Weed management in glyphosate-resistant and non-glyphosate-resistant soybean grown continuously and in rotation

Management inputs that maximize economic return from the early soybean [Glycine mar (L.) Merr.] production system have not been evaluated fully. Field studies were conducted near Stoneville, MS (33 degrees 26' N lat.), to determine the effect of rotating maturity group (MG) IV and V glyphosate [N-(phosphonomethyl)glycine]-resistant (GR) and non-GR cultivars on weed populations, soybean seed yields, and net returns from nonirrigated plantings. 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 [pre-emergent followed by postemergent weed management (PRE + POST) and postemergent-only weed management (POST)] were grown each year. Glyphosate-resistant cultivars using POST-only glyphosate was the most economical system each year. Maturity group effect on yield and net return resulted from weather differences during reproductive development. Rotating GR and non-GR cultivars had no consistent effect on weed populations and no significant effect on yield or net return in this 4-yr study. Using GR cultivars resulted in net returns that were greater than those from non-GR cultivars. These results indicate that production systems using either GR or non-GR cultivars grown continuously or in rotation with each other in this region can be utilized effectively with no effect on weed population shifts or reductions in seed yield and net return.