An aquatic risk assessment for cyhalofop-butyl: A new herbicide for control of barnyard grass in rice

Cyhalofop-butyl is a new herbicide used for control of barnyard grass in ri ce paddies. It is applied at a maximum rate of 300 g as/ha at the two to fo ur leaf stage to drained paddies. The environmental fate and ecotoxicology of cyhalofop-butyl has been extensively studied and the potential impact on rice paddies and adjacent ecosystems has been evaluated. Cyhalofop-butyl i s non-toxic to birds, terrestrial vertebrates, arthropods, earthworms and o ther soil organisms. It is mote toxic to aquatic organisms such as fish and Daphnia. However, very rapid and extensive degradation of the parent eater occurs in soil and sediment/water systems (half-life approximately two hou rs) resulting in the formation of acid, amide and diacid metabolites. Almos t complete degradation to carbon dioxide and soil-bound residues is achieve d within two weeks in soil and within three months in sediment/water. The m ajor metabolite, cyhalofop diacid, is non-toxic to fish, Daphnia and algae. A risk assessment has been carried out taking into account agricultural pr actices, environmental fate and ecotoxicological data and it has been concl uded that there is very little risk to the aquatic environment from normal use of cyhalofop-butyl in rice paddies.