Persistence and leaching of picloram applied to a clay soil on the Darling Downs.

Abstract

Picloram (4-amino-3,5,6-trichloropicolinic acid) was applied at six rates (0, 23.3, 70, 210, 630 and 1890 g ha-1) to a montmorillonitic clay soil. Rate of degradation of the herbicide and its vertical movement in the soil profile were determined by bioassays of field samples
taken at four times (3.7, 7.4, 16.8 and 28.4 months) after application. Wheat, lucerne and soybeans were sown at various times following application to determine the effect of phytotoxic residues on the growth of these crops. At all four sampling times the zone of highest concentration of picloram was in the surface 15 cm of soil. Picloram was not detected below 30 cm. At selective weed control rates of 23.3 g ha-1 and 70 g ha-1 less than 10% of the applied picloram was present in thesoil 7.4 months after application. At the latter rates, the growth of neither wheat nor lucerne was affected when planted 7.4 months after application.