Effects of protein supplementation of grass silage on the performance and carcass quality of beef cattle

Two experiments (E1 and 2) were carried out in Northern Ireland in 1988/89 to examine the effects of a wide range of supplementary protein intakes given with grass silage-based diets on the performance and carcass composition of beef cattle, and to compare diets based on silage and dried forage. The five treatments used consisted of grass silage offered an libitum and supplemented with 2.5 kg dry matter (DM) of barley-based concentrates containing (1) zero, (2) 0.2, (3) 0.4 and (4) 0.6 extracted soyabean meal and (5) artificially dried grass and hay supplemented with 3.3 kg concentrate DM. Experiments E3 and 4 were carried out in 1991/92 to examine the responses in ration digestibility, silage intake and animal performance to supplementary protein in relation to the requirements recommended by the United Kingdom Metabolizable Protein System. The two treatments consisted of grass silage offered ad libitum and supplemented with 3.2 kg DM of (1) barley and (2) 0.8 barley and 0.2 soyabean meal. The silages used were well preserved, containing on average 200 g DM/kg; 143 g crude protein (CP)/kg DM; 79 g ammonia-N/kg total N and 734 g digestible organic matter/kg DM. The cattle were castrated males of medium to late-maturing breed type and were initially 13 months old and 396 kg liveweight. Increasing protein intake did not affect ration digestibility, silage DM intake or animal performance in any of the four experiments, despite the fact that the effective rumen degradable protein (ERDP) intakes in the diets supplemented with barley alone were only 80-85% of ERDP requirements. However, increasing protein intake tended to increase carcass fatness in all experiments, the effect being significant (P < 0.05) for the lipid concentration in the M. longissimus dorsi muscle in E1 and 2. Offering the dried forage-based diet rather than an isonitrogenous silage-based diet did not affect carcass fatness. It is concluded that supplementing well preserved grass silages with mixtures of barley and soyabean meal, rather than with barley alone, did not affect ration digestibility, silage intake or animal performance but tended to increase carcass fatness, especially marbling fat, despite the fact that ERDP intakes for the diets with the lowest protein concentration were only 80-85% of requirements given by the United Kingdom Metabolizable Protein System.