Field decay of wheat straw and its effects on metribuzin and S-ethyl metribuzin sorption and elution from crop residues.

Best agricultural management practices (BMP) are needed for minimizing the potential off-site transport of agricultural chemicals while retaining their chemical efficacy. Field and laboratory studies of herbicide-crop residue interactions in conservation tillage systems were conducted to determine weather-induced changes in wheat (Triticum aestivum L.) straw chemical composition, the capacity of plant residues to retain metribuzin 14-amino-6-(1,1 dimethyl-ethyl)-3-(methyl-thio)-1,2,4-triazin-5(4H)-one and its S-ethyl analog (S-ethyl metribuzin), and the changes in herbicide sorption characteristics of these residues as they decay in the field. At harvest, undecayed wheat straw exhibited Freundlich straw-water partition coefficients of 13.1 and 24.2 micromole kg-1/(micromole L-1)1/n, where 1/n = 0.78 and 0.60 at 20 degrees C for metribuzin and S-ethyl metribuzin, respectively. Lignin accounted for the majority of the sorption capacity of intact wheat straw. The purified cellulose fraction did not significantly sorb the herbicides. Increased sorption capacity of decaying straw was associated with a decline in cellulose concentration or conversely the lignin enrichment of the straw. Field decay reduced straw cellulose concentration at the rate of 1.56 X 10-1 d-1 for straw in partial contact with the ground, following grain harvest in 1983, and 3.97 X 10-4, and 4.39 X 10-4 d-1 for standing straw in 1985, and 1986, respectively. Observed asymmetrical breakthrough curves of both chemicals from packed straw columns also suggested a slow mechanism of release of the herbicides from crop residues. Wheat straw's affinity could reduce herbicidal efficacy of soil-activated chemicals if they were applied as a surface spray in conservation tillage systems. Although metribuzin and S-ethyl metribuzin are primarily soil-activated herbicides, they would still be effective for postemergence control of Bromus spp. in winter wheat due to foliar absorption and slow release from the straw mulch. By the same token, the straw mulch is a temporary storage medium to alter these herbicides' environmental dispersion pattern and may be a significant mechanism of retardation of agricultural chemical movement to the subsurface of lands under conservation tillage.