Under Northern Ireland conditions, until recently, it was difficult to achieve high starch maize (greater than 200 g/kg DM) more often than one year in fifteen. However, 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 starch maize. As there is considerable variability in the quality of grass silage produced on Northern Ireland dairy farms, the objective of the present study was to examine the effects of maturity of maize silage at harvest on the performance of lactating dairy cattle offered two contrasting feed value grass silages. The potential concentrate sparing effect of contrasting maize silages was also examined.