Objectives

• Identify and measure environmental factors that govern movement and chemical fate of rice pesticides.
• Estimate their relative importance.
• Improve pesticide management.
• Apply the results to meeting regulatory requirements.

The Department of Environmental Toxicology at the University of California, Davis, does research to improve environmental safety and effectiveness of rice pesticides. This requires knowledge of the degradation products and their environmental effects. Data are obtained when necessary to meet regulatory requirements for new pesticides and for safer applications of those already in use.

Research in 1986 emphasized enhancement of Bolero degradation with zinc oxide and the dissipation rates for two new rice herbicides, Londax and Whip.

Field and laboratory studies showed that zinc oxide rapidly reduced herbicide levels in Bolero-treated water. A zinc oxide spray was applied at rates of 5 and 10 pounds per acre. Under normal growing conditions, the 10-pound rate applied six days after the Bolero reduced herbicide levels in water to about 100 ppb, or as much as 57 percent. In these trials, the untreated controls also showed reductions which have not been explained.

Bolero was adsorbed by the soil, and levels in the sediment were 10 to 30 times greater than those in water after zinc treatment. The soil levels decline slowly, and the herbicide is retained even after the water is drained.

In other field-scale trials on three major rice soil types, zinc oxide immediately reduced Bolero residues by up to 60 percent. However, natural dissipation offsets most of the advantage of using zinc oxide when water is held for the required 14 days.

Studies on the effects of sunlight intensity and pH of the water on the degradation of Bolero in the presence of zinc oxide showed that zinc oxide was most effective in full sunlight. Some cloudiness could be tolerated, but it slowed degradation. Zinc oxide was ineffective at pH 7.0 or below, but most rice field water is above pH 7.0. If zinc oxide treatment is being considered, a simple indicator strip test should be used first to determine the pH level.

Safe use of zinc oxide in rice fields requires assurance that aquatic animals, particularly fish, are not injured and that levels of dissolved zinc do not accumulate. Fish, snails, frogs and tadpole shrimp were enclosed in fields that had been treated with Bolero and zinc. Neither Bolero nor the combination of Bolero and zinc oxide had any significant effect on any of the organisms within 48 hours. The field zinc levels seldom exceeded 1 ppm, but they also seldom returned to the pre-zinc oxide levels within the holding period.

In the laboratory test, 15 ppm of dissolved zinc killed mosquito fish (Gambusia affinis) and 1.25 ppm killed tadpole shrimp (Triops longicaudatus) within 12 hours. However, the field tests indicated the fish and shrimp could survive the first few hours at elevated zinc levels.

The general conclusion is that zinc oxide might be valuable as an emergency treatment to degrade Bolero to safe levels in rice drainage waters but that widescale use of the treatment is not advisable at this time.

Londax, a rice herbicide that has given very promising results the past several years, has an experimental use permit. Field persistence studies were conducted at the Rice Experiment Station to find out if this herbicide presented any residue problems. Londax was applied to flooded-fields at the recommended rate of 1 ounce per acre (70 ppb in water 4 inches deep), and samples were collected for residue analysis over an 8-day period. The herbicide was almost undetectable after 8 hours and was not detectable after 8 days. Londax is not degraded readily by either microorganisms or sunlight. Its rapid disappearance from the field within 8 hours must, therefore, result from uptake by plants and/or the soil, probably an advantageous characteristic.

Whip, another new rice herbicide effective in the control of grass weeds, is not persistent in either water or soil under California rice field conditions. Soil adsorption contributes to its dissipation from water, and processes of hydrolyses, microbial metabolism and photolysis all contribute to rapid degradation.

Basagran, although an effective herbicide, has persistence problems, and conditions for its use are quite restrictive. It dissipates primarily by photolysis and movement in water. Because it is only weakly bound by soil, it presumably could move through wet dikes and levees. During the holding periods following application, Basagran would generate several stable and nonvolatile photoproducts that would release with the field water.