Adjusting management practices using glyphosate-resistant soybean cultivars

Glyphosate [N-(phosphonomethyl)glycine]-resistant soybean [Glycine max (L.) Merr.] cultivars have increased drastically in usage and acceptance. Little information exists to see how glyphosate-resistant soybean cultivars should be managed. The objective of this study was to evaluate different row-spacing and plant population systems using three weed management systems. A field study was conducted from 1997 through 1999 at six locations in Wisconsin. Soybean was planted in 19-, 38-, and 76-cm rows at a recommended (optimum), low, and high plant population for each row-spacing system with three weed management systems [glyphosate-resistant soybean cultivars with glyphosate (GRS/G), glyphosate-resistant soybean cultivars with conventional herbicides (GRS/CN), and conventional soybean cultivars with conventional herbicides (CN/CN)]. In northern Wisconsin, soybean yield in a GRS/G system did not respond to plant population while GRS/CN and CN/CN systems yielded 6% more in high than in low plant population. Additionally, soybean yield responded positively to plant population in 76-cm row CN/CN and GRS/CN systems in northern Wisconsin. In southern Wisconsin, GRS/G and GRS/CN systems yielded 6% less than the CN/CN system. No differences were observed among weed management systems in central and northern Wisconsin. Averaged across weed management systems and plant population, 19- and 38-cm rows yielded 7, 9, and 10% more than 76-cm rows in southern, central, and northern Wisconsin, respectively. No yield differences were observed between optimum and high plant population across Wisconsin, averaging 4% greater yield than the low plant population. The results demonstrated that it might be beneficial to alter management practices when using glyphosate-resistant soybean in some production environments in Wisconsin.