The effect of weed management systems and location on arable weed species communities in glyphosate-resistant cropping systems.

Question How do different weed management systems based on glyphosate-resistant ( GR) crops affect weed community structure and composition? Location A total of 156 agricultural fields across six US states. Methods Fields were selected and categorized into three weed management systems based on a 3-yr 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. Weed species population density was surveyed in 20 0.5-m2 plots per field four times each season from 2006 to 2010. Weed population density, species richness and diversity were analysed using repeated measures mixed models to test the effect of year, geographic location (based on plant hardiness zone) and weed management system (as described above). Weed community composition was analysed using NMDS ordination. The importance of geographic location, planted crop and weed management system were analysed using analysis of similarity and permutational ANOVA based on the Bray- Curtis similarity among sites calculated from the maximum weed species population density per site per year. Results A total of 329 weed species were identified across the six states and all time periods. Weed communities were most strongly correlated with geographic location and secondarily to the crop planted. Weed management system affected similarity among weed communities through an interaction with site location but not every year. The weed management systems of crop rotation and GR trait rotation generally reduced weed population density and species diversity, but the effect of crop rotation varied by geographic location. Conclusions The overall effects of weed management systems in fields including the GR trait on arable weed communities reinforce the need for locally adapted weed management programmes to steward the GR crop trait to manage for high weed diversity while reducing crop-weed competition and maximizing yield. [ABSTRACT FROM AUTHOR]