Increasing the level of concentrate fed to lactating ruminants reduces forage intake, but increases the yield of milk and milk constituents (Aston et al. 1991). However, the proportion of the total response due to additional metabolisable energy (ME) or crude protein supply (CP) is unclear. Recent work with dairy cows (Aston et al. 1992) suggests that additional ME or CP fed as concentrate reduced and slightly increased silage intake respectively. Additional CP had twice the effect on milk and three times the effect on protein yield than additional ME. However, ME and CP were equally important in determining fat yield. The objective of the trial was to investigate whether dairy sheep respond to additional ME or CP supply as concentrate in a similar way to dairy cows.

Three concentrate diets were formulated to contain 13.0 MJ/kg DM ME and either 100 (A), 200 (B) or 400 (C) g/kg DM CP. After weaning (6 weeks), 24 mature Freisland ewes were housed individually, and allocated by liveweight and initial milk yield to receive concentrate DM at daily rates of 0.8 kg, providing 10.4 MJ/day ME and 160 (B) or 320 (C) g/day CP (LLP, LHP), or 1.6 kg, providing 20.8 MJ/day ME and 160 (A) or 320 (B) g/day CP (HLP, HHP). The trial lasted 8 weeks and was analysed as a 2 X 2 factorial design.