Glyphosate-resistant soybean at 494,000 seeds/ha was drill seeded with conventional soybean at 0 to 1,976,000 seeds/ha, and conventional soybean controlled at 2, 4, or 6 wk after soybean emergence with glyphosate at 0.84 kg ae/ha. Weed control increased exponentially with soybean groundcover prior to treatment with glyphosate and was maximized at 69 and 84% for Palmer amaranth and Florida pusley, respectively. When glyphosate was applied 2 wk after soybean emergence, yields were improved 390 kg/ha at 494,000 glyphosate-resistant soybean plus 247,000 conventional soybean seeds/ha compared with glyphosate-resistant soybean alone. Further increases in seeding rate of the conventional soybean often had a negative effect on soybean yields due to interference between glyphosate-resistant and conventional soybean prior to applying glyphosate. Glyphosate applied 6 wk after soybean emergence was generally less effective in controlling Palmer amaranth and Florida pusley than at earlier timings because of weed size at application and the overlying soybean canopy. This research indicates the mix-seeded soybean system may suppress early-season weeds, but glyphosate needs to be applied no later than 4 wk after soybean emergence to ensure optimum yields and prevent interference between glyphosate-resistant and conventional soybean.

Field studies were conducted in 1997 and 1999 to determine the effect of a broadcast-seeded glyphosate-tolerant/conventional soybean seed mix on soybean seed yield, gross profit margin from weed management, and early-season weed suppression prior to a single glyphosate application. Tests were established by broadcasting and incorporating 494,000 seeds/ha glyphosate-tolerant soybean in combination with 0 to 1,976,000 seeds/ha conventional soybean. Glyphosate at 1.12 kg ai/ha was applied at the V5 to V6 soybean growth stage to kill all conventional soybean plants and emerged weeds. The broadcast planting technique was effective in establishing a soybean stand on a silt loam but was ineffective on a silty clay soil. Hemp sesbania and pitted morningglory control, resulting from interference with soybean, were correlated with percent soybean groundcover (R 2 = 0.961 to 0.971) and population (R 2 = 0.821 to 0.860) at the V5 to V6 soybean growth stage. Although not significant, increased yield and gross profit margin of dryland soybean occurred as the total seeding rate of the seed mix increased from 494,000 to 741,000 seeds/ha. A dryland seeding rate of 741,000 compared to 494,000 seeds/ha resulted in 438 kg/ha and $93.78/ha increase in seed yield and gross profit margin, respectively. Interference between the two soybean cultivars at 1,976,000 seeds/ha reduced yield. Irrigated soybean at 988,000 seeds/ha produced the greatest seed yield and gross profit margin at 2,987 kg/ha and $555.86/ha, respectively. Maximum yields were attributed to early-season weed suppression.