Effects of forage to concentrate ratio in the diet and protein intake on the performance and carcass composition of beef heifers

Abstract

SUMMARYIsoenergetic diets containing grass silage and concentrates in the ratios 75:25; 50:50 and 30:70 on a dry matter (DM) basis were offered to 42 beef heifers, which were initially 361 kg liveweight, for 115 days in 1987 at Hillsborough, Northern Ireland. Half of the animals given each forage concentrate ratio were given one of two crude protein (CP) intakes in a 3 × 2 factorial design. The silage contained 152 g CP/kg DM, 94 g ammonia-N/kg total N and 734 g digestible organic matter/kg DM. Concentrates were based on barley and soyabean meal and were formulated to equalize CP intakes for the three forage: concentrate ratios. For diets containing 75, 50 and 30% silage, metabolizable energy intakes were 94, 92 and 87 MJ/day; liveweight gains 0·99, 1·07 and 1·12 (S.E. 0·051) kg/day; carcass gains 0·61, 0·64 and 0·67 (S.E. 0·033) kg/day; carcass lean concentrations (estimated from dissection of fore-rib joints) 636, 642 and 648 (S.E. 4·7) g/kg and carcass fat concentrations 204, 199 and 194 (S.E. 5·3) g/kg. Increasing protein intake did not affect performance, but increased carcass fat concentration at all three forage concentrate ratios. It was concluded that increasing the proportion of concentrates in grass silage-based diets increased carcass weight gain per MJ of metabolizable energy consumed and tended to reduce carcass fatness. Contrary to previous findings with young growing cattle, increasing protein intake did not alleviate the problem of greater fat deposition in finishing cattle given high-silage diets, but rather significantly increased carcass fat concentration.