The use of glyphosate-resistant (GR) cropping systems is predominant in the US. Increasing our knowledge of the environmental effects of GR cropping systems will help us better understand how to conserve diversity in these agroecosystems. We compare weed communities across cropping systems following surveys from 2006 through 2010 of the weed flora 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 329 species were identified across all states and time periods. Weed communities were analyzed using Nonmetric Multidimensional Scaling ordination. The importance of environmental data, including geographical location, planted crop, and crop management system (crop rotation, and rotation of continuous GR traits or rotation with a non-GR crop) were analyzed using Analysis of Similarity and Permutational ANOVA on the Bray-Curtis resemblance site by species matrix based upon maximum weed species density per site per year.
Results/Conclusions
Similarity among weed communities across the six-state region was most strongly related to location and secondarily to the crop planted each year. Crop management system had a relationship to similarity among weed communities through an interaction with site location but not every year of the study. However, when management system was analyzed by crop rotation and GR crop trait rotation, the effects of the rotation of the GR crop trait were evident in two (MS and NC) of the six states in the study, whereas crop rotation affected weed communities in these and two additional states (IL and IN). These effects were independent of any interaction with time. Crop rotation and GR trait rotation generally reduced weed density and diversity, but the effect of crop rotation varied by location. Therefore, we conclude that crop management systems including the GR trait may affect weed communities reinforcing the need for locally-adapted weed management programs to successfully steward the GR crop trait to manage for high weed diversity while reducing crop-weed competition and maximizing yield.