Recent developments in maize breeding and in agronomic practices, particularly the development of degradable full cover plastic mulch, have resulted in the possibility of consistently producing high yields of high starch maize silage in Northern Ireland. However, there is considerable variability in the quality of maize and grass silages produced. In a recent study Keady et al. (2002) concluded that the highest yield of fat and protein from dairy cows was obtained from maize silage ensiled at approximately 30% dry matter (DM) when offered as 40% of the forage component of the diets consisting of either medium or high feed value grass silages supplemented with concentrates. Also Keady et al. (2002) concluded that replacing 40% of the grass silage component of the diet with maize silage had a concentrate sparing effect, as determined for milk yield, of up to 1.9 kg/cow/d. The objective of the current study was to examine further the effect of maturity of maize silage at harvest on the performance of dairy cattle offered grass silages differing in feed value. The potential concentrate sparing effect of contrasting maize silages was also examined.