Weed management systems for conventional and glyphosate-resistant soybean with and without irrigation

Management inputs that maximize economic return from the early soybean [Glycine mar (L.) Merr.] production system have not been evaluated fully. The objective was to determine the effect of weed management on yield and net return from early planted maturity group (MG) IV and MG V glyphosate [N-(phosphonomethyl)-glycine]-resistant (GR) and conventional (CONV) soybean cultivars grown in the early soybean production system with (IRR) and without (NI) irrigation. Field studies were conducted from 1996 through 1999 on Sharkey day (very-fine, smectitic, thermic Chromic Epiaquert) at Stoneville, MS (33degrees26' N lat). Weed management systems were (i) pre-emergent (PRE) broadleaf followed by postemergent (POST) broadleaf and grass weed management (PRE + POST) and (ii) POST broadleaf and grass weed management (POST). Use of POST-only weed management was cheaper, yielded more, and resulted in greater net returns than did use of PRE + POST weed management with both CONV and GR cultivars. Under the conditions of this study, use of GR vs. CONV cultivars in an NI or low-yield environment resulted in greater profit ($52 vs. -$17 ha(-1) 3-yr average, respectively). Use of CONV vs. GR cultivars resulted in greater profit in an IRR or high yield environment ($382 vs. $266 ha(-1) 3-yr average, respectively). These results indicate that use of GR cultivars with POST-only weed management will result in greater profit in an NI environment while use of CONV cultivars with POST-only weed management will result in greater profit in an IRR environment.