|Environmental Fate of
Rice Pesticides - 98
Project Leader and Principal UC Investigators
Donald G. Crosby, professor, Department of Environmental Toxicology, UC Davis
Ken Ngim, graduate research assistant, Department of Environmental Toxicology, UC Davis
|Knowledge of what happens to rice pesticides in the environment
is crucial to decisions affecting their use and regulation. The guiding objectives of this
ongoing project are to investigate the factors governing the movement of rice pesticides;
to estimate the relative importance of such factors; and to apply research results toward
meeting regulatory requirements and improved management practices.
New herbicide examined
Research continued on the promising new rice herbicide carfentrazone-ethyl (Shark®). Previous laboratory experiments showed that carfentrazone-ethyl dissipated rapidly from treated rice fields. However, more persistent acidic byproducts were still detectable at substantial levels in water as much as a month after herbicide application. While the breakdown products are not considered to be particularly toxic to animals, one of them (a chloropropionic acid) is somewhat toxic to plants. Further study has confirmed this finding. Carfentrazone-ethyl, once applied to a flooded rice field, quickly converts to a product that is unreactive, nonvolatile and mobile in water. In other words, this byproduct does not break down well and is cause for concern.
A field test of the persistence of carfentrazone-ethyl at the Rice Experiment Station showed that chloropropionic acid migrated with subsurface flow underneath dikes and appeared in downstream plots at substantial levels. This outdoor study of the "terminal residue" the most stable endproducts to remain in a field showed that the acid byproduct was the only herbicide-related substance detectable after 33 days. Interaction with rice straw and other naturally occurring compounds in field conditions may be manipulated to help with the breakdown of the carfentrazone acid.
RWW insecticide examined
Previous research on Fipronil (Icon®), a promising new insecticide for control of rice water weevil, showed that this chemical dissipated in less than two days. However, a highly persistent and toxic breakdown product formed by sunlight, raised concerns over Fipronil's proposed use on paddy rice. Controlled laboratory tests showed that the byproduct is degraded by sunlight in field water in about 18 days. The compound definitely persists longer than the parent insecticide but is far from permanent and no more persistent than many common pesticides.
Toxicologists previously determined that the continuing addition of copper (bluestone) to control algae and tadpole shrimp could raise soil copper levels to crop-damaging levels. In addition to several previously identified potential algicide replacements, a chemical literature search revealed several additional leads. The search for an alternative will continue.