Recent experiments have shown consistent improvements in grass silage intake and milk yield by cows in response to increasing concentrations of crude protein (CP) in the concentrates. Such results have led to recommendations that a strategy for winter feeding of dairy cows should be based on relatively small amounts of high-protein concentrates with grass silage ad libitum. There is, however, little information about the possible effects of such a strategy on milk nitrogen fractions. The purpose of this experiment was to examine the effects of two approaches to increasing the CP concentration of the concentrates on milk yield and composition, particularly nitrogen fractions.

From calving, 44 muciparous Holstein-Friesian cows were given 3 weeks on a standard diet and then were allocated to one of five concentrate treatments with grass silage ad libitum from weeks 4 to 18 of lactation. Concentrates were formulated to contain 200, 300, 400 or 600g CP/kg DM (Table 1). The silage contained 312g DM/kg and, per kg DM, 483g neutral detergent fibre, 286g acid detergent fibre, 168g CP, 16lg lactic acid, 14g acetic acid and l.lg butyric acid. Ammonia-N was 69g/kg total N and pH was 3.9.