 Technical - Final Report (FMC Study No, A82-75, Toxigenics Study No. 450-1095). Unpublished report prepared by Toxigenics, Inc. for FMC Corp., Philadelphia, PA.
  8. Witkonton, S. July 13, 1984. Mobility of FMC 57020 residues in soil (FMC Report No. P-0904). Unpublished report prepared by FMC Corp., Philadelphia, PA.
  9. Froelich, L.W. June 7, 1982. Soil adsorption/desorption characteristics of FMC 57020 (FMC Report No. M-4861). Unpublished report prepared by FMC Corp., Philadelphia, PA.
  10. Dziedzic, J.E. June 21, 1982. Hydrolysis study (FMC Report No. P- 0465). Unpublished report prepared by FMC Corp., Philadelphia, PA.
  11. Wu, J. May 11, 1984. Photodegradation of FMC 57020 in water (FMC Report No. P-0869). Unpublished report prepared by FMC Corp., Philadelphia, PA.