|Environmental Fate of
Rice Pesticides - 2005
Project Leader and Principal UC Investigators
Ronald S. Tjeerdema,professor, Dept. of Environmental Toxicology, UC Davis
The mission of this ongoing project is to study the environmental disposition of pesticides important to rice culture. Studies in 2005 concentrated on penoxsulam, a new post-flood herbicide marketed under the name Granite® and clomazone, a watergrass herbicide marketed under the name Cerano.®
Granite® is a new herbicide designed for post-emergence control of annual grasses, sedges and broadleaf weeds. It is an ALS inhibitor that is effective in very low doses yet displays low toxicity to mammals, birds, fish, amphibians and invertebrates.
Toxicologists began studying penoxsulam in laboratory tests in 2004 to gain a better understanding of how quickly it volatilizes into the atmosphere; whether and for how long it adheres to soil particles; and the nature of microbial and photochemical degradation.
As a general rule, rice pesticides have a greater tendency to volatilize in the Central Valley, with its hot, dry summers, than in other growing regions with cooler climates. However, as reported in 2004, adherence to soil and volatilization of penoxsulam were minimal in rice field conditions. What this indicates is that the herbicide will remain dissolved in water and thus be more likely to dissipate from microbial degradation or the action of sunlight.
The potential for penoxsulam to photodegrade was simulated in laboratory conditions using both field water and the four soils most commonly used for rice cultivation in the Sacramento Valley. An experiment evaluating the impact of sunlight showed that under typical summer California rice field conditions, the herbicide effectively degrades into numerous products believed to be less toxic than the parent within 15 to 25 days.
A second experiment to evaluate microbial degradation of penoxsulam under field conditions showed that anaerobic soil microbes, producing products similar to those formed by photodegradation within 21 to 48 days, rapidly degrade the herbicide. Thus, researchers concluded that this new product would be very non-persistent in the environment.
Studies continued in 2005 on the metabolism, comparative toxicity and potential safeners of Cerano® (clomazone) in rice and watergrass. These two plants showed the largest difference in sensitivity to the herbicide in prior work. Watergrass absorbs the herbicide at twice the rate of rice and produces more than three times the toxic metabolites, potentially explaining the selectivity of Cerano® toward watergrass.
This metabolic difference could be exploited to develop herbicide safeners for use with Cerano.® However, application of metabolic inhibitors or safeners in combination with Cerano® reduced its effectiveness on late watergrass. This may indicate a limited usefulness of safeners in rice culture, as it would have the unwanted effect of protecting watergrass. Further research is needed to understand this mechanism.