Environmental Fate of
Rice Pesticides-97



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Project Leader and Principal UC Investigators

Donald G. Crosby - Professor, Department of Environmental Toxicology UC Davis

Ken Ngim - Graduate Research Assistant, Department of Environmental Toxicology UC Davis

Theresa Pedersen - Postgraduate Researcher, Department of Environmental Toxicology UC Davis

Albert Brandon - Student Assistant Department of Environmental Toxicology UC Davis

Knowledge of what happens to rice pesticides in soil and water is crucial to decisions affecting their use and regulation. Envoronmental toxicologists working with this ongoing project are continually searching for improved ways of detecting and measuring pesticides; estimating the relative importance of factors affecting the use of rice pesticides; and applying research results toward meeting regulatory requirements and improved management practices. The narrative below highlights what researchers have learned about a promising new herbicide and an equally promising new insecticide.

New Herbicide Examined

DitchWater.jpg (68114 bytes)Toxicologists continued their examination of the promising new rice herbicide carfentrazone-ethyl (Shark®). To evaluate its environmental persistence and fate under California conditions, trace analysis methods were developed and principal breakdown products in field water and soil were sampled at the Rice Experiment Station and at a Rice Growers Association facility in Richvale.

In this experiment, carfentrazone dissipated within about two days. However, more persistent acidic byproducts were still detectable at substantial levels in water as much as a month after herbicide application. While the breakdown products are not considered to be particularly toxic to animals, one of them (a chloropropionic acid) is somewhat toxic to plants and may warrant further investigation. Nonetheless, carfentrazone thus far appears environmentally suitable for rice field use.

RWW Insecticide Examined

Fipronil (Icon®) is a promising new insecticide for control of rice water weevil. Previously, toxicologists had observed a highly persistent and toxic breakdown product formed by sunlight, raising concerns over fipronil's proposed use on paddy rice. However, in field experiments more indicative of commercial practice, a combination of environmental and physical factors appeared to lower fipronil and byproduct residues in water below detectability. Fipronil dissipated by half within about 20 days in flooded soil. The insecticide's environmental properties are acceptable thus far.

Bluestone Alternative

Toxicologists previously determined that continuing addition of copper (bluestone) to control algae and tadpole shrimp could raise soil copper levels to crop-damaging levels. Several potential algicide replacements have now been identified. One of these in the right concentration may provide algae control without harming fish or the rice field environment.

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