The use of glyphosate-resistant (GR) cropping systems is increasing.  Improving our knowledge of the environmental effects of GR cropping systems will help us conserve biodiversity in these agroecosystems.  We compare weed diversity across cropping systems, while considering the risks of developing GR weed species within each cropping system.  Surveys of weed flora were conducted in 156 agricultural fields across six states, Illinois, Indiana, Iowa, Mississippi, Nebraska, and North Carolina.  Fields were selected and categorized into three cropping systems based on a three year history:  1) a single continuous GR crop, 2) a rotation of two GR crops, and 3) a GR crop rotated with a non-GR crop.  A total of 216 species were identified across all states and time periods. Species richness, evenness, and Shannon’s H’ diversity, were calculated for each site.  Eight weed species with known GR biotypes were present in the dataset.  The frequency of species with GR biotypes and diversity measures were analyzed in relation to environmental data, including geographical location categorized by USDA plant hardiness zone, crop type in 2006, crop rotation, and rotation of GR crop trait (continuous GR trait or rotation with a non-GR crop).    
Results/Conclusions

Data suggest patterns in weed diversity by cropping system across all USDA plant hardiness zones, with the highest diversity in the single continuous GR crop.  However, when cropping system was analyzed by its components, crop rotation and GR crop trait rotation, the effects of the rotation of the GR crop trait were dependent upon plant hardiness zone; Shannon’s H' was highest in zones four and six when the GR trait was continuously cropped.  The frequency of weed species with GR biotypes was found to vary by USDA plant hardiness zone and cropping system.  USDA plant hardiness zone six had the highest number of GR weed species in the single continuous GR cropping system, while other zones did not show consistent patterns across the three cropping systems.  Therefore, we conclude that the overall effects of cropping system on weed diversity are independent of geography, though GR trait rotation was geographically dependent for Shannon’s H'.  However, the occurrence of weed species with GR biotypes varies among individual plant hardiness zones, suggesting the need for geographically based management decisions.