The potential long-term impacts of glyphosate-resistant crops on biodiversity of the agricultural landscape have been the subject of controversy. A new article in the journal Weed Science describes a large-scale study analyzing the effects of glyphosate-resistant crops on the diversity of agricultural weeds. This study examined 156 field sites with at least a 3-year history of growing glyphosate-resistant crops in six states: Illinois, Indiana, Iowa, Mississippi, Nebraska, and North Carolina. Researchers analyzed the effects on weed communities of cropping system and crop rotation, including frequency of planting glyphosate-resistant crops.

In total, 139 weed species were identified across all sites. Three species were common to all states, 79 were unique to one state, and 46 were unique to a single site. Diversity of the weed flora and weed soil seedbank was more strongly influenced by geographic location and hardiness zone than by any other factor. The previous year's crops and cropping systems also affected weed community composition, but deployment of the glyphosate-resistant crop trait did not.

“A primary conclusion from this research,” author Bryan G. Young said, “is that the diversity of weed communities is not driven solely by the glyphosate-resistant crop trait. Rather, the overall crop production management system and geography in which the glyphosate-resistant crop is integrated will have a greater impact on the diversity of agricultural weeds in the soil seedbank.”

Full text of the article, “Seedbank and Field Emergence of Weeds in Glyphosate-Resistant Cropping Systems in the United States,” Weed Science, Vol. 63, No. 2, April-June 2015, is now available.