Bensulfuron methyl dissipation in california rice fields, and residue levels in agricultural drains and the Sacramento River

Abstract

Herbicides applied to rice fields in the Sacramento Valley, CA, are released into agricultural drains via runoff water. Major agricultural drains then empty into the Sacramento River where contaminants may affect the quality of drinking water and aquatic wildlife habitat. Bensulfuron methyl (BSM) (Londax| methyl 2-[[[[[(4,6-dimethoxypyrimidin-2-yl)amino]carbonyl]amino]sulfonyl] methyl]benzoate), a highly active sulfonylurea, was registered for use in California in 1989 as a rice herbicide. Although toxicity of BSM is low for mammals and fish (rat oral LDso >5000 mg/kg; rainbow trout 96-hr LCso >150 mg/L) (Beyer et al. 1988), the effect of residues on the aquatic environment, in particular plant life, is unknown. Persistence of BSM in rice paddy water has been examined in the laboratory (Yuyama et al. 1987b) but not in the field. According to label recommendations, rice paddy water should be held on the field for at least 5 days after application of BSM. Residue levels of BSM in agricultural drains and rivers, resulting from runoff releases after this holding time, have not been determined. This study examines dissipation of BSM from paddy water of commercial rice fields and measures residue levels in major agricultural drains and the Sacramento River. MATERIALS AND METHODS Three commercial rice fields located in Colusa and Glenn Counties, CA, were selected to examine dissipation of BSM in paddy water. Fields 1, 2 and 3 had total areas of 24, 16 and 17 ha, respectively. Only the last paddy of each of these fields was sampled. Paddy areas were 1.3, 1.2 and 3.1 ha, for Fields 1, 2 and 3, respectively. A 60% dry flowable formulation of BSM was aerially applied to Fields 1, 2 and 3 on May 2, 12 and 16, respectively, in 1989. Applications were made to standing water, 8 to 12 d after seeding, at the nominal rate of 0.07 kg/ha, for all three fields. Water