Dissipation of the herbicide benzofenap (Taipan 300) in a rice field ecosystem

The fate of benzofenap {2-[4-(2,4-dicholoro-m-toluoyl)-1,3-dimethylpyrazol- 5-yloxy]-4′-methylacetophenone} applied to flooded rice was studied at two locations in New South Wales (Australia). Solid-phase extraction (SPE) was compared with liquid-liquid extraction (LLE) for the determination of the commercial chemical in water samples. SPE performed well as compared to LLE (84 vs 80%) in irrigation waters. However, at the lower end of the concentration range (3 μg/L), LLE achieved higher recoveries than SPE (72 vs 59%). Rates of dissipation (DT50) from floodwaters and soils were measured. Dissipation of the herbicide from water and soil occurred fairly erratically in both mediums and can be best explained by a first-order decay process. The DT50 value for benzofenap was <1 day in irrigation water due to rapid deposition of the suspension concentrate formulation. The DT50 in surface soil was 44 days. The maximum measured concentration of benzofenap in a rice field floodwater was 39 μg/L, taking approximately 32 days to dissipate to <1 μg/L.