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).
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 (21).
Breakdown on Chemical in Water
Precautions should be taken to avoid contaminating water with bentazon from cleaning application equipment or through disposal of its wastes (12).
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 (11).
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 temperatures (12).
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)|
|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).|