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In order to assess the effects of future elevated atmospheric CO2 concentrations on yield, mineral content and the nutritive value of mixed swards of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.), both species were grown as monocultures and as different mixtures and were exposed season-long to ambient (380 p.p.m.) and elevated (670 p.p.m.) CO2 concentrations in open-top chambers. Mini-swards were cut four times at about monthly intervals at a height of 5 cm, dry-matter yields were determined and content of macroelements (N, P, K, S, Mg, Ca, Na) and crude fibre, crude protein and ash content were measured. The CO2-related increase in seasonal yield amounted to 16–38% for white clover monocultures, 12–29% for mixed swards and 5–9% for ryegrass monocultures. The white clover content of all swards was significantly enhanced by elevated CO2. The K and Na content of total yield was decreased by high CO2 but did not fall below the minimum requirements for ruminants. As the Ca content of total yield was increased by elevated CO2 and the P content was not changed, the Ca/P ratio of total yield was increased and exceeded values required for animal nutrition. The crude protein content of total yield was reduced by high CO2 at the beginning of the growing season only and was increased by elevated CO2 in the course of the experiment, whereas crude fibre content was decreased throughout the season, sometimes falling below the minimum requirement for ruminants. Removal of N, P, S, Mg and Ca by cutting was significantly enhanced because of CO2 enrichment. The results show that, besides the positive effect of rising atmospheric CO2 on dry-matter yield of white clover/ryegrass swards, impacts on the nutritive value should be expected. Possible changes in species composition and implications for grassland management are briefly discussed.