E X T O X N E T
Extension Toxicology Network
A Pesticide Information Project of Cooperative Extension Offices of
Cornell University, Michigan State University, Oregon State University, and
University of California at Davis. Major support and funding was provided
by the USDA/Extension Service/National Agricultural Pesticide Impact
Publication Date: 3/94
TRADE OR OTHER NAMES
Some trade names include Sinbar, Compound 732, DuPont 732 and Geonter.
Terbacil is registered by the U.S. Environmental Protection Agency (EPA)
as a general use herbicide. The EPA requires the signal word "Caution" on
containers of formulated terbacil (19). Check with specific state regulations
for local restrictions that apply.
Terbacil is a selective herbicide, an agricultural chemical used for
control of both annual weeds and perennial grasses, in sugarcane, apples,
alfalfa, peaches, pecans, and mints (16). It is sprayed on soil surfaces
preferably just before, or otherwise during, the period of active weed growth.
It should not be used on sandy or gravelly soils. Terbacil works in plants by
interfering with photosynthesis, the process by which plants derive energy
from the sun (13). It is part of a family of chemicals called substituted
uracils. Terbacil is available in wettable powder formulations (19).
Terbacil has low acute toxicity in humans (16). Clinical signs of
poisoning in rats included initial weight loss, pallor, prostration and rapid
breathing. In dogs, a single dose of 5 mg/kg caused repeated vomiting (21).
Terbacil may irritate the skin, eyes, and mucous membranes of the nose
and throat. Breathing of its dust or spray mist should be avoided (3).
During the manufacturing of this herbicide, human exposure is limited due to
normal control measures. However, during mixing and application of this
herbicide, industrial and agricultural workers are exposed to the wettable
powders and aqueous emulsions. The two primary routes of exposure are (a)
respiratory, that is, inhalation of dusts and sprays and (b) skin contact with
dusts, emulsions, or sprays (2). Respiratory exposure could be minimized by
wearing an approved respirator. Skin and/or eye exposures could be minimized
with the use of gloves, goggles, and other protective clothing during terbacil
handling (16). Terbacil is not a skin sensitizer (22).
Terbacil is of low toxicity to animals. The lethal dose fifty, or LD50,
is the amount of a chemical that is deadly to one-half of experimental animals
exposed to it. When it is given by mouth, the oral LD50 of terbacil for rats
is 5,000 to 7,500 milligrams per kilogram (mg/kg) (5, 8). Signs of acute
terbacil poisoning in rats include weight loss, paleness, lack of movement and
rapid respiration. Six out of six male rats survived ten daily doses of 1,000
mg/kg (17). The dermal LD50 in rabbits is greater than 5,000 mg/kg (the
maximum feasible dose). These rabbits did not show clinical signs of
toxicity, nor any obvious gross changes caused by disease. No skin irritation
and only mild eye irritation was seen in rabbits at this dose (2, 17).
Similarly, there was no skin irritation or sensitization in terbacil-treated
guinea pigs (17). Dogs that are given five grams per kilogram (g/kg) of
terbacil exhibit vomiting and a lack of eye pupil responsiveness (2).
No evidence of toxicity was seen in two-year feeding studies of rats fed
doses as high as 12.5 mg/kg or in dogs fed doses as high as 6.25 mg/kg of
terbacil. At 125 to 500 mg/kg there was a lower rate of weight gain, liver
enlargement and other liver changes in rats. The high dose produced a slight
increase in liver weight in dogs (2).
The EPA has established a Lifetime Health Advisory (LHA) level of 90
micrograms per liter (ug/l) for terbacil in drinking water. This means that
EPA believes that water containing terbacil 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 terbacil at high levels
well above the LHA level over a long period of time has been shown to cause
liver damage, reduced fetal weight, and disturbances in fetal development in
laboratory animals (23).
There were no adverse effects on lactation, fertility, birth rate, pup
survival, or any other aspect of reproduction in rats fed 2.5 and 12.5
mg/kg/day of terbacil for 3-generations (2).
The average number of live fetuses per litter and the average final
maternal body weight were significantly lowered in the 103 and 391 mg/kg/day
dosage groups (24).
Terbacil was not teratogenic when tested on rats and rabbits (22). When
doses of 0, 30, 200, or 600 mg/kg/day were administered by gavage to pregnant
rabbits on days 7 to 19 of gestation, adverse effects on the fetuses appeared
only at the highest dose tested. This dose also produced maternal toxicity
and increased maternal mortality. No adverse effects on the mothers or the
pups were observed at lower doses. The NOAEL for this study was 200
mg/kg/day. In another study, pregnant rats were fed doses of 0, 23, 103, or
391 mg/kg/day on days 6 to 15 of gestation. Abnormalities occurred in the
renal pelvis, and ureter dilation was found in pups from all the treatment
Terbacil was not mutagenic in several screening tests (2, 22).
EPA has determined that terbacil does not increase the risk of cancer in
humans (23). No evidence of carcinogenicity was found in rats fed 2.5, 12.5,
125 or 500 mg/kg/day of terbacil for two years nor in dogs fed as much as 250
mg/kg/day for 2.5, 62.5 or 250 mg/kg (2). When mice were fed dietary doses of
0, 2.5, 62.5 or 250 mg/kg for 2 years, no increased incidence of cancer was
Various liver changes have been seen in experimental rats exposed to high
doses of terbacil (2).
Fate in Humans and Animals
In general, the uracil herbicides, the chemical class in which terbacil
is included, are rapidly excreted in urine by mammals (9). This may account
for their reportedly low toxicity (4). When given in the feed of lactating
cows at five and 30 ppm, terbacil was excreted in the milk at levels up to
0.03 and 0.08 ppm, respectively. No herbicide was detected in the cows' urine
and feces (9).
Effects on Birds
Terbacil is slightly toxic to birds (19). The eight-day LD50 for
terbacil is more than 56,000 ppm for Peking ducklings and greater than 31,450
ppm for pheasant chicks (17). The LD50 for terbacil in quail is greater than
2,250 mg/kg (22).
Effects on Aquatic Organisms
Terbacil is not toxic to fish (19). The median tolerance limit (TLM) is
the concentration of a chemical that will kill 50% of exposed organisms in a
given time period. The 48-hour TLM of terbacil in sunfish is 86 ppm (2). The
LC50 for terbacil in bluegill sunfish is 102.9 ppm (practically non-toxic) and
46.2 (slightly toxic) in rainbow trout. Terbacil is slightly toxic to
freshwater invertebrates, with an LC50 of 65 ppm in Daphnia, a small
freshwater crustacean. The LC50 for terbacil in marine oysters is greater
than 4.9 ppm (moderately toxic) and 49 ppm (slightly toxic) in shrimp (22).
Terbacil does not bioaccumulate in bluegill sunfish (24).
A study on grass shrimp with an 84.7% formulated terbacil product was
sufficient to characterize the herbicide as slightly toxic to marine
invertebrates. Estuarine and marine organisms may be exposed to terbacil due
to its use as a sugarcane herbicide (16). The 48-hour TLM of terbacil in
fiddler crabs is 1,000 ppm (2).
Effects on Other Animals (Nontarget species)
Terbacil is not toxic to bees and can be used around bees with minimal
injury (3, 7)
Breakdown of Chemical in Soil and Groundwater
In most soil types, terbacil has a relatively low tendency to be adsorbed
to soil particles (Koc = 55 g/ml). It also is highly soluble in water and
highly persistent in soils. Soil half-lives of 120 days or 2 to 5 months have
been reported. This information indicates that terbacil is likely to be
moderately mobile in soil and can potentially pollute groundwater (4, 14, 17,
20, 24). It should not be used on sandy or gravelly soils that have less than
1% organic matter, particularly if the water table is near the soil surface
(12, 16). A potential exists for residues of terbacil to contaminate
groundwater beneath sandy soils treated with terbacil, particularly when deep
tillage is used (14). When used in orchards, terbacil accumulates and moves,
or 'leaches,' in soils, with most movement occurring in coarse textured soils
(13). Leaching may be slower in soils that are finer textured and/or have
higher organic matter content (16). Terbacil was not detected in a national
groundwater survey conducted by EPA (15).
In moist soils, terbacil is subject to microbial degradation. However,
data suggest that recommended rates of terbacil use may result in its
persistence for more than one growing season (13). Data from field
dissipation studies showed that terbacil persistence in soil varied with
application rate, rainfall, soil type, and mobility (16). In topsoil, 50% of
terbacil and its breakdown products still remained five to seven months after
application (3). The soil half-life of radio-labeled terbacil was 5 to 6
months when 4 lb/acre were applied to the surface of a silt loam soil (17).
Terbacil was extremely persistent, with soil half-lives of 520 day under
aerobic conditions and 178 days under anaerobic conditions (22).
Breakdown of Chemical in Water
Contamination of surface waters near terbacil-treated areas, and
subsequent exposure of humans and nontarget organisms, is possible due to
terbacil's mobility in soil and its high water solubility (16). Terbacil is
stable to hydrolysis and photo degrades slowly in water (22). Terbacil should
be kept out of lakes, streams, and ponds. Water can be contaminated by
inappropriate cleaning of equipment or disposal of wastes (1).
Breakdown of Chemical in Vegetation
At normal application rates, terbacil has residual phytotoxicity in
treated soils for one to two years (16). Terbacil residues were phytotoxic to
oats planted three years after a previous application of the herbicide (6).
In alfalfa, 12% of terbacil plus its metabolites are still found six to eight
months after application (3). Since treated cover crops contain residues of
terbacil, there is a general restriction against the grazing or feeding of
livestock on treated orchard cover crops (16).
Terbacil is most readily absorbed through the root system of plants to
which it is applied. Less is absorbed through the leaves and stems of plants.
Studies of sugarcane plants indicate that terbacil is moved, or
'translocated,' upward into the leaves after absorption by the roots (17).
PHYSICAL PROPERTIES AND GUIDELINES
Terbacil is a white, crystalline, odorless solid which is noncorrosive
and nonflammable (2, 18). Terbacil is stable under normal temperatures and
pressures, but may pose a slight fire hazard if exposed to heat or flame (21).
It is stable in water, aqueous bases and common solvents, but poses a fire and
explosion hazard in the presence of strong oxidizers. Thermal decomposition
products may include toxic oxides of nitrogen and carbon and toxic and
corrosive fumes of chlorides (2, 21).
Technical terbacil is 95% pure active material. Terbacil formulations are
compatible with most herbicides with which they might be mixed (13, 17). It
should be stored in a cool, dry place. The active ingredient is non-volatile
and will remain chemically stable under normal storage conditions (1, 17).
Keep terbacil out of reach of children and do not contaminate water, feed, or
food by its storage or disposal. Empty terbacil containers should be disposed
of or buried away from water supplies in accordance with federal, state and
local regulations. Open dumping is prohibited (17).
Protective clothing is required, along with good sanitary practices, when
terbacil is used (1). Care should be taken to prevent drifting terbacil over
neighboring crops (3). Due to residues of terbacil contained in cover crops,
grazing or feeding of livestock with such crops is restricted in pecan, apple,
pear, and peach orchards, as well as in citrus groves (16).
No occupational exposure limits have been established for terbacil by
OSHA, NIOSH or ACGIH (21).
|TWA (8-hour): ||10 mg/m3 (2)
|NOEL: ||50 ppm in dog; 250 ppm in rat (in chronic feeding studies) (16)
|ADI: ||0.0125 mg/kg/day (16)
|MPI (Maximum Permissible Daily Intake): ||0.75 mg/day (based on 60 kg. body weight (16)
|CAS #: ||5902-51-2
|Specific gravity: ||1.34 (25/25 degrees C) (21)
|H20 solubility: ||710 ppm at 25 degrees C (17)
|Solubility in other solvents: ||Barely soluble in mineral oils and aliphatic hydrocarbons.
Slightly soluble in methyl isobutyl ketone, butyl acetate and xylene.
Readily soluble in cyclohexanone, dimethylformamide, xylene and strong aqueous alkalis (3, 19).
|Solubility at 25 degrees C: ||Dimethylformamide - 252,000 ppm; Cyclohexanone - 180,000 ppm; Methyl isobutyl ketone - 121,000 ppm; Butyl acetate - 88,000 xylene - 61,000 ppm (17)
|Melting Point: ||175-177 degrees C (347-351 degrees F) (17)
|Boiling point: ||Terbacil is a solid at room temperature (22)
|Vapor pressure: ||4.7 x 10 to the minus 7 mmHg at 29.5 degrees C ; 5.4 x 10 to the minus 6 mmHg at 54 degrees C (17)
|Kow: ||log Kow = 1.89 (11); 78 (14)
|Koc: ||55 g/ml (20)
|Kd: ||46 (10)
|Chemical Class/Use: ||Substituted uracil herbicide
Du Pont Agricultural Products
Walker's Mill, Barley Mill Plaza
PO Box 80038
Wilmington, DE 19880-0038
Review by Basic Manufacturer:
Comments solicited: October, 1992
Comments received: November, 1992
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