Dairy cow genetic merit continues to increase in the dairy herd. These animals require a higher level of nutritional input than lower genetic merit cows if higher performance levels are to be sustained. The high cost of ingredients for ruminant diets highlights the need for greater precision in ration formulation in order to improve the efficiency of feed utilisation. This experiment was carried out to assess the effect of DUP supplementation on the performance of dairy cows with ad libitum access to grass silage.

An experiment was conducted to investigate the response by high genetic merit Holstein Friesian dairy cows (PTA90 fat plus protein = 61 kg) to both source and concentration of protein in the concentrate, given in addition to ad libitum grass silage (metabolisable energy (ME) = 10.9 MJ/kg DM; crude protein (CP) = 166 g/kg DM). Twenty-four cows, including 12 first parity animals (mean days calved 37.5, s.d. 22.3), were blocked according to milk yield, calving date and parity. The experimental design was a balanced changeover with four 4-week periods.