Microbial Degradation of
Pesticides Important to
Rice Culture-01 



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Project Leader and Principal UC Investigators

Ronald S. Tjeerdema, professor, Department of Environmental Toxicology, UC Davis

Donald G. Crosby, professor, Department of Environmental Toxicology, UC Davis



An investigation into the cause of Delayed Phytotoxicity Syndrome (DPS), a serious problem in some parts of the Central Valley, was initiated in 2001. DPS is characterized by stunted plants. Dechlorinated byproducts created by the anaerobic breakdown of the rice herbicide thiobencarb (Bolero®) had previously been implicated. This research project was established to reveal the mechanisms at work and what growers could do to avert DPS.
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The results of these experiments suggest that
rolling straw onto rice fields rather than disking it in
would produce the most rapid degradation of
thiobencarb - regardless of soil type.

Two Sacramento Valley rice field soils with different physical and chemical properties—one with a history of the DPS problem (East side) and one without it (West side)—were examined. In addition, to determine the influence of organic carbon on the byproduct (deschloro-
thiobencarb), various amounts of rice straw were incorporated into each soil. Samples analyzed at a lab in the UC Davis Department of Environmental Toxicology show rice straw plays a crucial role in the creation of deschlorothiobencarb.

Deschlorothiobencarb showed up in the east-side soil in as little as five days after herbicide application, whereas west-side soils took longer, about three weeks or more. The byproduct was produced in both soils—with and without straw amendments. Although DPS symptoms have not been reported in west side soils, these experiments show that it is possible and therefore cause for concern. This is contrary to previous studies that showed DPS was limited to specific soil conditions.

Deschlorothiobencarb production in east side soils was correlated with the percentage of straw mixed into the soil. In the west side samples the relationship between straw amendments and lag time is not as clear.

What does this mean for field management? The results of these experiments suggest that rolling straw onto rice fields rather than disking it in would produce the most rapid degradation of thiobencarb—regardless of soil type. On the east side, the best scenario would be to remove straw from the field, which would prolong the action of the herbicide, delay its degradation and thus minimize DPS.

Researchers also determined in laboratory experiments that deschlorothiobencarb formation can continue for up to a week after water has been drained from a field. The reason for this phenomenon is anaerobic conditions continue in the rice field for some time. Only after a week do soils become aerobic enough to oxidize deschlorothiobencarb.

Another part of this research sought to determine the threshold at which deschlorothiobencarb causes DPS. Researchers developed a greenhouse hydroponic dosing method to analyze these levels. Rice plants were observed for decreased growth, color changes and increased tillering. A complete toxicity assessment should be completed in 2002.

Although soil organic carbon is strongly correlated with production of deschlorothiobencarb, thus also likely DPS, other natural soil characteristics may be at work. Additional soil samples will be taken and analyzed in 2002 so verifiable conclusions can be drawn.


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