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.

Nomenclature: Dwarf sorghum, Sorghum bicolor cv. ‘Martin,’ redroot pigweed, Amaranthus retroflexus L. AMARE; sorghum, Sorghum bicolor L. Moench; velvetleaf, Abutilon theophrasti Medicus ABUTH.