Breeding programmes for Holstein Friesian (HF) animals have been based on improved milk production with little emphasis on functional traits such as fertility. This has resulted in a rapid increase in milk production potential of the national dairy herd but unfortunately this has been at the expense of issues such as longevity, especially due to poor reproductive performance. In contrast Norwegian (NC) dairy cattle have been bred via a multi-trait selection procedure for 25 years and there is evidence that fertility has improved during that period. These differences in selection procedures for the two breeds may have major effects on overall herd output and profitability within the grass-based systems of milk production employed in Northern Ireland. The present study is part of an overall study comparing the effects of HF and NC genotypes on food intake, animal performance, nutrient utilisation, behaviour, health, fertility and longevity. The objective of the present paper is to present the effects of breed on food intake and utilisation, and animal performance when offered two levels of nutrient inputs based on grass silage-based diets.