Post-Applied Residual Herbicides in an Early-Planted Glyphosate Resistant Soybean System

Prince Czarnecki, J. M., Shaw, D. R., Farno, L. A., Poston, D. H., Koger, C. H., Eubank, T. W., & Gray, C. J. (2006). Post-Applied Residual Herbicides in an Early-Planted Glyphosate Resistant Soybean System. Proceedings Southern Weed Science Society. San Antonio, TX. 59, 28.

Field experiments were conducted during 2004 and 2005 at sites in Brooksville, Starkville, and Stoneville, MS, to evaluate the effects of residual grass herbicides applied mid-POST in tank-mixes with glyphosate on early-planted glyphosate-resistant soybean [Glycine max (L.) Merr.] at two row spacings for annual grass control. Glyphosate was also applied alone at early- or mid-POST, as well as sequential applications at these timings. It was thought that tank mixtures of glyphosate with metolachlor, flufenacet, dimethenamid, or pendimethalin might provide added residual control for subsequent weed flushes that would arise after the initial control from glyphosate. Treatments for each spacing varied by the application rate and application tank-mix. Weed control was evaluated at two and six weeks after mid-POST application, except in 2005 at Stoneville where a six-week rating was not taken. Results were pooled across location and year. At two weeks, the only significantly different treatment was a 0.69 kg/ha early-POST application of glyphosate with no tank-mix or M-POST treatment, which was not as effective as the other treatments. No differences in row spacing with regard to control were noted at this evaluation timing. At six weeks, all treatments containing a tank mixture were more effective than applications of glyphosate alone, but were not different from one other. A 0.69 kg/ha early-POST application of glyphosate was not different than the untreated check. At this evaluation timing weed control in the 38-cm row spacing was better than control in the 76-cm row spacing. At two weeks, the highest injury (~20%) resulted from applications of glyphosate tank-mixed with flufenacet in a mid-POST application. With the exception of 12% injury resulting from a 0.69 kg/ha glyphosate plus 0.64 kg/ha pendimethalin application, all other treatments were not different, with injury being < 7%. Soybean injury at six weeks was < 7%. Soybean yields in the 36-cm row spacing were higher than in the 76-cm row spacing. However, no significant differences in yield were seen for any chemical treatments, regardless of tank-mix or application timing.