Deep and Shallow Fall Tillage for Irrigated Soybean Grown with Different Weed Management Systems in the Midsouthern USA

Abstract

Management inputs that maximize economic return from early soybean [Glycine max (L.) Merr.] production system (ESPS) plantings in the midsouthern USA have not been evaluated fully. The objective was to determine the effect of different weed management systems on yield and net return from irrigated ESPS plantings of soybean following deep (DT; 40–45 cm deep) and shallow (ST; <10 cm deep) fall tillage. Adjacent experiments receiving either DT or ST were conducted in 1999 and 2000 on Sharkey clay (very‐fine, smectitic, thermic Chromic Epiaquert) near Stoneville, MS (33°26′ N lat). Weed management systems were (i) glyphosate‐resistant (GR) cultivar with pre‐emergent (PRE) nonglyphosate [N‐(phosphonomethyl)glycine] herbicides followed by one postemergent (POST) application of glyphosate timed to control grasses, (ii) GR cultivar with two POST applications of glyphosate timed to control both grasses and broadleaves, (iii) GR cultivar with PRE nonglyphosate herbicides followed by two POST applications of glyphosate timed to control both grasses and broadleaves, (iv) non‐GR cultivar with PRE herbicides followed by one POST application of a grass herbicide, (v) non‐GR cultivar with POST application of herbicides timed to control both grasses and broadleaves, and (vi) non‐GR cultivar with PRE herbicides followed by POST applications of herbicides timed to control both grasses and broadleaves. Fall DT was more expensive than ST but resulted in taller soybean plants and less weed cover at harvest. Average yields and net returns from DT and ST were 4286 and 4085 kg ha−1 and $364 and $362 ha−1, respectively. Thus, the investment in equipment for fall DT for irrigated ESPS plantings is not justified. Postemergent‐only weed management was the cheapest for both GR and non‐GR cultivars. Weed management that used POST‐only glyphosate resulted in the greatest yield and profit from GR cultivars. With non‐GR cultivars, yields were not affected by weed management, but net returns were lower when the most intensive weed management was practiced.