Picloram residue levels for the control of leafy spurge regrowth

The persistence of picloram was measured in the soil following a single application of 2.24 kg/ha and four consecutive annual applications of 0.56 kg/ha for the control of leafy spurge. The concentration of picloram in the surface 7.5 cm of soil decreased from 820 ppb immediately following the application of 2.24 kg/ha to range between 17 and 52 ppb in the third year after treatment. Picloram residues ranged between 45 and 165 ppb during each of the 4-yr periods of annual applications of 0.56 kg/ha. The critical level of picloram that must remain in the soil to prevent leafy spurge re-establishment from seed is about 50 ppb.