Michael D. Owen

Using a Grower Survey to Assess The Benefits and Challenges of Glyphosate-Resistant Cropping Systems for Weed Management in U.S. Corn, Cotton, and Soybean

Abstract Over 175 growers in each of six states (Illinois, Indiana, Iowa, Mississippi, Nebraska, and North Carolina) were surveyed by telephone to assess their perceptions of the benefits of utilizing the glyphosate-resistant (GR) crop trait in corn, cotton, and soybean. The survey was also used to determine the weed management challenges growers were facing after using this trait for a minimum of 4 yr. This survey allowed the development of baseline information on how weed management and crop production practices have changed since the introduction of the trait. It provided useful information on common weed management issues that should be addressed through applied research and extension efforts. The survey also allowed an assessment of the perceived levels of concern among growers about glyphosate resistance in weeds and whether they believed they had experienced glyphosate resistance on their farms. Across the six states surveyed, producers reported 38, 97, and 96% of their corn, cotton, and soybean hectarage planted in a GR cultivar. The most widely adopted GR cropping system was a GR soybean/non-GR crop rotation system; second most common was a GR soybean/GR corn crop rotation system. The non-GR crop component varied widely, with the most common crops being non-GR corn or rice. A large range in farm size for the respondents was observed, with North Carolina having the smallest farms in all three crops. A large majority of corn and soybean growers reported using some type of crop rotation system, whereas very few cotton growers rotated out of cotton. Overall, rotations were much more common in Midwestern states than in Southern states. This is important information as weed scientists assist growers in developing and using best management practices to minimize the development of glyphosate resistance. Nomenclature: Glyphosate; corn, Zea mays L.; cotton, Gossipium hirsutum L.; rice, Oryza sativa L.; soybean, Glycine max (L.) Merr

Weed diversity and soybean yield with glyphosate management along a north–south transect in the United States

Abstract There are many concerns about the effects of repeated use of glyphosate in glyphosate-resistant (GR) crops, including two that are seemingly contradictory. These are (1) weed escapes and (2) loss of weed diversity. Weeds that escape glyphosate treatment represent species that likely will become troublesome and difficult to control in the future, and identifying these future problems may allow more effective management. In contrast, complete weed control directly reduces the weed component of agroecosystem biodiversity and may lower other components indirectly (e.g., weed-dependent granivores). During 2001 and 2002 effects of glyphosate and conventional weed control treatments on weed community composition and GR soybean yields were studied. Field studies were conducted along a north–south transect of sites spanning a distance of 1600 km from Minnesota to Louisiana. Low-intensity use (single application yr−1) of glyphosate allowed more escapes and maintained higher weed diversity than high-intensity use (two applications yr−1) of glyphosate, and it was equivalent to or even higher than diversity in non-GR systems. Although the same weeds escaped from low- and high-intensity glyphosate treatments, frequency of escapes was higher with less intensive use. These results suggest that limited use of glyphosate would not have profound effects on weed diversity. In addition, crop yield did not differ between GR and non-GR treatments at high latitudes, but below 40° N latitude, with a longer cropping season, yields with low-intensity glyphosate use decreased by about 2% per degree latitude because of competition from escaped weeds. Nomenclature: Soybean, Glycine max (L.) Merr.

Benchmark study on glyphosate-resistant crop systems in the United States. Economics of herbicide resistance management practices in a 5 year field-scale study

BACKGROUND Since the introduction of glyphosate-resistant (GR) crops, growers have often relied on glyphosate-only weed control programs. As a result, multiple weeds have evolved resistance to glyphosate. A 5 year study including 156 growers from Illinois, Iowa, Indiana, Nebraska, North Carolina and Mississippi in the United States was conducted to compare crop yields and net returns between grower standard weed management programs (SPs) and programs containing best management practices (BMPs) recommended by university weed scientists. The BMPs were designed to prevent or mitigate/manage evolved herbicide resistance. RESULTS Weed management costs were greater for the BMP approach in most situations, but crop yields often increased sufficiently for net returns similar to those of the less expensive SPs. This response was similar across all years, geographical regions, states, crops and tillage systems. CONCLUSIONS Herbicide use strategies that include a diversity of herbicide mechanisms of action will increase the long-term sustainability of glyphosate-based weed management strategies. Growers can adopt herbicide resistance BMPs with confidence that net returns will not be negatively affected in the short term and contribute to resistance management in the long term.