Improvement of herbicide selectivity with the increase of flooding water depth

Selectivity of thenylchlor [2-chloro-N-(3-methoxy-2-thienyl) methyl-2′, 6′-dimethylacetanilide], pyrazosulfuron-ethyl[ethyl 5-[[3-(4, 6-dimethoxypyrimidin-2-yl) ureido] sulfonyl]-1H-1-methylpyrazole-4-carboxylate] and bifenox [methyl 5-(2, 4-dichlorophenoxy)-2-nitrobenzoate] between transplanted rice plant (Oryza sativa L., cv. Hoshinohikari) and pre- or post-emerged early watergrass (Echinochloa oryzicola Vasing.) was evaluated by a pot test conducted in a greenhouse. Crop safety and control of early watergrass by thenylchlor and pyrazosulfur-on-ethyl were significantly improved when the flooding water depth increased from 2cm to 10cm at 1 or 24 hours after herbicide application as compared to the shallow flooding conditions under which the flooding water depth was maintained at 2cm throughout the experiments (Figs. 1, 2, 5 and 6). In case of thenylchlor, similar results were obtained irrespective of formulation types tested (Fig. 4). Control of early watergrass by bifenox was also enhanced under these flooding conditions but crop safety significantly decreased (Figs. 3, 5 and 6). These results suggested that the changes in the flooding water depth from shallow to deep shortly after the herbicide application seemed to be a suitable management for the selectivity of thenylchlor and pyrazosulfuron-ethyl between transplanted rice plants and early watergrass.