Leaching of clopyralid and metamitron under conventional and reduced tillage systems

The influence of conventional tillage (CT) and reduced tillage (RT) on leac hing of clopyralid (3,6-dichloropyridine-2-carboxylic acid) and metamitron (4-amino-4,5-dihydro-3-methyl-6-phenyl-1,2,4-triazin-5-one) was monitored i n a laboratory experiment with undisturbed soil columns from two plots wher e CT or RT practices have been used. Herbicides were applied at the rate of 5 kg ha(-1) to duplicate water-saturated columns that were leached with 60 0, 1200, or 1800 mL or 4000 mL of water. Clopyralid leaches more rapidly th an metamitron due to its lower sorption. Higher amounts of clopyralid were detected in leachates from RT columns than in leachates from CT columns. Br eakthrough of clopyralid occurred earlier in RT columns due to preferential now, although the total amounts leached (78%) were lower than in CT column s (94%). Residual clopyralid in soil was only slightly lower in RT than in CT. Metamitron rendered similar BTCs in RT and CT although the total recove ry in leachates were higher in CT columns (10%) than in RT columns (5%), Me tamitron residues in soil were much lower in RT than in CT columns. Additio nal experiments with handpacked soil columns showed no differences in RT an d CT for clopyralid, but higher leaching of metamitron in CT (16%) than in RT (8%). The lower recoveries (soil residue and leachates) observed for bot h herbicides under RT has been attributed to more rapid degradation in this system. These differences were much more pronounced for metamitron due to higher sorption and degradability in soil.