Effect of glyphosate on common cocklebur, velvetleaf, and common waterhemp seed production, viability, germination, and dormancy
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Environmental and economic factors have caused growers to increase the use of reduced glyphosate rates and alternate application timings. Field experiments were conducted at Ames, Iowa in two locations in 2002 and one location in 2003 to evaluate the effects of glyphosate application timings and rates in soybeans. Four application timings were used (EPOST, MPOST, LPOST, FLOWER) along with five glyphosate rates (0, 143, 285, 570, 1140 g ai/ha) in a randomized complete block split-plot design. The seed of weeds surviving treatments were collected and analyzed to determine glyphosate effects on seed production, viability, germination, and dormancy. Glyphosate rates had a significant effect on soybean yield, weed control, and weed growth while application timing effects were inconsistent across all variables. Glyphosate rates from 285 to 1140g ai/ha provided equivalent soybean yields. The highest soybean yields were obtained with the MPOST and LPOST application timings. Common cocklebur and common waterhemp growth was consistently decreased and mortality consistently increased with rates from 285 to 1140 g ai/ha. Season-long efficacy was equal with glyphosate rates from 285 to 1140 g ai/ha in common cocklebur. Glyphosate reduced common cocklebur, velvetleaf, and common waterhemp seed production. Common cocklebur viability was decreased by 10 to 20% while common waterhemp viability was decreased by 16% with glyphosate rates of 143 g ai/ha. Common cocklebur and common waterhemp germination and dormancy, as a percent of viable seed, were not affected by glyphosate. Glyphosate reduced velvetleaf germination 69 to 79% with all rates in one location. At another location glyphosate rates of 143 and 285 g ai/ha reduced velvetleaf germination by 56% to 32% respectively. Velvetleaf dormancy was increased at both locations. Velvetleaf viability was not affected by glyphosate. The most consistent effect of glyphosate on common cocklebur, velvetleaf, and common waterhemp was a reduction in seed production. Velvetleaf control was better with early glyphosate application timings while common cocklebur and common waterhemp control was better with later glyphosate applications.