Recent trials with cows fed grass silage have shown significant increases in intake and in yields of milk and milk solids when the crude protein (CP) concentration in a supplementary concentrate was raised. Giving additional CP in the concentrate was a more effective strategy for improving yields of milk and milk protein than giving extra energy (Aston et al 1992). The objective of this trial was to examine the influence of changes to the amount and pattern of distribution of CP supplied in a fixed concentrate ration given with grass silage ad libitum.

Fifty-five Holstein-Friesian cows in their second and subsequent lactations were given a standard diet for two weeks from calving and then were used in a continuous feeding trial from weeks 4 to 21. The cows received 5 kg of fresh concentrate daily containing 156 (LP), 245 (MP) or 338 (HP) g CP per kg dry matter (DM), MP comprised equal amounts of LP and HP. Concentrate carbohydrate sources were cereals and digestible fibre and CP was increased by a mixture of 3:1 soya:fish meals. Grass silage contained 253 g toluene DM/kg, 162 g CP/kg DM, D value in vivo 0.723, pH 3.7, fermentation acids 135 g/kg DM of which 0.85 was lactic acid and NH3-N 91 g/kg total N.