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E X T O X N E T

Extension Toxicology Network

Pesticide Information Profiles

A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Oregon State University, the University of Idaho, and the University of California at Davis and the Institute for Environmental Toxicology, Michigan State University. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.

EXTOXNET primary files maintained and archived at Oregon State University

Revised 7/95

Clethodim

INTRODUCTION: Clethodim is a selective post-emergence cyclohexenone herbicide used to control annual and perennial grasses in a wide variety of broad leaf crops including soybeans, cotton, flax, peanuts, sunflowers, sugarbeets, potatoes, alfalfa and most vegetables (211, 1, 58). It is available in both the technical form and an emulsifiable concentrate (1). Unless otherwise stated, data presented herein refer to the technical product. Clethodim has shown antagonism when used with Basagran (bentazon) or Blazer (acifluorfen-sodium) (1, 58), and should be used with a non-phytotoxic crop oil concentrate (211,1). Clethodim is very well-tolerated by broad-leaved crops, showing little if any phytotoxic effect (1, 58, 294). Formulations of clethodim may include trimethylbenzene, xylenes, cumene and emulsifier as inert ingredients (295).

TOXICOLOGICAL EFFECTS

• Acute Toxicity: Clethodim is moderately toxic by ingestion. The reported oral LD50s are 1,630 mg/kg and 1,360 mg/kg in male and female rats, respectively (1). Reported LD50s for Select 2 EC formulation are 3,610 mg/kg and 2,920 mg/kg in male and female rats, respectively (211, 58). Clethodim is practically non-toxic by dermal absorption. The reported dermal LD50 is greater than 5,000 mg/kg in rabbits for the technical product as well as the formulation (211, 1, 58). The technical product did not cause skin irritation in the rabbit (58), but the formulation (Select) caused moderate skin as well as eye irritation in the rabbit (58). Eye irritation was reversible within 8-21 days (58). Select formulation caused no skin sensitization in guinea pigs (58). No data regarding skin sensitization or eye irritation were available for the technical product.Clethodim is practically non-toxic by the inhalation route as well. The reported rodent 4-hour inhalation LC50s for clethodim technical and Select formulation are greater than 3.9 mg/L and 4.4 mg/L, respectively (58, 294).Effects of acute exposure to clethodim or Select may include eye or skin irritation or central nervous system effects, e.g., salivation, decreased motor activity, incoordination, unsteady gait and hyperactivity (58, 295). These latter effects may be in large measure due to the aromatic constitutents of the formulation, as these effects commonly occur upon exposure to such compounds(296).
• Chronic Toxicity: In a one-year feeding study of dogs, doses of 75 mg/kg/day resulted in increased relative and absolute liver weights, with anemia-like alterations in blood chemistry such as reduced hemoglobin, erythrocyte and hematocrit counts (294, 295). In a two-year chronic study of rats, no compound-related effects on the structure and function of the liver were observed, and no changes in liver weights were observed at the highest dose tested , approximately 100 mg/kg/day (58,294). Reduced body weight gain was observed in another study on rats at 350 mg/kg/day, but not at 100 mg/kg/day, over an unspecified period (58, 294).
• Reproductive Effects: No effects on fertility, length of gestation or growth and development of offspring were observed at doses up to and including the highest dose tested, 263 mg/kg/day (58, 294). No other data were available regarding reproductive effects; while these data are insufficient, it appears unlikely that reproductive effects would occur in humans under normal circumstances.
• Teratogenic Effects: Reductions in fetal body weights and increases in skeletal abnormalities were observed in rats at doses of 350 mg/kg/day (58,294) and higher. In another study of rats, there were significant reductions in fetal body weight, litter size and significant increases in cervical rib deformation at doses of 700 mg/kg/day, but not at lower doses (58,294). In rabbits, no teratogenic or developmental effects were seen in offspring at doses up to and including the highest dose tested, 300 mg/kg/day (58,294).The evidence suggests that while there have been documented teratological effects in animal studies, such effects are unlikely in humans under normal conditions of exposure.
• Mutagenic Effects: Results of the Ames mutagenicity assay indicated that clethodim did not show mutagenic potential (58,294). Testing for unscheduled DNA synthesis in mouse liver cells following oral administration of 5,000 mg/kg were negative (297). Tests for structural chromosomal damage in rat bone marrow cells after oral administration of 1,500 mg/kg were also negative (298). The available data for mutagenicity and genotoxicity yield no evidence for mutagenic or genotoxic activity.
• Carcinogenic Effects: No carcinogenic effects were observed in mice administered clethodim at doses of 24 mg/kg/day over an 18 month period (294). No carcinogenic effects were observed in rats fed up to the highest dose tested, approximately 100 mg/kg/day, in a two-year carcinogenicity study (294). Based on the available data, it appears that clethodim is not carcinogenic.
• Organ Toxicity: The liver was the primary organ affected in chronic animal studies. Although potential effects associated with acute exposure are reported to include central nervous system effects (58, 295), no available chronic data pointed to such effects.
• Fate in Humans & Animals: Clethodim is readily absorbed in the gastrointestinal tract, with approximately 90% absorption of oral doses (294). It is rapidly metabolized and eliminated (primarily sulfoxide metabolites, ca 63%) with less than 1% recoverable unchanged (294).

ECOLOGICAL EFFECTS

• Effects on Birds: Clethodim is practically non-toxic to birds. Reported 8-day dietary LC50s are greater than 6,000 ppm in the mallard duck and bobwhite quail (211) and greater than 5,000 ppm for the Japanese quail (1). Under likely conditions of use, it is unlikely to pose a hazard to avian species.
• Effects on Aquatic Organisms: Clethodim is slightly toxic to fish and aquatic invertebrate species. Reported 96-hour LC50s ranged from 18 mg/L (58) to 56 mg/L in rainbow trout (1), and 33 mg/L in bluegill sunfish (58). A 48-hour LC50 of 20.2 mg/Lhas been reported for Daphnia species (58) for the formulation. No effects were seen at concentrations of 5.5 mg/L in Daphnia (294). No significant bioaccumulation has been observed in fish (294). Under likely conditions of use, it is unlikely to pose a hazard to aquatic species.
• Effects on Other Animals (Nontarget species): Clethodim is practically non-toxic to honeybees with reported LD50s of greater than 100 ug/bee for both the technical product and Select formulation (211, 58). EPA has stated that "available...wildlife data indicate that the proposed uses on cotton and soybeans will result in minimal hazard to nontarget and endangered beneficial insect, avian and freshwater fish and mammalian species" (294). Clethodim is selectively toxic to plants, affecting only grass species (294).

ENVIRONMENTAL FATE

• Breakdown of Chemical in Soil and Groundwater: Clethodim is of low persistence in most soils with a reported half-life of approximately 3 days (58). Breakdown is mainly by aerobic processes, although photolysis may make some contribution (294). Volatilization loss and hydrolysis are probably not important processes in the soil breakdown of clethodim (58). The main breakdown products in soils under aerobic conditions are sulfoxide, sulfone and oxazole sulfone (294).Clethodim and these degradates are weakly bound to soils, with reported soil Kd (soil-water partition coefficient unadjusted for soil organic matter) values of 0.05 and 0.23 over a range of five soils (58). Thus, while it may be somewhat mobile in the soil environment , it is very short-lived. EPA has stated "under present use patterns and under most circumstances clethodim does not appear to threaten groundwater" (294). In field studies, no vertical movement of the parent compound or residues was observed below the top 20 cm of the soil (294).
• Breakdown of Chemical in Surface Water: Clethodim may be highly persistent in the aquatic environment. Reported half-lives for clethodim in the aquatic environment are 128 days in the aqueous phase and 214 days in the sediment (294). The reported hydrolysis half-life at pH 7-9 is approximately 300 days. The main pathway for degradation of clethodim in the aquatic environment is anaerobic metabolism by microorganisms (294). However, due to the low persistence and mobility of the compound, it is unlikely to be found in surface waters.
• Breakdown of Chemical in Vegetation: Clethodim is rapidly degraded on the leaf surfaces by an acid-catalyzed reaction and photolysis (58). Remaining clethodim will rapidly penetrate the cuticle and enter the plant. (58). Little information is available regarding translocation and accumulation, but it is hypothesized that it may translocate and accumulate at growing points (58). Within soybeans, cotton and lettuce it is rapidly metabolized (58).

PHYSICAL PROPERTIES AND GUIDELINES

Physical Properties:

• Appearance: Clethodim is a clear, viscous, amber liquid at room temperature (1).
• Chemical Name: (+/-)2-{(E)-1-{3-chloroallyloxyimino]propyl]-5-{2-(ethylthio)propyl}-hydroxycyclohexen-2-one(1)
• CAS Number: 99129-21-2 (1)
• Molecular Weight: 359.9 (1)
• Water solubility: Highly dependent on pH (1)
• Solubility in Other Solvents: Soluble in most organic solvents (1)
• Melting Point: Not Available
• Vapor Pressure: <1 x 10-2 mPa (20 degrees C)
• Partition Coefficient: Not Available
• Adsorption Coefficient: 0.49 @ pH=9; 40 @ pH=7; >3,000 @ pH=5 (299)

Exposure Guidelines:

• ADI: 0.01 mg/kg b.w. (1)
• MCL: Not Available
• RfD: Not Available
• PEL: Not Available
• HA: Not Available
• TLV:1 mg/meters cubed (8-hour) (for Select 2 EC; manufacturer suggested; 5)

BASIC MANUFACTURER

Valent USA Corp.
1333 N. California Blvd.
Walnut Creek, CA 94596-8025

• Telephone: 510-256-2700
• Emergency: 800-892-0099

 REFERENCES

References for the information in this PIP can be found in Reference List Number 10