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Dissipation of Bentazon in Flooded Rice Fields

California Dep. of Food and Agric., Sacramento, CA 95814;

B. Buechler

California Analytical Lab., West Sacramento, CA 95691.

^* Corresponding author.

ABSTRACT

The dissipation of bentazon (3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide) in water, soil and vegetation from three flooded rice (Oryza sativa L.) fields was investigated in California. Regression analyses were used to describe dissipation and to determine half-lives. Maximum bentazon concentrations in water, soil and vegetation were 2100 µg/L, 960 µg/kg and 23,000 µg/kg, respectively. Concentrations in water showed a gradient from the upper to lower end of the fields over time and were related to movement of bentazon downfield in flood water. Bentazon mass recovered in water samples was not linear over time but was related to degradation, dilution, application rate and mass flow within a field. In water, the time estimated for a 50% reduction in initial bentazon mass ranged between 11 and 32 d after application for the three fields. Bentazon mass in soil declined exponentially over time and the dissipation half-life was calculated to be 5 d. Bentazon mass recovered from vegetation samples was not linear over time but was related to degradation and wash-off from plant surfaces. The time estimated for a 50% reduction in initial bentazon mass in and/or on vegetation was between 2 and 3 d after application. Dissipation rates determined in this study were different than published results from small field plots and from laboratory data, indicating the need for field measurements in the development of regulatory strategies.