Herbicidal Activity and Residual States of Metolachlor in Upland Soils

The residues of metolachlor [2chloro-2'-ethyl-N-(2-methoxy-l-methylethyl)6'-methylacetanilide] applied to a soybean field were determined separately as to water-soluble and water-insoluble ingredients and were found to fluctuate with precipitation. The half life of all metolachlor was calculated as 40 to 60 days in 1990 and 4 to 5 days in 1991. The half life of the water-soluble ingredients, however, was as 6 to 12 days in 1990 and 3 days in 1991. Plant growth of Italian ryegrass in soils collected periodically after metolachlor application was inhibited, and this inhibition was closely related to the concentration of the water-soluble ingredients but little to that of the water-insoluble ingredients. This confirmed that the inhibition of plant growth by soil-applied metolachlor was induced by its water-soluble ingredients, while concentration of the herbicide in the soil water fluctuated little with the amount of precipitation, as different amounts of precipitation caused little variation in activity.