Weed community responses to cropping system factors in glyphosate-resistant corn and soybean

Research was conducted from 1998 through 2005 to determine the effects of crop sequence, tillage system, and glyphosate use intensity on weed community composition and management risks in glyphosate-resistant corn and soybean. The most rapid change in weed plant community composition occurred during the first 4 years in treatments that included a non-glyphosate-component, and was associated with the relatively low efficacy of specific herbicides on giant ragweed and shattercane. In contrast, changes in weed plant community composition in glyphosate-based treatments developed more slowly over time, becoming apparent after 5 to 6 years, and were associated with weed emergence patterns that extended beyond glyphosate postemergence timings; such patterns were most apparent for giant foxtail and giant ragweed. Results suggested that glyphosate-based weed management was not associated with a decline in weed species number and diversity over time relative to non-glyphosate weed management, as has been predicted previously. Increases in giant ragweed and shattercane densities over time were associated with chisel plow and no-tillage systems, respectively; in contrast, weed densities were relatively low over time in moldboard plow, and were associated with a relatively low risk of corn and soybean yield loss. Crop yield loss associated with giant ragweed and shattercane typically occurred in treatments that included a non-glyphosate component; however, increased densities of both species and associated yield loss occurred by 2005 in treatments that included glyphosate applied once postemergence. Treatments that included glyphosate postemergence followed by cultivation or glyphosate late postemergence provided the lowest weed management and economic risks over time. Soybean-corn rotation was typically associated with fewer instances of high levels of crop yield loss than continuous corn. The lowest probabilities of common lambsquarters, redroot pigweed, giant foxtail, and velvetleaf resistance to glyphosate were typically associated with the integration of a soil-residual herbicide applied prememergence and glyphosate postemergence or the annual rotation of glyphosate with non-glyphosate herbicides. In contrast, the lowest probabilities for giant ragweed resistance were associated with glyphosate applied sequentially or glyphosate integrated with interrow cultivation, suggesting that such tactics were important for effective management of giant ragweed densities over time.