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Diclofop-methyl

Publication Date: 9/95

TRADE OR OTHER NAMES

The active ingredient diclofop-methyl is found in a variety of commercial herbicides. Trade names for products containing diclofop-methyl include Hoelon, Illoxan, Hoe-Grass,and One Shot (diclofop plus bromoxynil plus MCPA) (1, 3).

REGULATORY STATUS

Some or all uses of diclofop-methyl may be classified as a restricted use pesticide (RUP). Check with specific state regulations for local restrictions which may apply. Restricted products containing diclofop-methyl must carry the Signal Word "Danger" on their label; others may carry the Signal Word "Caution" (4).

INTRODUCTION

Diclofop-methyl is a selective post-emergence herbicide for control of wild oats and annual grassy weeds found among brassicas, carrots, celery, field beans, french beans, broad beans, barley, wheat, parsnips, peas, potatoes, soy beans, oilseed rape, onions, sugar beet, and lettuce (1).

The herbicide comes in emulsifiable concentrate formulations (1, 2). It is compatible with herbicides derived from urea, i.e. asulam, ioxynil, etc., although mixing with dinoseb and growth-regulating herbicides leads to decreased effectiveness (1).

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

The acute oral LD50 for rats ranged between 563-693 mg/kg (in sesame oil). The acute dermal LD50 for female rats was greater than 2,000 mg/kg. The no effect level (NEL) for a 90-day feeding to rats was 12.5 mg/kg and 8 mg/kg for a 15-month feeding to dogs (1, 2, 3). The acute percutaneous LD50 for rats was greater than 5,000 mg/kg (2). The acute inhalation toxicity for rats exposed to technical diclofop-methyl was greater than 3.83 mg/l/hour (3).

Rats exposed to the product Hoelon 3EC (contains diclofop-methyl) had an oral LD50 value of greater than 2,000 mg/kg. The acute dermal LD50 for exposure to the same product was greater than 5,000 mg/kg (3). The inhalation LC50 in rats for a 36%EC formulation of diclofop-methyl over a four-hour period was 8.3 mg/l air (1).

Inhalation: a rate of 4,800 ml of a 5% solution in a 4 cubic meter container for four hours was studied. Deaths occurred with rabbits, but not with rats, guinea pigs or cats at this high rate. The 4-hr LC50 for rats was 8,274 mg/m3 (3).

The acute dermal LD50 for rabbits was reported to be 640 mg/kg. A diclofop-methyl study on rabbits indicated no skin irritation at 24, 48 and 72 hours. Eye irritation in rabbits was found to be zero at 3, 7, or 24 hours (3).

Other studies have reported eye irritation in rabbits as corneal opacity spots and conjunctival irritation in both rinsed and non-rinsed groups. Reversibility was observed in all but one animal in each group at 7 days. A 10% solution produced corneal opacity in some of the animals that was completely reversible in 72 hours (3, 5).

The acute dermal LD50 for rabbits was found to be 640 mg/kg (3).

CHRONIC TOXICITY

In 2-year feeding trials the NEL for rats was 20 mg/kg diet. The 15 month NEL for dogs was 8 mg/kg diet (2). Repeated absorption of chlorinated diphenyl ethers has resulted in liver damage in animals (6).

Reproductive Effects

The NEL in a three-generation study of technical diclofop-methyl in rats was greater than 30 ppm (3, 8).

Teratogenic Effects

In a rat teratology study, the teratogenic No-Observable-Effect-Level (NOEL) was 100 ppm, the highest dose tested.

A rabbit teratology study reported a teratogenic NOEL of 3 mg/kg/day, the highest dose tested, and a NOEL for fetotoxicity of 3.0 mg/kg/day (8).

Mutagenic Effects

Ames assay testing revealed no mutagenic effects using four bacterial strains with and without enzyme activation in dose ranges up to 5 miligrams (3).

A micronucleus test in mice indicated no mutagenic effect in a strain with known sensitivity. In a dominant lethal assay, the NOEL was greater than 100 mg/kg. No impairment in fertility of the male mice and no difference in the number of live and dead implantations in the female animals were noted (3).

Carcinogenic Effects

No information was available.

Organ Toxicity

No information was available.

Fate in Humans and Animals

Diclofop-methyl is metabolized in mammals via hydroxylation. Chlorophenoxy compounds are absorbed across the gut wall, lung and skin. They are not significantly stored in fat and urinary excretion is the principal route of elimination. Elimination as a conjugate is within 96 hours (1).

ECOLOGICAL EFFECTS

Effects on Birds

The acute oral LD50 to bobwhite quail was 4,400 mg/kg (3); and greater than 10,000 mg/kg for Japanese quail (2). The eight-day dietary LC50 value for coturnix quail was greater than 20,000 ppm; 13,000 ppm for bobwhite quail; and greater than 20,000 ppm for mallard ducks (3).

Effects on Aquatic Organisms

The 96-hour LC50 for technical diclofop-methyl in rainbow trout was 0.35 mg/l water (2). The 96-hour LC50 in rainbow trout for the formulated product Hoelon 3EC was 1.38 ppm; and 2.60 ppm for carp (3).

The 48-hour LC50 in the crustacean Daphnia for the formulated product Hoelon 3EC was 4.03 ppm (3).

Effects on Other Animals (Nontarget species)

The LD50 for honeybees in a lab test of the formulated product Hoelon 3EC indicated it was nontoxic at the highest dose tested; 48 kg/ha (43 lb ai/acre) (3).

ENVIRONMENTAL FATE

Breakdown of Chemical in Soil and Groundwater

Under aerobic conditions, diclofop-methyl hydrolyzes in a matter of days in the soil to 2-[4-(2',4'-dichlorophenoxy)phenoxy] propanoic acid which in turn is degraded relatively quickly with a half-life of 10 days in sandy soils and about 30 days in sandy clay soils. Small amounts of 4-(2,4 dichlorophenoxy)phenol are also produced (3, 7).

Field studies of application rates up to 3.4 kg active ingredient per hectare (3.0 lb ai/acre) showed very low finite residues in soil. At harvest, small finite residues were present in the 0-7.5 cm (0-3 in) soil level and rare small residues were present above the 15 cm (6 in) level. These studies indicate that diclofop-methyl does not leach downward or move laterally, and dissipates quickly in soil (3).

Breakdown of Chemical in Surface Water

No information was available.

Breakdown of Chemical in Vegetation

Diclofop-methyl is absorbed via the leaves and in damp soil there is slight absorption via the roots. The compound inhibits root growth (1).

PHYSICAL PROPERTIES AND GUIDELINES

Physical Properties:

CAS No.: 51338-27-3 (1)
Chemical name: methyl 2-[4-(2,4-dichlorophenoxy)phenoxy]propanoate (1)
Chemical Class/Use: diphenylether/chlorophenoxy herbicide, preemergence and postemergence herbicide (5)
Specific gravity: 1.3 gm/cm3 at 40 degrees C (3)
Solubility in water: 50 mg/l at 22 degrees C (1)
Solubility in other solvents: readily soluble in common organic solvents, e.g. acetone 40, xylene 50, methanol 40 (all in g/100 ml at 22 degrees C) (1)
Melting point: 39-41 degrees C (1, 3)
Boiling point: 173-175 degrees C at 0.1 mbar (1)
Decomposition temperature: 288.4 degrees C (3)
Flashpoint: 38 degrees C (closed cup) (4)
Vapor pressure: 3.4 x 10 to the minus 7 mbar at 30 degrees C; 3.8 x 10 to the minus 5 mbar at 54.3 degrees C (1)
Koc: 4.89 (7)

BASIC MANUFACTURER

Hoechst-Roussel Agri-Vet
P.O. Box 2500
Rt. 202-206
Somerville, NJ 08876-1258
Fax: 908-231-4452
Telephone: 908-231-2000
Emergency: 800-228-5635

Review by Basic Manufacturer:

Comments solicited: October, 1994
Comments received:

REFERENCES

  1. The Agrochemicals Handbook. 1983. The Royal Society of Chemistry, The University, Nottingham, England.
  2. Worthing, C. R. (ed.). 1983. The Pesticide Manual: A World Compendium. Seventh edition. Published by The British Crop Protection Council.
  3. Herbicide Handbook of the Weed Science Society of America. 1989. Sixth edition. Champaign, IL.
  4. Farm Chemicals Handbook. 1994. Meister Publishing Co. Willoughby, OH.
  5. Thomson, W. T. 1992. Agricultural Chemicals. Book II: Herbicides. Thomson Publications, Fresno, CA.
  6. OHS Database. June, 1994. MSDS for Diclofop-Methyl. MDL Information Systems Inc., San Leandro, CA.
  7. Montgomery, J. H. 1993. Agrochemicals Desk Reference: Environmental Data. Lewis Publishers. Chelsea, MI.
  8. U. S. Environmental Protection Agency. 1986. Pesticide Tolerance for Diclofop-Methyl. Federal Register. Vol. 51, No. 102. Rules and Regulations. Wednesday, May 28, 1986.