Influence of soybean population on the efficacy of glyphosate
Studies were conducted at three Iowa locations in 2005 and 2006 to test if a reduction in soybean seeding rate would reduce weed suppression and increase glyphosate efficacy. The cultivar H-2162 RR was planted at four seeding rates (240,000, 300,000, 360,000, and 420,000 seed ha-1) for both experiments. In the first experiment, treatments included glyphosate applied at V2, V4, V6, and V2+V6 soybean stage at Boone, Hancock and Story. Weed control efficacy was influenced more by application timing than by soybean seeding rate, while grain yield was influenced more by soybean population than application timing. Weed control was almost 100% at Boone and Story, while at Hancock delaying application until V6 weed control was only 88%. At the highest seeding rate compared to the lowest seeding rate at Boone, Hancock, and Story soybean yield increased 11, 20, and 11% respectively. In the second experiment, treatments included application of two glyphosate rates 0.5 and 1.0 kg ae ha-1 when common lambsquarters was 10 cm tall and soybean was at V6 stage. Herbicide interception was affected by plant population, whereas weed control was not affected. Common lambsquarters control was 100% at 21 DAA for both glyphosate rates. These results suggest that soybean population influenced grain yield under favorable or drought conditions. Soybean population also did not affect the efficacy of glyphosate. At places with high weed densities no single glyphosate application guarantees high grain yield. Under these conditions a second glyphosate application may be needed to protect yields.