Elevated Atmospheric Carbon Dioxide and Weed Populations in Glyphosate Treated Soybean

Although rising atmospheric carbon dioxide (CO2) is known to stimulate the growth of agronomic weeds, the impact of increasing CO2 on herbicide efficacy has not been elucidated for field‐grown crops. Genetically modified soybean [Glycine max (L.) Merr.] (i.e., Round‐up Ready soybean) was grown over a 2‐yr period at ambient and projected levels of atmospheric carbon dioxide (CO2, 250 μmol mol−1 above ambient), with and without application of the herbicide, glyphosate [N‐(phosphonomethyl)glycine], to assess the impact of rising atmospheric carbon dioxide concentration [CO2] on chemical efficacy of weed control. For both years, soybean showed a significant vegetative response to elevated [CO2], but no consistent effect on seed yield. For 2003, weed populations for all treatments consisted entirely of C4 grasses, with no [CO2] effects on weed biomass (unsprayed plots) or glyphosate efficacy (sprayed plots). However, in 2004, weed populations were mixed and included C3 and C4 broadleaves as well as C4 grasses. In this same year, a significant increase in both C3 broadleaf populations and total weed biomass was observed as a function of [CO2] (unsprayed plots). In addition, a [CO2] by glyphosate interaction was observed with significant C3 broadleaf weed biomass remaining after glyphosate application. Overall, these data emphasize the potential consequences for CO2–induced changes in weed populations, biomass, and subsequent glyphosate efficacy in Round‐up Ready soybean.