The quality of protein supplied from grass silage is poor in relation to it's ability to sustain satisfactory levels of milk production and it is therefore necessary to use protein supplements in order to increase protein supply. However responses at the highest level of inclusion can be marginal with excess nitrogen being excreted and causing an increased environmental threat. The aim of this experiment was to examine the effect of six different concentrates fed in conjunction with grass silage, supplying two levels of dietary crude protein (CP) at three levels of degradability, on nitrogen partitioning by lactating cows.