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Terbutryn

Publication Date: 9/95

TRADE OR OTHER NAMES

Terbutryn is also called terbutryne. Trade names include Prebane, Igran, Shortstop, Clarosan, GS 14260, Plantonit, Gesaprim Combi (with Atrazine 1:1), Senate (with trietazine), and Igrater 50WP (with metobromuron 1:1)(3).

REGULATORY STATUS

Terbutryn is a General Use Pesticide (GUP). The U.S. Environmental Protection Agency has proposed the revocation of all tolerances for residues of terbutryn in or on barley, sorghum, and wheat. They do not plan to recommend action levels to replace the tolerances (Federal Register 59(138):37019). Many terbutryn product registrations have been cancelled. The EPA has classified it as Toxicity Class III-slightly toxic. Products containing terbutryn bear the Signal Word "Caution".

INTRODUCTION

Terbutryn is a selective herbicide and a triazine compound. It is absorbed by the roots and foliage and acts as an inhibitor of photosynthesis. Terbutryn is a preemergent and postemergent control agent for most grasses and many annual broadleaf weeds in winter wheat, winter barley, sorghum, sugarcane, sunflowers, peas, and potatoes (6). It is also used as an aquatic herbicide for control of submerged and free-floating weeds and algae in water courses, reservoirs, and fish ponds (1). Terbutryn is available as a wettable powder or a soluble concentrate (3, 6).

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

Terbutryn is slightly toxic. It affects the central nervous system in animals leading to incoordination, convulsions, or labored breathing (5). At extremely high dosages, the animals showed swelling and fluid in the lungs and central nervous system (5).

The dose which kills half of the test animals, the LD50, is 2450-2500 mg/kg for rats and 3884 mg/kg for mice (2, 7). The LD50 for dermal exposure is greater than 2,000 mg/kg for rabbits (3, 4).The concentration in air which kills half of the test animals by inhalation, the LC50 (4 hours), is greater than 8 mg of an 80% formulation per liter of air for rats (1). Terbutryn is not a skin sensitizer (4).

CHRONIC TOXICITY

Reproductive Effects

A three generation reproduction study of rats showed that doses of 150 mg/kg/day of terbutryn caused decreased fertility indices in both male and female rats. (4, 7).

Teratogenic Effects

Above doses of 500 mg/kg/day, pregnant rats produced offspring with reduced weight and reduced bone formation in the front and rear paws. Pregnant rabbits exposed to doses of 75 mg/kg/day also had offspring with reduced bone formation (6).

Mutagenic Effects

In tests of terbutryn, no mutagenic effects were observed (4).

Carcinogenic Effects

In a two-year feeding study of rats, doses of 150 mg/kg of terbutryn caused cancerous tumor growth. However, there is no evidence of carcinogenicity in mice. Terbutryn has been classified as a possible human carcinogen by the U.S. EPA (4).

Organ Toxicity

Long-term feeding at high doses of terbutryn can cause growth retardation, kidney damage, liver damage and a decreased number of white blood cells (5, 7).

Fate in Humans and Animals

When given orally to mammals, 73 to 85% of a terbutryn dose is eliminated in metabolized form in the feces within 24 hours (1).

ECOLOGICAL EFFECTS

Effects on Birds

Terbutryn is only slightly toxic to birds. The LC50 (8-day dietary) is greater than 20,000 mg/kg for bobwhite quail and pheasant and greater than 4640 mg/kg for mallard ducks (1, 4).

Effects on Aquatic Organisms

Terbutryn is moderately toxic to fish (3). However, except when applied directly to 6 inches of water, residues have been calculated to be insignificant even to the most sensitive aquatic animal species. The concentration which is lethal to fish in water, the LC50 (96 hours), is 3 mg/kg for rainbow trout and 4 mg/kg for bluegill sunfish, carp, and perch (1).

Effects on Other Animals (Nontarget species)

Terbutryn is not toxic to bees (1). It is practically non-toxic to upland game birds (3).

ENVIRONMENTAL FATE

Breakdown of Chemical in Soil and Groundwater

Terbutryn is readily adsorbed in soils with high organic or clay content (3). The half-life in soil is 14-28 days (1). Depending on the application rate, the residual activity of terbutryn in soil is 3 to10 weeks. It is slightly mobile to immobile in soils. Data indicate that it will not leach in agricultural soils. However, its major breakdown product, hydroxy terbutryn, is more mobile and persistent and has potential to leach to groundwater (3).

Breakdown of Chemical in Water

In water, terbutryn is not volatile. It will adsorb to sediment and suspended particulate matter. Half-lives of 180-240 days have been reported for degradation of terbutryn in pond and river sediment. It may be subject to very slow hydrolysis and biodegradation in water (6).

Breakdown of Chemical in Vegetation

Terbutryn interferes with carbon dioxide assimilation and sugar formation in plants. It is degraded to a breakdown product which is quite stable (1). Further degradation in plants is slow (4).

PHYSICAL PROPERTIES AND GUIDELINES

Terbutryn is a white or colorless crystalline powder with no distinct odor. It is stable to dilute aqueous alkaline and acidic solutions. It is hydrolyzed in strong acid or basic solutions. It is decomposed by ultraviolet irradiation. Terbutryn is stable for about three years at room temperature when dry (4). It is not corrosive (6).

Physical Properties:

CAS#: 886-50-0
Chemical Name: 2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine
Solubility in water: 25 mg/l at 20 degrees C (6)
Solubility in other solvents: Readily soluble in isopropanol, xylene, dioxane, chloroform, carbon tetrachloride, and acetone (6)
Melting point: 104-105 degrees C (6)
Boiling point: 154-160 degrees C (6)
Partition Coefficient (octonol/water): 3070 (1)
Chemical class/Use: triazine/herbicide

Exposure Guidelines:

ADI (provisional): 0.01 mg/kg (4)
NOEL: 0.1 mg/kg/day (female rats) (8)
RfD: 1 x 10 to the minus 3 mg/kg/day (8)

BASIC MANUFACTURER

Ciba Geigy Agricultural Division
P.O. Box 18300
Greensboro, NC 27419-8300
Telephone: 919-632-2000

Review by Basic Manufacturer:

Comments solicited: October, 1994
Comments received:

REFERENCES

  1. The Agrochemicals Handbook, Third Edition. 1994. Royal Society of Chemistry Information Systems, Unwin Brothers Ltd., Surrey, England.
  2. Bohmont, Bert. 1981. The New Pesticide Users Guide. B & K Enterprises. Fort Collins, CO.
  3. Meister, R.T. 1994. Farm Chemicals Handbook '94. Meister Publishing Company. Willoughby, OH.
  4. U.S. Environmental Protection Agency. 1986. Fact Sheet Number 104: Terbutryn. U.S. EPA. Washington, DC.
  5. Wagner, Sheldon. 1981. Clinical Toxicology of Agricultural Chemicals. Oregon State University. Corvallis, OR.
  6. U.S. Department of Health and Human Services. 1993. Hazardous Substance Data Base. HHS. Washington, DC.
  7. National Institute for Occupational Safety and Health (NIOSH). 1993. Registry of Toxic Effects of Chemical Substances (RTECS). NIOSH. Cincinnati, OH.
  8. U.S. Department of Health and Human Services. 1994. U.S. EPA Integrated Risk Information System (IRIS) database. HHS. Washington, DC.