Dwarf sorghum (C4) was grown at ambient and at projected levels of atmospheric carbon dioxide (250 mol mol−1 above ambient) with and without the presence of a C3 weed (velvetleaf) and a C4 weed (redroot pigweed), to quantify the potential effect of rising atmospheric carbon dioxide concentration [CO2] on weed–crop interactions and potential crop loss. In a weed-free environment, increased [CO2] resulted in a significant increase in leaf weight and leaf area of sorghum but no significant effect on seed yield or total aboveground biomass relative to the ambient CO2 condition. At ambient [CO2] the presence of velvetleaf had no significant effect on either sorghum seed yield or total aboveground biomass; however, at elevated [CO2], yield and biomass losses were significant. The additional loss in sorghum yield and biomass was associated with a significant (threefold) increase in velvetleaf biomass in response to increasing [CO2]. Redroot pigweed at ambient [CO2] resulted in significant losses in total aboveground biomass of sorghum but not in seed yield. However, as [CO2] increased, significant losses in both sorghum seed yield and total biomass were observed for sorghum–redroot pigweed competition. Increased [CO2] was not associated with a significant increase in redroot pigweed biomass (P = 0.17). These results indicate potentially greater yield loss in a widely grown C4 crop from weedy competition as atmospheric [CO2] increases.
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