Technical Abstract: The continuing rise in atmospheric carbon dioxide (CO2) concentration may increase plant growth relatively more than the uptake of soil-derived chemical elements that are essential for herbivores. Rising CO2 also may alter the nutritional quality of forage or fodder derived from multi-species vegetation by changing plant species abundances or composition. We measured concentrations of 11 nutritionally important elements in above-ground tissues of three C4 grasses that had been exposed for two growing seasons to a continuous gradient in CO2 from 250 to 500 umol mol-1. The three grasses, Bouteloua curtipendula, Schizachyrium scoparium, and Sorghastrum nutans, are competitive dominants in assemblages of tallgrass prairie vegetation growing on each of three soil types along a field CO2 gradient located in central Texas, USA. Our objective was to determine whether CO2 effects on levels of chemical elements in grass mixtures result mainly from changes in element concentrations of individual species or from shifts in species abundances. Increased CO2 had little effect on element concentrations in C4 grasses compared to differences observed among the three grass species and three soil types. Increasing CO2 from the pre-Industrial to near the current and to elevated levels reduced the phosphorus concentration in grasses grown on a clay soil and a sandy loam soil, respectively. Concentrations of most other elements either did not respond to CO2 treatment or increased at higher CO2. Cover of the mid-grass Bouteloua declined at higher CO2 levels as cover of the taller grass Sorghastrum increased. Concentrations of several elements were lower in Bouteloua than Sorghastrum, hence this exchange of species as CO2 increased improved the overall nutritional status of grasses. Several of the elements for which concentrations were smallest in Bouteloua, including P, Cu, and Se, occurred at levels that are inadequate to support the growth of beef cattle. By favoring one grass species over another, the pre-Industrial to elevated rise in CO2 increased concentrations of several elements important to herbivore nutrition. Results highlight the under-appreciated impact that CO2 enrichment may have on ecosystem services by changing plant composition.