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Pesticides
Product name

clomazone

Other name 3-Isoxazolidinone,2-[(2-chlorophenyl)methyl]-4,4-dimethyl-
Chemical name 2-(2-chlorobenzyl)-4,4-dimethyl-1,2-oxazolidin-3-one
Cas number 81777-89-1
EC#
Molecular Formula C12H14ClNO2
Molecular Weight 239.72
Molecular Structure
Technical Data Specs orCOAMSDSInquriy

Clomazone is an agricultural herbicide, and has been the active ingredient of products named "Command" and "Commence". The molecule consists of a 2-chlorobenzyl group bound to a N-O heterocycle called Isoxazole. It is a white solid.

Clomazone was first registered by the USEPA on March 8, 1993 and was commercialized by FMC Corporation. It is used for broadleaf weed control in several crops, including soybeans, peas, maize, oilseed rape, sugar cane, cassava, pumpkins and tobacco. It may be applied pre-emergence of incorporated before planting the crop. Clomazone is relatively volatile (vapor pressure is 19.2 mPa) and vapors induce striking visual symptoms on non-target sensitive plants. Clomazone undergoes biological degradation, exhibiting a soil half life of one to four months. Adsorption of the herbicide to soil solids slows degradation and volatilization. Encapsulation helps reduce volatility and therefore reduces off-target damage to sensitive plants.

Clomazone regulatory status

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

Clomazone 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.
The herbicide clomazone (2-(2-chlorophenyl)methyl-4,4-dimethyl-3-isoxazolidinone; CAS 81777-89-1; Fig. 1) was first approved for use in 1986 (US EPA 2007). It is produced by the FMC Corporation under the trade names that include Command® and Cerano® 5 MEG(Tenbrook et al. 2006).
Clomazone is the only isoxazolane herbicide registered for use within the United States (US EPA 2007). It is used for annual control of broad-leaf and grassy weeds such as barnyard grass (Echinochloa crus-galli), crab grass (Digitaria spp.), foxtails (Setaria spp.), and others that infest soybean, tobacco, rice and other row crops (Scott et al. 1995; Lee et al. 2004; Schocken 1997).
Within the USA, approx. 503,487 kg of active ingredient is applied each year (US EPA 2007). It is formulated as an emulsifiable concentrate and microencapsulated flowable granule (5% clomazone) and is applied either pre- or post-emergence (CDPR 2003; US EPA 2007).
Clomazone is highly water soluble and weakly to moderately persistent in soils with half-lives (t1/2s) ranging from 5 to 60 days. Because of its water solubility, the potential impact of clomazone on surface water, groundwater and aquatic organisms is of great concern. In this paper, we have reviewed the relevant literature and address clomazone’s chemistry, environmental fate and toxicity.

Clomazone toxicological effects

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

Clomazone 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. 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. 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.

Clomazone 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.
Teratogenic Effects

Clomazone is not teratogenic. 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.

Clomazone 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.

Clomazone carcinogenic effects

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

Clomazone ecological effects

Clomazone 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.
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.
Effects on Other Animals (Nontarget species)

No information was found.

Clomazone environmental fate

Clomazone is relatively stable to degradation by UV light. 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.

Clomazone 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. The product Command has low mobility in sandy loam, silt loam and clay loam soils. It is moderately mobile in fine sand.
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. In field studies, the half-life of clomazone was 28 to 84 days, depending on soil type and the organic matter content.

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.

Clomazone breakdown of chemical in surface water

Under laboratory conditions, clomazone was not readily hydrolyzed in sterile water. 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.

Clomazone 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.

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

Clomazone 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.
MPI: 0.6 mg/kg/day for a 60 kg person.

Clomazone 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
H20 solubility: 1100 ppm
Solubility in other solvents: soluble in acetone, acetonitrile, chloroform, cyclohexanone, dimethyl formamide, dioxane, heptane, hexane, methanol, methylene chloride, toluene, and xylene.
Melting point: 25 degrees C (FMC)
Boiling point: 275.4 degrees C at atmospheric pressure
Decomposition temperature: > 200 degrees C (Fe catalyst)
Flashpoint: 314 degrees F for technical clomazone; 106-109 degrees F for Command 4EC.
Vapor pressure: 1.92 x 10-2 Pa at 25 degrees C (1.44 x 10-4 mm Hg)
Koc: 274

Clomazone Chemistry and Physicochemical Properties

Clomazone is an isoxazolane herbicide containing a chloroaromatic ring (Fig. 1). When pure,clomazone is a crystalline solid (CDPR 2003). At room temperature it is highly soluble in water 3 and has a low-to-moderate affinity for soil. This herbicide is denser than water, and is susceptible to microbial degradation. The physiochemical properties of clomazone are presented in Table 1.

Clomazone Environmental Chemodynamics

Clomazone at Soil

Clomazone is not expected to bind to soils strongly given its relatively low Kd and its hydrophilic nature. However, sorption to various soil types (with varying temperature and moisture) has been investigated. Mervosh et al. (1995b) observed that a concentration of ca. 9 mg/kg of 14Cclomazone sorbed to a silty clay loam soil; such sorption was independent of temperature, and soil moisture content had a minor sorbtive effect. Although overall soil sorption is low, the agent has a higher affinity for binding to humic acid than to whole soil (Gunasekara et al. 2009).
Furthermore, it appears that the presence of black carbon or burned residues, in fire-affected locations, increase the sorption of this herbicide (Gunasekara et al. 2009). Loux et al. (1989) determined that adsorption was dictated by organic matter rather than by clay content; Kd values for clomazone ranged from 0.47 for silt loam to 5.3 for loamy sand.
Half-lives and desorption coefficients were determined for clomazone in four Tasmanian soils. A first-order half-life (t1/2) for ferrosol (clay loam), kurosol (loamy sand), sodosol (silt loam), and vertosol (light clay) soils ranged from 79 to 124 d, respectively. Half-lives derived from the Hoerl equation ranged from 6 to 59 d, respectively; this equation provided a good fit to the measured concentrations (Cumming et al. 2002). Desorption also varied with soil type; Kd values ranged from 1.7 to 3.6, respectively.
The persistence of clomazone was examined under both conventional and no-tillage practices. Following an initial application rate of 1.4 kg/ha, measurable amounts of clomazone were detected at a soil depth of 0-10 cm, 120-days later (Mills et al. 1989). Soil concentrations of 124±54 and 30 ±12 ng/g, respectively, were measured following conventional and no-till practices (Mills et al. 1989).
Quayle et al. (2006) applied clomazone to simulated flooded rice plots and measured resulting soil concentrations. Analytical results varied 7.5-fold between the 4 and 48 day post-application samplings. In addition, a measured t1/2 of 14.6 days was attributed to anaerobic conditions. Halflives of 32.9 and 37.4 d, respectively, in Montana loam and silty clay loam soils were noted by Gallandt et al. (1989). Field half-lives for Tennessee clay loam and loam soils ranged from 5-to-29 d and a t1/2 of 34 d resulted under laboratory conditions (Kirksey et al. 1996); this indicates that environmental conditions affect clomazone’s dissipation rate.

Clomazone at Water

Due to its high water solubility (1,102 mg/L) and relatively low Kow value, clomazone is expected to concentrate within the aqueous phase; thus, concerns exist for potential impacts on drinking water systems. To investigate risks posed to drinking water, Byers et al. (1995)measured clomazone concentrations in vadose zone waters at depths of 0.3, 0.6 and 1.5 m using tension lysimeters. They found concentrations to decrease (ca. 3-fold) as soil depth increased. In addition, soil treatments no mulch was used, or plastic mulch was used had measurable clomazone concentrations respectively of 0.09 and 0.04 ppb (Byers et al. 1995). 5 Since flooded fields discharge excess water into surrounding creeks and rivers, there is potential for applied residual organics to contaminate surrounding water bodies. The dissipation of clomazone from floodwaters was studied by Quayle et al. (2006). When Quayle et al. (2006) applied clomazone to small replicated rice plots at a rate of 0.5 L/ ha (i.e., as commercially formulated Magister® containing 480 g/L a.i.), an initial measured mean water concentration of 202 ug/L was produced. However, within 4 d the concentration had decreased to 83 ug/L, and by 19 d the concentration declined to 3 ug/L (Quayle et al. 2006). The t1/2 for chlomazone in this study was 7.2 d. Furthermore, the releasing waters contained 3 ug/L clomazone, which was assessed as having a low toxicity hazard. Two Brazilian rivers, the Vacacaí-Mirim River and the Vacacaí River, were monitored for residues of clomazone, particularly sourced from rice field
irrigation. An average level of 4.5 ug/L was measured within 41% of collected samples from the Vacacaí River whereas the Vacacaí-Mirim River had measurable concentrations of 3.7 ug/L in 33% of samples (Marchesan et al. 2007). The higher rate and level of detections in the Vacacaí River were attributed to its larger surrounding drainage area and plot acreage. Zanella et al.
(2002) reported residual clomazone concentrations in samples collected from experimental rice fields in the central region of the Rio Grande do Sul, Brazil. During both December 1999 and 2000, samples collected 130 days post application were found to contain clomazone concentrations of 0.9 and 0.2 ug/L, respectively (Zanella et al. 2002).

Clomazone at Air and Volatilization

The volatility of various formulations of clomazone from Flanagan silt loam was studied under both moist soil and simulated rainfall conditions. Mervosh et al. (1995c) reported that each of the granular formulations reduced volatilization; small granules (20 to 30 mesh) produced greater volatilization than did those of 14 to 20 mesh. In addition, they found that soil-water content greatly affected volatilization flux; highly saturated soil resulted in increased flux rates.
Compared to others, starch-based formulations reduced off-site movement (Mervosh et al.1995c).Thelen et al. (1988) observed volatilization up to 2 weeks post-application in both surface applied or soil-incorporated treatments; surface application resulted in higher volatilization. In addition, the presence of rainfall increased clomazone’s overall tendency to volatilize. The offsite movement of vapors from extremely wet soil was observed by Halstead and Harvey (1988).
Such vapors traveled as far as 32 m from the application site as measured by phytotoxicity to sunflower and wheat chlorosis. The application rate was a major factor in producing phytotoxic effects at this distance; however soil moisture and wind speed may have contributed to clomazone’s transport (Halstead and Harvey 1988). Mervosh et al. (1995a) observed increased volatilization from increasing temperature, but not from soil moisture. Schummer et al. (2010)determined that air samples, collected from a farming site in Northeastern France, contained gasphase concentrations of clomazone ranging from 0.14 to 0.68 ng/m³.




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