Relative dissipation of acetochlor, alachlor, metolachlor, and SAN 582 from three surface soils

The dissipation of four commonly used soil-applied herbicides was examined in a standardized field experiment in three southern states (Kentucky, Miss issippi, and Tennessee). Averaged over the three soils and 2 yr, the relati ve order of increasing half-life defined as time for 50% disappearance in d ays (DT50) was acetochlor (6.3 d) = alachlor (6.3 d) = SAN 582 (7.3 d) < me tolachlor (13.7 d). Metolachlor was the most persistent in the soil surface , and this could potentially translate into greater duration of weed contro l into the growing season. All examined herbicides had a DT50 that averaged less than 14 d in all states in both years, so full-season weed control of susceptible species would not be expected. Rapid herbicide degradation was encouraged in these field sites by adequate to excessive soil moisture and warm temperatures throughout the sampling interval. The soils also were li ght textured, and the lower adsorption of the herbicide allowed for degrada tion ease and perhaps leaching below the sampling depth.