Environmental Fate of
Rice Pesticides-03



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Project Leader and Principal UC Investigators

Ronald S. Tjeerdema, Dept. of Environmental Toxicology, UC Davis


DPS Study Continues

An investigation continued into the cause of Delayed Phytotoxicity Syndrome (DPS), a serious problem in the rice fields of the eastern Sacramento Valley.

DPS, characterized by stunted rice plants, is thought to be caused by a dechlorinated byproduct of Bolero®. This byproduct, deschlorothiobencarb (DTB), is created by anaerobic soil bacteria and is exacerbated by the presence of organic carbon, a concern for growers because of increasing amounts of incorporated stubble and straw in rice fields.

One area of research examined how well copper might deter formation of microbes that facilitate production of DTB. Since copper is also used in rice fields to control shrimp and algae, it could be modified to inhibit the microbes implicated in DTB formation. Concentrations above 100 ppm were found to reduce production of the DPS-producing toxic agent.

Experiments were also conducted on two types of DPS-susceptible soil to see whether high phosphate concentrations might prevent formation of deschlorothiobencarb by disrupting the energy metabolism of the microbes. Initial results indicate this strategy holds promise and that high phosphate concentrations may avert DPS.


Work also continued on the herbicide clomazone. Researchers are exploring the physiological reasons for this herbicide's selectivity against watergrass.

Early and late watergrass were exposed to this herbicide. Rice was considerably - and in many cases significantly - less sensitive than either of the watergrasses.

A plant metabolite called ketoclomazone is believed to be the primary mode of toxicity for clomazone. Comparing differences in uptake and biotransformation of the herbicide between rice and watergrass could lend insight into the mechanism behind the selective toxicity. Such information would facilitate ongoing collaborative research to identify safeners for use with the herbicide to reduce rice injury. Data analysis is continuing.


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