Weed management in nonirrigated glyphosate-resistant and non-resistant soybean following deep and shallow fall tillage

Management inputs that maximize economic return from the early plantings of soybean Glycine max (L.) Merr. in the midsouthern USA have not been evaluated fully. The objective was to compare perennial weed control in and yields and economic returns from plantings of maturity group (MG) IV and V soybean cultivars grown in the field under different weed management systems (WMS) following shallow (ST) and deep (DT) fall tillage. Adjacent experiments were conducted on Tunica clay (clayey over loamy, smectitic, nonacid, thermic Vertic Haplaquept) near Stoneville, MS (lat. 33 degrees 26'N). Weed management systems were (i) glyphosate N-(phosphonomethyl)glycine-resistant (GR) cultivars with preemergent (PRE) nonglyphosate herbicides followed by postemergent (POST) glyphosate; (ii) GR cultivars with POST glyphosate; (iii) non-GR cultivars with PRE plus POST nonglyphosate herbicides; and (iv) non-GR cultivars with POST nonglyphosate herbicides. Control of perennial redvine Brunnichia ovata (Walt.) Shinners declined in the ST environment when non-GR cultivars were used, but this did not result in a yield decline. Control of perennial johnsongrass Sorghum halepense (L.) Pers. at the end of the study period averaged 93% when GR cultivars were used regardless of tillage treatment, and this was associated with lower yield. Use of PRE + POST vs. POST-only weed management sometimes resulted in lower profits regardless of fall tillage treatment. The fall tillage treatment x WMS interaction was not significant for yield or net return, which indicates that use of DT for perennial weed management is not economical.