Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/2004
Publication Date: 3/15/2005
Citation: Heatherly, L.G., Reddy, K.N., Spurlock, S.R. 2005. Weed management in glyphosate-resistant and non-resistant soybean grown continuously and in rotation. Agronomy Journal. 97:568-577. Interpretive Summary: There is concern in the soybean industry that continuous use of postemergent-applied glyphosate to control weeds in glyphosate-resistant varieties will lead to weed resistance to glyphosate. If this occurs, alternative weed managment strategies such as rotation of glyphosate-resistant and non-resistant varieties must be used. In four years of nonirrigated studies conducted on Tunica clay soil near Stoneville, MS, using glyphosate-resistant varieties resulted in net returns that were equal to or greater than those resulting from using non-resistant varieties. Using preemergent plus postemergent vs. using postemergent-only weed management was not necessary for achieving highest yield or net return from either glyphosate-resistant or non-resistant varieties. Rotating glyphosate-resistant and non-resistant varieties had no effect on soybean seed yield. In the fourth and final year of the study, rotating glyphosate-resistant and non-resistant varieties significantly affected net return only when preemergent plus postemergent weed management was used with later-maturing (maturity group V) varieties. When early-maturing (maturity group IV) varieties were used, rotating glyphosate-resistant and non-resistant varieties did not affect net returns. These results indicate that a system that utilizes glyphosate-resistant and non-resistant varieties grown in rotation in the midsouthern USA can be used without concern for lower yield or net return. Thus, if weed resistance to glyphosate develops, rotation of glyphosate-resistant and non-resistant varieties is a feasible production option.
Technical Abstract: Management inputs that maximize economic return from the Early Soybean [Glycine max (L.) Merr.] Production System (ESPS) have not been evaluated fully. Field studies were conducted to determine the effect of rotating glyphosate-resistant (GR) and non-GR maturity group (MG) IV and MG V soybean cultivars on weed populations in and soybean seed yield and net return from nonirrigated plantings on Tunica silty clay (clayey over loamy, smectitic, nonacid, thermic, Vertic Haplaquept) near Stoneville, MS (33 26'N lat.). Eight management systems, each containing a MG IV or MG V GR or non-GR cultivar grown continuously or in rotation with each other, and two weed management treatments [preemergent followed by postemergent weed management (PRE + POST) and postemergent-only weed management (POST)], were grown each year. Use of GR cultivars and POST-only glyphosate was cheaper in all years. Maturity group affected yield and net return, but this resulted from weather differences during reproductive development. Rotating GR and non-GR cultivars had no consistent affect on weed populations and no significant affect on yield or net return in this 4-yr study. Using GR cultivars resulted in net returns that were greater than or equal to those from non-GR cultivars. These results indicate that a system that utilizes GR and non-GR cultivars in this region can be used without concern for rotating them. This conclusion is made without considering selection pressure for weed resistance to glyphosate that may develop with its sole use in POST-only weed management systems for GR cultivars.