Annual weed control by glyphosate in glyphosate-resistant soybean (Glycinemax)

Control of natural infestations of common lambsquarters and giant foxtail i n 1993, 1994, and 1995 and of velvetleaf in 1994 and 1995 by postemergence application of glyphosate to glyphosate-resistant soybean planted in narrow (20 cm) and wide (76 cm) rows was evaluated. Planting glyphosate-resistant soybean in narrow rows and applying reduced rates of glyphosate when commo n lambsquarters, giant foxtail, and velvetleaf were at their actively growi ng stage 3 to 18 cm, 5 to 28 cm, and 3 to 20 cm tall, respectively, resulte d in > 90% control. The effect of time of herbicide application was greater than the rate of herbicide application, especially within the wide-row soy bean plantings. Applying imazethapyr in combination with glyphosate did not improve weed control or soybean yield compared with glyphosate alone.