Incorporating Residual Herbicides into Early-planted Glyphosate-resistant Soybean Production

Prince Czarnecki, J. M., Shaw, D. R., Gray, C. J., Givens, W. A., & Poston, D. H. (2005). Incorporating Residual Herbicides into Early-planted Glyphosate-resistant Soybean Production. Proceedings of the Southern Weed Science Society. Charlotte, NC. 58.

Field experiments were conducted 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. 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. Results from Brooksville and Starkville were pooled when possible. In most instances, no significant differences were shown between row spacings. At two weeks, no differences were noted for any tank-mix treatments on johnsongrass [Sorghum halepense (L.) Pers.]. Control was 98% or better for any treatment other than early-POST application of glyphosate alone at 0.69 kg ae/ha. At six weeks, at least 75% control was observed for most treatments of glyphosate plus a residual grass herbicide. The best johnsongrass control was obtained with mid-POST application of glyphosate plus 0.69 kg ai/ha pendimethalin, or glyphosate plus 1.2 kg ai/ha metolachlor. Barynardgrass [Echinochloa crus-galli (L.) Beauv.] control at 2 weeks was 89% or more for most tank-mix treatments. At Starkville and Brooksville, the only applications that resulted in less control were those where glyphosate was applied alone (early and mid-POST at 0.69 kg ae/ha each time). At Stoneville, however, glyphosate applied at early and mid-POST controlled barnyardgrass at least 89%. At 6 weeks, row spacing and treatment interaction effects occurred at Brooksville and Starkville. Control was 70% or more for approximately half of the treatments. The highest control resulted from 38-cm row spacing with mid-POST application of glyphosate plus metolachlor at either 1.2 or 1.5 kg ai/ha. At Stoneville, the 38-cm row spacing was better than the 76-cm row spacing for suppressing barnyardgrass. Control was 84 % for all tank-mix treatments, with the best results coming again from mid-POST application of glyphosate with either rate of metolachlor. Soybean injury at the Brooksville and Starkville sites was as high as 32%, but most injury was below 8%. Injury at Stoneville was 16% at most, with most treatments resulting in 13% injury or less. The use of glyphosate at 0.69 kg ae/ha with flufenacet at 0.49 kg ai/ha or 0.64 kg ai/ha resulted in the highest amount of injury at both sites, with glyphosate alone at 0.69 kg ae/ha producing the lowest rates of injury. Yield differences were not significant at Brooksville and Starkville. Large populations of broadleaf weeds lowered yields. At Stoneville, the 38-cm row spacing produced higher yields than 76-cm row spacing. Yields were highest following glyphosate plus metolachlor at either rate, or glyphosate plus pendimethalin.