DICAMBA MOBILITY IN CONVENTIONALLY TILLED AND NON-TILLED SOIL

Root zone leaching losses of dicamba (3,6-dichloro-2-methoxybenzoic acid) were evaluated over 5 years (1984-1988) under conventional tillage (CT) and no-tillage (NT) corn (Zea mays L.) management from postemergence application date in June of each year through December. Pan lysimeters embedded 1.2 m deep were used to collect root zone percolates. Dicamba losses in runoff water were evaluated through September. Land slope ranged from 3 to 5% within tillage areas. Dicamba was applied at the recommended rate (0.56 kg a.i. ha-1) on Hagerstown silty clay loam (Typic Hapludalf ). Leaching events, sample numbers and leachate volumes were greater under NT than CT management. In general, dicamba concentrations in leachates were greater under NT conditions, and dicamba residues were detected in leachates from both tillage systems as late as 6 months after application during several seasons. Entrapment and translocation within the micropore matrix of this well-drained soil were postulated as being critical factors involved in the atypical residence time and late season 'breakthrough' of this anionic, low persistence herbicide. Mean areal leaching losses ranged from 0 to 3.3 g ha-1 (CT) and 2.2 to 31.2 g ha-1 (NT), accounting for maximum losses of 0.6% (CT) and 5.7% (NT) of the amount applied. More runoff was recorded from the CT area and dicamba losses ranged from 0.01 to 4.6 g ha-1 (CT) and from less than 0.01 to 0.7 g ha-1 (NT). Maximum runoff losses were 0.8% and 0.1% of the rate applied for CT and NT, respectively. Yearly differences in the magnitude of losses by each transport process were strongly related to rainfall distribution and number of leaching and runoff events proximal to the spray date of dicamba.