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CROP ECOLOGY, MANAGEMENT & QUALITY

Elevated Atmospheric Carbon Dioxide and Weed Populations in Glyphosate Treated Soybean

Crop Systems and Global Change Laboratory, USDA, Agricultural Research Service, Building 1, Room 323, 10300 Baltimore Avenue, Beltsville, MD 20705

^* Corresponding author (lziska{at}asrr.arsusda.gov)

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.

Abbreviations: AHI, apparent harvest index • ai, active ingredient • DAS, days after sowing