Shifts in Weed Problems Following Adoption of Roundup Ready Technology in Continuous Soybean Cropping Systems

Prince Czarnecki, J. M., Shaw, D. R., Farno, L. A., Givens, W. A., Gerard, P. D., Wilcut, J. W., Young, B. G., Wilson, R. G., Owen, M. D. K., & Weller, S. C. (2007). Shifts in Weed Problems Following Adoption of Roundup Ready Technology in Continuous Soybean Cropping Systems. Proceedings Southern Weed Science Society. Nashville, TN.

A multi-state effort is being conducted to assess the long-term viability of Roundup Ready technology as a foundation for cropping systems. The specific research objectives of this study are to 1) determine the practices used by growers when adopting glyphosate-based cropping systems; 2) evaluate the sustainability and profitability of the continued use of glyphosate-based cropping systems; 3) assess the risks of glyphosate-based cropping systems for the evaluation of glyphosate-resistant weed populations and weed community shifts; and 4) discover alternative tactics that can be used to support the glyphosate-based cropping systems as a foundation for managing herbicide resistance and weed shifts. Survey questions covered the severity of weed pressure before and after adoption, which specific weeds were problems before and after adoption, and what the biggest challenges were in this system. Results represent the compilation of survey data regarding weed pressure shifts in soybean systems from four states – Illinois, Indiana, Mississippi, and North Carolina. Changes in weed pressure were evident after the adoption of a continuous Roundup Ready soybean system overall, and within each state. On a scale of 1 to 10, most respondents reported decreases in pressure, with a range of 5-7 prior to adoption and a range of 2-5 after adoption. Level of pressure from specific weeds was surveyed as well. There were obvious regionalisms encountered when examined specific species. Ipomoea spp. affected only southern states, while Ambrosia and Setaria spp. affected only Midwestern states. Tall waterhemp (Amaranthus tuberculatos) was listed as problematic only in Illinois. Significant differences in weed pressures were seen between current weed pressures, and weed pressures prior to the adoption of a continuous Roundup Ready soybean system overall and for individual states. Prior to implementation, on a scale of 1 to 10, 26% felt that pressure was a 5 out of 10, and 19% felt that pressure was a 7 out of 10. After adoption 23% felt that pressure was a 2 out of 10, while a close 22% felt that pressure was a 5 out of 10. On an individual state basis, all states saw a decline in pressure following adoption. The majority of values for all states in the current situation were 5 out of 10 or less. The most pronounced weed shifts were seen for johnsongrass and common cocklebur, which decreased substantially following adoption. On an individual state basis, there are regional differences in weed shifts. In Illinois, pressure from Setaria spp. and Ambrosia spp. has remained constant, while pressure from common cocklebur has decreased and pressure from tall waterhemp (Amaranthus tuberculatos ) has increased. In Indiana, pressure from Ambrosia spp. has remained consistent, but pressure from Setaria spp. has declined significantly. In both Mississippi and North Carolina, pressure from Ipomoea spp. remains relatively constant. Pressure from common cocklebur and johnsongrass has declined in Mississippi, while pressure from prickly sida has remained virtually unchanged. North Carolina has also seen a decline in pressure from common cocklebur, but increased pressure from grasses in general. Pressure from Amaranthus spp. and sicklepod has remained constant. The biggest challenge survey participants identified was Ipomoea spp. (20%). The next highest response (8%) was prickly sida. All other responses were 4% or less. For individual states, results varied. In Illinois the largest percentage of respondents felt that tall waterhemp was the biggest challenge (14%). Ambrosia spp. (12%) was identified in Indiana as the biggest challenge. Both Mississippi and North Carolina chose Ipomoea spp. (23% for both states), with North Carolina additionally identifying sicklepod (14%) as a challenge. The range of answers for the question included not only weeds, but also management issues. However, the percentages for any of these answers did not generally exceed 2% overall or for individual states.