Environmental Fate of Rice Pesticides-04



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Project Leader and Principal UC Investigators

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


New herbicide analyzed

Penoxsulam is a new post-flood herbicide for control of annual grasses, sedges and broadleaf weeds.  It was scheduled for release in 2005 under the trade name Granite.

This herbicide belongs to a class of ALS inhibitors that are effective at extremely low doses.  These herbicides are comparatively safe for birds, fish amphibians and invertebrates.  Due to its novelty, however, little is known about the environmental fate of penoxsulam.

Using four representative rice field soil samples from the Sacramento Valley, scientists conducted a laboratory analysis at UC Davis.  They concluded that penoxsulam is highly mobile in water and will not be significantly retained in rice field soils.

Biodegradation and photodegradation experiments are also in progress on penoxsulam to gain knowledge of how this compound breaks down in flooded field soils.  One of the findings from this work is that penoxsulum is virtually non-volatile.

Researchers anticipate that photodegradation by sunlight and biodegradation from microorganisms will be the primary means by which penoxsulam will breakdown in Sacramento Valley rice fields.

DPS studies continue

Scientists also looked more deeply into Delayed Phytotoxicity Syndrome (DPS), a serious problem in the rice fields of the eastern Sacramento Valley.  DPS, characterized by stunted rice plants, is caused by a dechlorinated byproduct of Bolero.®  This byproduct, deschlorothiobencarb (DTB), is created by anaerobic soil bacteria.

One area of research examined how well copper or phosphate 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. 

Results of this work showed that at currently acceptable field rates, phosphate application could both enhance thiobencarb degradation and inhibit the formation of DTB.  Copper addition was deemed a less effective, less acceptable method for dealing with the problem.

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 soils 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.

Clomazone studies

Studies of clomazone, the active ingredient in Command and Cerano, neared completion in 2004.  Researchers sought a better understanding of this herbicide’s selectivity against watergrass compared to rice.  Rice is considerably — and in many cases significantly — less sensitive than early watergrasses.

A plant metabolite called ketoclomazone is believed to be the primary mode of toxicity for clomazone. Early watergrass is capable of absorbing clomazone at almost twice the rate as rice, possibly explaining the differential sensitivity toward rice.  These findings will be integrated with a greenhouse study that is examining the use of safeners in combination with clomazone. 


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