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Tebuthiuron

Publication Date: 9/93

TRADE OR OTHER NAMES

Some trade names include Spike, Brush Bullet, EL-103, Graslan, Perflan, Herbec, Herbic and Reclaim.

REGULATORY STATUS

Tebuthiuron is registered by the U. S. Environmental Protection Agency (EPA) as a general use herbicide. Products containing tebuthiuron must bear the signal word "Caution" (10). Check with specific state regulations for local restrictions which may apply.

INTRODUCTION

Tebuthiuron is a broad spectrum herbicide used to control weeds in noncropland areas, rangelands, rights-of-way and industrial sites (10). It is effective on woody and herbaceous plants in grasslands and sugar cane (2). Weeds that are controlled by tebuthiuron include alfalfa, bluegrasses, chickweed, clover, dock, goldenrod, mullein, etc. Woody plants take a period of two to three years to be completely controlled (5).

Tebuthiuron is sprayed or spread dry on the soil surface, as granules or pellets, preferably just before or during the time of active weed growth. Rainfall enhances the initial control provided by this herbicide (8).

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

Skin, eye or clothing contact with the herbicide should be avoided (8). It is classified as a moderately poisonous herbicide. Symptoms of tebuthiuron poisoning in rodents include lack of energy, loss of appetite, muscular incoordination and death. Vomiting occurred in cats and dogs (2, 11).

Tebuthiuron has high acute oral toxicity in experimental animals. The amount of the herbicide that causes death to one-half (50%) of test animals to which it is given is referred to as its lethal dose fifty, or LD50. The oral LD50 for tebuthiuron is 644 mg/kg in rats, 57.9 mg/kg in mice, and 286 mg/kg in rabbits. The dermal LD50 for tebuthiuron in rabbits is greater than 5,000 mg/kg. Neither skin irritation nor general overall intoxication were produced in rabbits that had 200 mg/kg of the material applied to their skin (2, 3, 8). 67 mg of the herbicide in the eye of rabbits produced short-term conjunctivitis, inflammation of the lining of the eye, but no irritation to other eye parts, the cornea and iris (8). The inhalation by animals of 3.7 g/m3 technical tebuthiuron for 4 hours did not cause death or toxicity. Tebuthiuron did not induce allergic reactions when tested on the skin of guinea pigs.

CHRONIC TOXICITY

Decreases in body weight gain and red blood cell counts, along with minor effects on the pancreas were seen in rats fed 125 mg/k/g/day for three months (2). Exposure of rats to dietary doses of tebuthiuron as high as 80 mg/kg for 2 years was well tolerated with no indication of cumulative toxicity or serious effects. Similarly, no toxic effects were observed in mice exposed to doses as high as 200 mg/kg for most of their lifetime, or in dogs given doses of 25 mg/kg for one year (8).

The EPA has established a Lifetime Health Advisory (LHA) level of 500 micrograms per liter (ug/l) for tebuthiuron in drinking water. This means that EPA believes that water containing tebuthiuron at or below this level is acceptable for drinking every day over the course of one's lifetime, and does not pose any health concerns. However, consumption of tebuthiuron at high levels well above the LHA level over a long period of time has been shown to cause excessive weight loss and damage to the pancreas in animals studies (14).

Reproductive Effects

The reproductive capacity of rats fed dietary concentrations of tebuthiuron as high as 56 mg/kg was unimpaired through three successive generations, and no abnormalities were detected in either parents or offspring. Tebuthiuron administered to pregnant rabbits at doses as high as 25 mg/kg, and to rats at doses as high as 180 mg/kg produced no adverse effects on either the mothers or offspring (8).

Teratogenic Effects

No teratogenic effects were observed when rats were fed tebuthiuron at 800 mg/kg body weight (3). A rat teratology study of tebuthiuron was negative at 1,800 ppm (90 mg/kg), the highest dose tested. A rabbit teratology study was negative at 25 mg/kg, the highest dose tested (1).

Mutagenic Effects

A mutation test on bacteria, the Ames test, was negative (1).

Carcinogenic Effects

No tumor-producing effects were observed in a two-year rat feeding study, up to and including 1,600 ppm (80 mg/kg), the highest dose tested. A two-year oncogenic study on mice was negative at 1,600 ppm (240 mg/kg), the highest dose tested (1). Data from laboratory studies are inadequate for EPA to determine if tebuthiuron can increase the risk of cancer in humans (14).

Organ Toxicity

Long-term consumption of tebuthiuron at high levels well above the Lifetime Health Advisory level of 500 ug/l for drinking water has been shown to cause excessive weight loss and damage to the pancreas in animals studies (14).

Fate in Humans and Animals

In rats, rabbits, dogs, mallards and fish, tebuthiuron is readily absorbed into the bloodstream from the gastrointestinal tract, rapidly metabolized, and then excreted in the urine (8). Tests indicate that the herbicide is broken down and excreted within 72 hours, primarily as a variety of urinary metabolites (2).

ECOLOGICAL EFFECTS

Effects on Birds

Tebuthiuron is not hazardous to birds. The oral LD50 for mallard ducks is greater than 2,000 mg/kg (8). A 30-day feeding of 1,000 mg/kg tebuthiuron to hens had no effect (3).

Effects on Aquatic Organisms

Tebuthiuron is not hazardous to aquatic organisms (8). The lethal concentration fifty, or LC50, is that concentration of a chemical in air or water that kills half of the fish exposed to it for a set time period. The LC50 for rainbow trout is 144 mg/l, and 112 mg/l for bluegill (3, 10). The 96-hour LC50 is greater than 160 mg/l for goldfish and fathead minnow (9). The 48-hour LC50 for Daphnia, a small, freshwater crustacean, is 297 mg/l (8). The LC50 for fiddler crab is greater than 320 ppm, the LD50 for pink shrimp is more than 48 ppm (8).

Effects on Other Animals (Nontarget species)

If tebuthiuron is used correctly, it should not pose a threat to bees (3).

ENVIRONMENTAL FATE

Breakdown of Chemical in Soil and Groundwater

Tebuthiuron has all the characteristics of a material with a high potential for groundwater contamination. It is highly soluble in water (2,500 ug/ml), adsorbs only weakly to soil particles (Koc = 80 mg/l), and is highly persistent in soils (soil half-life = 360 days) (11). Tebuthiuron is easily moved with moisture in the soil. The EPA considers tebuthiuron to be one of a group of pesticide compounds that have the greatest potential for leaching into, and contaminating, groundwater. It was not found in groundwater in a U.S. groundwater survey conducted by the EPA (6).

In field studies, tebuthiuron has seldom been detected below the top 24 inches of soil. Little or no lateral movement has been observed after tebuthiuron has adsorbed to clay or organic matter in the soil (8).

In areas receiving 40 to 60 inches of annual rainfall, the time that it takes for half of the tebuthiuron to break down in soil is 12 to 15 months. This time period is known as its soil half-life. It takes longer for the herbicide to break down in areas that have less rainfall. The half-life for tebuthiuron is also greater in muck and other high organic soils, regardless of rainfall (8).

Some tebuthiuron is broken down in the soil by 'microbes,' through the process of microbial degradation. However, tests indicate that this may not be the primary way by which tebuthiuron is degraded. Photodecomposition, or breakdown by sunlight, is negligible, as is volatilization, by which it changes from a solid to a gaseous form (8).

Breakdown of Chemical in Water

No degradation was observed in a 33-day study of photolysis of tebuthiuron in water (13).

Breakdown of Chemical in Vegetation

Tebuthiuron is readily absorbed through roots. It is less likely to be absorbed by the leaves. Since the herbicide can be harmful to nontarget plants and trees, it should not be applied, nor any of its application equipment drained or flushed, on or near desirable trees or plants, nor on areas to which their roots extend. Tebuthiuron is readily translocated by the roots to other plant parts. It is effective by inhibiting photosynthesis, the process by which plants receive light from the sun and convert it into energy (8).

PHYSICAL PROPERTIES AND GUIDELINES

Tebuthiuron is an off-white to buff colored crystalline solid with a pungent odor (13). Tebuthiuron is compatible with other herbicides. It is stable under normal storage conditions, but may pose a fire hazard if exposed to heat or flame. It poses a fire and explosion hazard in the presence of strong oxidizers. Thermal decomposition of tebuthiuron may release toxic oxides of carbon, nitrogen and sulfur (12). It is stable in aqueous media at pH's between 5 and 9. At high temperatures, it is hydrolyzed by strong alkalis and strong acids. It is noncorrosive to metals and polyethylene (8).

Exposure Guidelines:

No occupational exposure limits have been established for tebuthiuron by OSHA, NIOSH, or ACGIH (12).

NOEL: 50 mg/kg in 90-day rat feeding study; 40 mg/kg in 2-year rat feeding study (systemic NOEL) (1)
TMRC (theoretical maximum residue contribution): 0.2232 mg/day based on a 1.5 kg daily diet (1)
MPI (Maximum Permissible Intake): 12 mg/day for 60-kg human (1)
ADI: 0.20 mg/kg/day based on a two-year rat feeding study (NOEL) of 20 mg/kg/day) and using 100-fold safety factor (1)

Physical Properties:

CAS: 34014-18-1
H20 solubility: 2,300 ppm (7); 860 ppm (7); 0.25 g/100 ml at 25 degrees C (8)
Solubility in other solvents (in g/100 ml): 25.0 in chloroform; 17.0 in methanol; 7.0 in acetone; 0.61 in hexane; 0.37 in benzene; acetonitrile 6.0 (8)
Melting point: 161.5 x 164 degrees C (with decomposition) (8)
Vapor pressure: 2 x 106 mmHg at 25 degrees C (8)
Hydrolysis half-life: greater than 9 weeks (7)
Soil/Field dissipation half-life: 360 days (11); highly persistent (7)
Koc: 80 g/ml (11)
Kd: 0.2-10.0; Class 5; very mobile (7)
Chemical Class/Use: Urea carbamate herbicide

BASIC MANUFACTURER

DowElanco
9002 Purdue Rd.
Indianapolis, IN 46268-1189

Review by Basic Manufacturer:

Comments solicited: November, 1992
Comments received: December, 1992

REFERENCES

  1. Federal Register. 48 FR 24396. June 1, 1983. U.S. Government Printing Office. Washington, DC.
  2. Gosselin, R. E., et al. 1984. Clinical toxicology of commercial products. Fifth edition. Baltimore, MD: Williams and Wilkins.
  3. Hartley, D. and H. Kidd (eds.). 1983. The Agrochemicals Handbook. Nottingham, England: Royal Society of Chemistry.
  4. McEwen, F. L. and G. R. Stephenson. 1979. The use and significance of pesticides in the environment. NY: John Wiley and Sons, Inc.
  5. Thomson, W .T. 1983. 1983. Herbicides. Agricultural Chemicals, Book II. Fresno, CA: Thomson Publications.
  6. U.S. Environmental Protection Agency. 1987 (Feb.). Environmental News. Office of Public Affairs (A-107). Washington, DC.
  7. _____. 1984. Memorandum from Stuart Z. Cohen. List of potential groundwater contaminants. Office of Pesticides and Toxic Substances. Washington, DC. Photocopy.
  8. WSSA Herbicide Handbook Committee. Herbicide Handbook of the Weed Science Society of America, 6th Ed. WSSA, Champaign, IL. 1989.
  9. Worthing, C. R. (ed.). 1983. The pesticide manual: A world compendium. Croydon, England: The British Crop Protection Council.
  10. Meister, R.T. (ed.). 1992. Farm Chemicals Handbook '92. Meister Publishing Company, Willoughby, OH.
  11. 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.
  12. Occupational Health Services, Inc. 1991 (Feb. 21). MSDS for Tebuthiuron. OHS Inc., Secaucus, NJ.
  13. DowElanco. Letter of Nov. 13, 1991 from Merlyn Jones, Product Registration Manager, DowElanco, Indianapolis, IN.
  14. U.S. Environmental Protection Agency. 1989 (Jan.). Health Advisory Summary: Tebuthiuron. US EPA, Washington, DC.