The potential impact of global warming and the enhanced atmospheric CO2 concentration on grassland management on dairy farms within the UK requires assessment. This has led to the development of a mathematical model of the grazing dairy cow. The model, that embraces grass and grass white clover swards, has been used to assess the effects that the projected increases in temperature and rainfall under global warming and the increased levels of CO2 might have on milk production and on silage conservation for a typical dairy farm. The results suggest that the impact on milk production for grass- based systems will vary depending on the locality. On the other hand, for herds grazed on grass white clover swards milk output might increase regardless of site, when the concentration of CO2 is enhanced. As regards silage production from grass white clover swards, under global warming and at current levels of CO2 there is an apparent tendency to increase the percentage of total silage yield obtained from the first cut, although this does not occur for grass swards. At the same time, there are also indications that global warming will increase the percentage of clover in the herbage cut for conservation.