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: 9/93
TRADE OR OTHER NAMES
Basagran, Bendioxide, Bentazone, Bas 351-H, Leader, Pledge. All
products currently marketed in the U.S. contain the sodium salt of
bentazon as the active ingredient, referred to as sodium bentazon (12).
As of 1985, bentazon and its sodium form were under special review
by the United States Environmental Protection Agency (EPA) for data
evaluation and development of a regulatory position (13). Products
containing bentazon must bear the signal word "Caution" (18). Check
with specific state regulations for local restrictions that may apply.
Bentazon was initially registered in 1975. It is a postemergence
herbicide used for selective control of broadleaf weeds and sedges in
beans, rice, corn, peanuts, and mint (1). Bentazon is a contact
herbicide, which means that it causes injury only to the parts of the
plant to which it is applied (4). It interferes with the ability of
susceptible plants to use sunlight in the production of energy for
survival, a process called photosynthesis. Visible injury to the
treated leaf surface usually occurs within 4 to 8 hours, followed by
plant death (11). It should not be used on blackeyed peas or garbanzo
beans (9, 17). It is available as a soluble concentrate (18).
Bentazon is moderately toxic by ingestion and slightly toxic by
dermal absorption (20). Human ingestion of this herbicide has caused
vomiting, diarrhea, trembling, weakness, and irregular or difficult
breathing. It is moderately irritating to the skin, eyes, and
respiratory tract, as well (7). Symptoms which have occurred in test
animals include apathy, incoordination, prostration, tremors, anorexia,
vomiting and diarrhea (15).
Bentazon has been determined to be moderately toxic to mammals
based on the moderate amounts of the herbicide that kill one-half (50%)
of test animals to which it is administered. These doses are referred
to as lethal doses fifty, or LD50s. The lower the lethal dose, the more
toxic is the chemical. The LD50 for bentazon in cats is 500 milligrams
per kilogram (mg/kg), in rabbits is 750 mg/kg, in mice is 400 mg/kg, and
in rats is 1,100 to 2,063 mg/kg (1, 2, 3, 4, 15).
When bentazon was applied to the shaven skin of rabbits, it did not
cause irritation (2, 7, 15, 17). Its dermal LD50 is 4,000 mg/kg in
rabbits. When it is injected under the skin (percutaneously) of rats,
the LD50 for bentazon is 2,500 mg/kg (8). Severe eye irritation from
this material healed after one week (17). No adverse diseased
(pathological) or poisoning (toxicological) effects were observed in
rats exposed to 200 mg of the dust of formulated material (BAS 3512-H)
in a liter of water for one hour daily, over a two-week period (15).
Prolonged or repeated exposure of the skin or eyes to bentazon may
cause dermatitis or conjunctivitis (20).
In 1983, an 18-month dietary feeding study with mice, and a two-
year feeding study with rats, both produced a no observable effect level
(NOEL) of 17.5 mg/kg. In 1985, the EPA decided that all previous
assessments of bentazon's chronic toxicity were invalid or questionable
because research practices did not meet EPA guidelines or data were
invalid. The replacement of all data was required (11). When dogs were
given 0, 2.5, 7.5, 25 or 75 mg/kg/day for 13 weeks, weight loss, general
ill health, and inflammation of the prostrate occurred at the highest
dose tested. The NOAEL for this study was 2.5 mg/kg/day (23).
The EPA has established a Lifetime Health Advisory (LHA) level of
20 micrograms per liter (ug/l) for bentazon in drinking water. This
means that EPA believes that water containing bentazon 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 bentazon at high levels well above the HAL level over a long period
of time has been shown to result in excessive weight loss and
inflammation of the prostate gland in animal studies (22).
The EPA also reported in 1985 that, although the herbicide was not
known to negatively affect reproduction, submitted test data were either
questionable or invalid, and further testing was required (11).
The available rat and rabbit teratogenicity studies were deemed
inadequate by the EPA even though they do not suggest that bentazon
causes birth defects. Birth defects called 'terata' were observed in
one rat study at a dose of 200 mg/kg/day, but the usefulness and
validity of these data are in question. Additional teratology studies
in the rat and rabbit were required by the EPA in 1985 (12).
At the time of its review in 1985, there was a data gap in
mutagenicity data on this herbicide. While a variety of mutagenicity
studies on bentazon were reviewed in 1985, none were thought to be
adequate for regulatory purposes (12).
The EPA reports that no tumor-causing (oncogenic) effects have been
seen from bentazon, but available data are invalid or questionable.
Tumors have been seen in rats at 200 mg/kg/day, but again, these results
are questionable. Additional testing was deemed necessary in 1985 (12).
EPA has not classified bentazon as a carcinogen because data from
laboratory studies are inadequate for EPA to determine if bentazon can
increase the risk of cancer in humans (21, 23).
Consumption of bentazon at high levels well above EPA's Lifetime
Health Advisory level (20 ug/l) over a long period of time has been
shown to result in excessive weight loss and inflammation of the
prostate gland in animal studies (22).
Fate in Humans and Animals
The metabolism of bentazon in animals is not completely understood.
It is rapidly absorbed and readily excreted, unchanged, in the urine.
About 91% of a 0.8 mg dose administered to rats by stomach tube was
excreted in the urine within 24 hours of ingestion, with less than 1% in
feces. This suggests that bentazon is almost completely absorbed from
the gastrointestinal tract into the bloodstream when it is ingested.
The unmetabolized part consists of at least one other metabolite, or
breakdown product (12, 23). One hour after dosing, radio-labeled
bentazon appeared in the stomach, liver, heart and kidneys, but not in
the brain or spinal chord of rats (23).
Effects on Birds
Technical and formulated bentazon are both slightly toxic to birds.
The acute oral LD50 of formulated bentazon (BAS 3510H) is 2,000 mg/kg
for mallard ducks and 720 mg/kg for Japanese quail (15).
Effects on Aquatic Organisms
Technical and formulated forms of bentazon are classified by EPA as
practically nontoxic to both coldwater and warmwater fish. Bentazon is
slightly toxic to aquatic invertebrates. Residues of the herbicide
accumulate in the tail meat and organs of crayfish. The lethal
concentration fifty (LC50) is that concentration of a chemical in air or
water that kills half of the experimental animals exposed to it for a
set time period. The 96-hour LC50 for bentazon in rainbow trout is 510
mg/l for wettable powder. In bluegill sunfish, the 96-hour LC50 for
technical bentazon is 616 ppm, and in rainbow trout it is 190 ppm. For
formulated bentazon (BAS 35107-H), the 96-hour LC50 in bluegills is
1,060 ppm, and in rainbow trout is 636 ppm (12, 4, 15).
Effects on Other Animals (Nontarget Species)
Applications of bentazon on registered use sites are not considered
by EPA to be hazardous to most nontarget organisms because of the
generally low toxicity of this material. A final risk assessment has
not been made by the Agency as of yet, however, due to missing test data
(12). Bentazon is not toxic to bees (3, 18).
Breakdown of Chemical in Soil and Groundwater
Bentazon does not bind to, or adsorb, to soil particles and it is
highly soluble in water. These characteristics usually suggest a strong
potential for groundwater contamination. Bentazon, however, is so
rapidly degraded in upper soil layers by sunlight and soil microbes that
it does not tend to move (leach) below the plow layer of the soil. Its
rapid degradation is expected to prevent the contamination of
groundwater (11, 19).
In the soil, bentazon is subject to both photodegradation,
breakdown by ultraviolet (UV) light from sun, and biodegradation, rapid
metabolism by soil bacteria and fungi. Bentazon's plant-killing, or
'herbicidal,' activity is brief, and there are no problems associated
with its persistence in the soil (4, 17). Under aerobic soil conditions
in the lab and in the field, the time that it takes for half of bentazon
to be broken down by natural processes is less than two weeks. This
time period is referred to as its half-life. Bentazon reaches
undetectable levels in soil six weeks after its application (12, 10).
At concentrations of 2 to 10 ppm, bentazon's half-life is 2 to 5 weeks
on soil (6).
Based on a national survey, EPA estimates that bentazon may be
found in about 0.1% of the rural drinking water wells nationwide.
Laboratory analysis is capable of detecting bentazon concentrations as
low as 0.25 ug/l (ppb). Bentazon was not detected in any community
water systems. It was also not detected at concentrations above EPA's
drinking water Lifetime Health Advisory Level of 20 ug/l in any well
Breakdown on Chemical in Water
Precautions should be taken to avoid contaminating water with
bentazon from cleaning application equipment or through disposal of its
Bentazon has the potential to contaminate surface water because of
both its mobility in runoff water from treated crops, and its pattern of
use on rice, which involves either direct application to water, or
application to fields prior to flooding. Technical grade bentazon has
low water solubility, meaning that it does not dissolve very easily in
water. Commercial formulations are readily water soluble (11).
Bentazon appears to be stable to hydrolysis, a chemical reaction
with water. It has a half-life of less than 24 hours in water because
it undergoes photodegradation (11).
Breakdown of Chemical in Vegetation
Bentazon is absorbed by plant leaves after it is applied as a
foliar spray. In resistant, or tolerant, plants, bentazon is rapidly
broken down, or metabolized, into natural plant components (metabolites)
(1, 15, 17).
When bentazon is absorbed by the roots, it is translocated from the
roots to other plant parts. Translocation does not occur after
absorption by leaves (15). The degree of translocation depends on the
type of plant. Whether translocated or not, bentazon is quickly
metabolized, reorganized and incorporated into natural plant components
Bentazon is absorbed through the green plant parts and has little
effect on germinating seeds. Studies of the interactions between
bentazon in resistant plants, those that are not harmed by the
herbicide, and susceptible plants, those that are sensitive to the
herbicide, indicate that the bentazon is able to kill only those plants
that do not have the capacity to metabolize this compound (6, 15, 17).
Bentazon inhibits photosynthesis (15). Some leaf-speckling and leaf-
bronzing may occur under certain conditions of bentazon usage. Crop
injury may result if bentazon is applied to crops that have been
subjected to stress conditions, such as drought or widely fluctuating
Upon its review by EPA in 1985, many restrictions were placed on
bentazon's use on vegetation. For example, treated corn fields should
not be grazed for at least 12 days from the last sodium bentazon
application. Product labels should be carefully read and complied with.
It is a federal violation to use bentazon products, or any other
pesticide, in a manner inconsistent with labeling (12).
PHYSICAL PROPERTIES AND GUIDELINES
Pure bentazon is a colorless, odorless crystalline solid.
Technical bentazon is a slightly brown solid (18). Bentazon is stable
under normal temperatures and pressures, but it may pose a slight fire
hazard if exposed to heat or flame. It poses a fire and explosion
hazard in the presence of strong oxidizers. Thermal decomposition of
bentazon will release toxic oxides of nitrogen and sulfur (20).
The stability of bentazon was unchanged after three months of
storage at 50 degrees C. It should be kept from freezing. It should be
kept out of reach of children. It is harmful if swallowed, and contact
with eyes or skin should be avoided. Spray mists should not be inhaled
(4). Contamination of lakes, streams or ponds should also be avoided.
The contamination of water, food or feed can be prevented by storing or
disposing of bentazon far away from any of the above. It should not be
applied when weather conditions favor drift from target areas (15).
Bentazon should not be applied to turf until it is well
established. It should not be applied to blackeyed or garbanzo beans.
Rain within eight hours of application of bentazon will reduce its
effectiveness. The effectiveness of bentazon increases with
temperature. Before application of bentazon, dry beans should have the
first trifoliate leaves fully extended, and peas should have three pairs
of leaves, or plant injury will result. In rice fields, only those
weeds which have emerged above the water level should be treated (9).
|NOEL: ||70 mg/kg body weight, or 3.5 mg/kg/day for 90-day feeding to rats; 300 mg/kg (7.5 mg/kg/day) for 90-day feeding to dogs (4)
|ADI: ||Estimated tolerable daily dose for humans is 0.035 mg/kg (4)
|CAS #: ||25057-89-0
|H20 solubility: ||0.05 g/100 g at 20 degrees C (12); 0.05 g/100 g at 20 degrees C (4)
Sodium bentazon is more water soluble than bentazon, with a solubility of 230 g/100 g in water. (12)
|Solubility in other solvents: ||in acetone, 150.7; ethanol, 86.1; ethyl acetate, 65.0; diethyl ether, 61.6; chloroform, 18.0; benzene, 3.3; cyclohexane, 0.02 (all in g/100 g at 20 degrees C) (4)
|Melting Point: ||137-139 degrees C (279-282 degrees F) (12)
|Decomposition temperature: ||200 degrees C (392 degrees F) (15)
|Flash point: ||100 degrees C (15)
|Vapor pressure: ||negligible; less than 1.3 x 10 to the minus 7 power mbar at 20 degrees C. (4)
|Kow: ||log Kow = 3.81 (5); 2.5 (16)
|Koc: ||log Koc = 3.40 (5); 35 g/ml (19)
140 soil-water distribution coefficient divided by the organic carbon content and calculated by: log Koc = 3.64 0.5 (log water solubility in ppm) +/-1.23 (calculated from a table) (Kenaga, 1980)
|Kd: ||very mobile; soil-water distribution coefficient or adsorption constant from column leaching or TLC studies (14)
|Chemical Class/Use: ||Thiadiazinol herbicide (7)
|Environmental fate: ||Neely partitioning - 86.21% in air; 5.64% in water; 4.21% in ground; 3.91% in hydrosoil (5).
|Predicted bioconcentration factor: ||19, based on calculated water solubility; 0, based on calculated soil-adsorption coefficient (10).
Agricultural Products Group
PO Box 13528
2505 Meridian Pkwy.
Research Triangle Park, NC 27709-3528
Review by Basic Manufacturer:
Comments solicited: November, 1992
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