Comparison of energy and protein sources offered at low levels in grass-silage-based diets for dairy cows

Four dietary treatments were based on a flat-rate (5 kg/day) of concentrates with ad libitum grass silage. The concentrates were iso-energetic and iso-nitrogenous, based on either barley or unmolassed sugar-beet pulp and either extracted rapeseed meal (RSM) or a 1 : 3 mixture of fish and soya-bean meals (FIS). These diets were offered to 61 multiparous Holstein-Friesian cows in a continuous design experiment from lactation weeks 4 to 22. Milk yields tended to be higher with RSM (25.1 v. 23.9 kg/day; s.e.d. = 0.64; P < 0.1), whilst milkfat (38.1 v. 40.0 g/kg; s.e.d. = 0.81) and milk protein (30.4 v. 32.3 g/kg; s.e.d. = 0.41) concentrations were significantly (P < 0.05) lower. There were no significant effects of treatments on the efficiency of conversion of food-nitrogen (N) to milk-N or on N-retention. A lower organic matter apparent digestibility (gig) was found for RSM-based diets (0.738 v. 0.763; s.e.d. = 0.0096; P < 0.05). The diets were also offered to four fistulated dairy cows in a Latin-square-design Experiment. Concentrate energy source had significant effects on rumen pH (P < 0.05) and ammonia-N concentration (P < 0.01), whilst protein sources had no effect; values were always in the optimal range (pH > 6 and ammonia-RT > 50 mg/l). There was a significant interaction effect (P < 0.05) such that the N-degradability of the whole diet, estimated in vivo, tons unaffected by energy source for RSM-based diets but highly dependent on energy source for FIS diets. Microbial protein yield was reduced on the RSM-based diets (179 v. 220 g/day; s.e.d. = 9.6; P < 0.001).