Behavior of controlled-release pyrimisulfan in the soil and its herbicidal activity on Echinochloa oryzicola seedlings under various irrigation conditions.

Pyrimisulfan is a highly effective, water-soluble, and paddy herbicide used against various annual weeds, such as Echinochloa spp. Pyrimisulfan was released into surface water, and it migrated into soil water; it formed a herbicide-treated layer within seven days of the water-holding period upon the application of granules with uncontrolled formulation under the flooded condition. Although pyrimisulfan efficacy declined under overflow conditions before forming the herbicide-treated layer, the decrease in herbicidal activity was suppressed after layer formation. In addition to uncontrolled formulation, a controlled-release formulation was tested. Although time for the formation of the herbicide-treated layer increased, water fluctuation resistance improved. As a benchmark, the concentration of pyrimisulfan in soil water on the target date, after adsorption equilibrium, was 57.6 ppb. Although herbicidal activity generally decreases in soils with high organic carbon content, pyrimisulfan showed no loss of activity. This study provides fundamental insights into the mechanism of herbicide-treated layer formation.