Leaching potential and decomposition of clopyralid in Swedish soils under field conditions

The mobility and decomposition of the herbicide clopyralid (3,6‐dichloro‐2‐pyridinecar‐boxylic acid) were studied under field conditions in a sandy soil and a clay soil. Leachate was collected in lysimeters with undisturbed soil (sand) and in tile‐drained plots (clay). Soil samples were also collected down to 1 m depth in both soils to characterize the temporal depth distribution of clopyralid in the profiles. The herbicide was applied at two rates, i.e., 120 and 240 g acid equivalents (a.e.)/ha, representing normal and double doses of the compound for sugar beets and oilseed rape. Some lysimeters received supplementary watering. Concentrations of clopyralid above the detection limit {0.4μg/L}, i.e., 0.5 (normal dose) and 6 μg/L (double dose) were found in only two drainage samples from the clay soil. Both samples were collected simultaneously approximately two months after the herbicide application, by which time less than 1 mm of drainage had been collected. The overwhelming part of applied clopyralid remained in the topsoil at both sites. Four months after spraying, no clopyralid was detectable (<2 μg/kg dry soil) in the soil.