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.
Nomenclature: Glyphosate; Palmer amaranth, Amaranthus palmeri S. Wats. #3 AMAPA; Florida pusley, Richardia scabra L. # RCHSC; soybean, Glycine max (L.) Merr.
Additional index words: Application timing, integrated weed management, soybean groundcover, transgenic crop.
Abbreviation: WAE, weeks after soybean emergence.