SUBSURFACE MOBILITY OF METRIBUZIN AS AFFECTED BY CROP RESIDUE PLACEMENT AND TILLAGE METHOD

The role of crop residues as a sorptive layer for herbicides applied in conservation tillage systems is not well understood. The organic mulch may retain herbicides and decrease chemical mobility and off-site transport. Field and laboratory experiments were conducted to determine the effects of surface placement of crop residues on herbicide mobility in Norge silt loam (fine-silty, mixed, mesic Udic Paleustoll). In the field, four treatments were established that included no-till plots with all crop residues removed (NTRR), residues left standing (NTRS), residues cut and laid Bat on ground (NTRF), and plots that were moldboard plowed to incorporate residues (MT). An application of 1.15 kg ha(-1) of metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazine-5(4H)-one] and 50 kg ha(-1) of potassium bromide was made. Soil cores were taken to 1-m depth following an initial sprinkler irrigation and natural rainfalls. Wheat residues intercepted metribuzin and attenuated subsurface movement in NT soils. Standing crop residues or residues placed Bat on the soil surface resulted in more metribuzin in the near-surface zone; centers of mass of concentration distributions in NTRS and NTRF soils showed that metribuzin moved only 47 to 62% of the distance observed in MT soil after 45 and 175 mm of precipitation, respectively. Addition of straw matter to soil and elevated organic C concentrations in the near-surface zone of no-till soils resulted in two- to fivefold increase in metribuzin retention. Therefore, placement geometry of crop residues and the retention capacity of the near-surface zone could be managed to attenuate metribuzin mobility in the field to achieve optimal herbicidal functions and dissipation.