Elevated carbon dioxide alters chemical management of Canada thistle in no-till soybean

Differential responses of crops and weeds to anthropogenic climatic change may alter competition and crop yields. Here we examine the role of current and projected increases in carbon dioxide concentration [CO2], on soybean growth and seed yield with and without competition from Canada thistle (Cirsium arvense, a common perennial weed in soybean farming systems), over a 3-year period using no-tillage (i.e., no physical cultivation for weed removal) practices. Weed control was implemented by applying herbicide (glyphosate) as a pre-emergent treatment at the beginning of each growing season. Under a weed-free condition, round-up ready soybean demonstrated a significant response of seed yield and total above-ground biomass to elevated [CO2], but no synergistic effect of no-till over time on the response of biomass or yield to [CO2] was observed. Average above-ground weight of Canada thistle was significantly greater at elevated [CO2] for 2008 and 2009, and establishment of thistle increased as a function of [CO2] over time even with pre-emergent applications of glyphosate. Although the presence of Canada thistle reduced seed yield and biomass of soybean for both CO2 treatments from 2007 to 2009, the reduction was higher for the elevated [CO2] treatment, and a significant [CO2] × Canada thistle interaction was observed for these parameters. Overall, these are the first data to indicate that perennial weeds associated with no-tillage practices could be a greater impediment to crop yields and harder to control chemically in response to rising levels of atmospheric carbon dioxide.