Environmental Fate of Rice
Pesticides - 2007



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Project Leader and Principal  Investigators

Ronald S. Tjeerdema, professor and chair, Dept. of Environmental Toxicology, UC Davis


The overall goal of this ongoing research project is to characterize what happens to rice pesticides under flooded California rice field conditions. Compounds dissipate in the environment through four processes – volatilization through air, adherence to soil, degradation by sunlight, and degradation by soil microbes. Studies in 2007 again concentrated on etofenprox, an experimental insecticide for Rice Water Weevil control, and clomazone, a popular grass herbicide.

Etofenprox study continues

Etofenprox belongs to a new class of ether-based pyrethroid pesticides and is not yet registered in California. It has been used effectively for pest control in rice grown in Spain and Japan. The water-insoluble insecticide is very effective against Rice Water Weevil.

Soil samples were taken from two rice fields – one from each side of the Sacramento Valley (Princeton and Richvale). Soils were characterized for a number of chemical properties at UC Davis. Laboratory analysis showed that volatilization into the air will not be a major dissipation pathway for this compound. Etofenprox did show a strong tendency to partition out of water and adhere to soil particles.

The Richvale soil showed a stronger potential for uptake of etofenprox than the Princeton soil. Further work is required to unravel the individual composition of each soil to better understand why this is so. The nature of soil organic matter may be a factor. The Princeton field had a history of burning rice straw after harvest, whereas the Richvale field incorporated rice straw after harvest.

This research showed that offsite movement of etofenprox through drainage water is unlikely. Microbial degradation is a more likely route for this compound in the environment. Studies in 2008 will focus on this pathway, as well as the effect of sunlight on degradation of waterborne etofenprox.

Clomazone studies

Studies continued in 2007 on clomazone (Cerano®), a popular and versatile herbicide used against barnyardgrass, sprangletop and water grasses. This compound was commercialized in the 1980s under the trade name Command® in corn, cotton, and soybeans. In the last decade clomazone has become a popular tool to control broadleaf weeds and grasses in rice.

Clomazone is relatively water soluble, so a better understanding of its migration through soils is important. Soil samples from Princeton and Richvale were also used in this study. The compound was found to absorb more effectively to soil organic carbon, regardless of field burning history.

This research showed that dissipation of clomazone from flooded rice fields is not likely to be through soil. It may very well be efficiently degraded through the action of sunlight or by the action of soil microbes. Future studies will examine those pathways.


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