Evaluation of the management practice for controlling herbicide runoff from paddy fields using intermittent and spillover-irrigation schemes
Two water management practices, an intermittent irrigation scheme using automatic irrigation system(AI) and a spillover-irrigation scheme (SI), were compared for the fate and transport of commonly used herbicides, mefenacet (MF) and bensulfuron-methyl (BSM) in experimental paddy plots. Maximum mefenacet concentrations in paddy water were 660 and 540 µgL−1 for AI and SI plot, respectively. The corresponding values for bensulfuronmethyl were 46.0 and 42.0 µg L−1. Dissipation of the herbicides in paddy water appeared to follow the first-order kinetics with half-lives (DT50) of 1.9–4.5 days and DT90 (90% mass dissipation) of 7.8–11.3 days. The AI plot had no surface drainage, hence no herbicide was lost through paddy-water discharge. However, SI plot lost about 38 and 49% of applied mefenacet and bensulfuron-methyl, respectively. The intermittent irrigation scheme using automatic irrigation system with a high drainage gate was recommended to be a best management practice for controlling the herbicide losses from paddy fields. The paddy field managed by spillover-irrigation scheme may cause significant water and herbicide losses depending on the volume of irrigation and precipitation. The water holding period after herbicide application was suggested to be at least 10 days according to the DT90 index.