EFFECTS OF FORAGE TO CONCENTRATE RATIO IN THE DIET AND PROTEIN-INTAKEON THE PERFORMANCE AND CARCASS COMPOSITION OF BEEF HEIFERS

Isoenergetic diets containing grass silage and concentrates in the rat ios 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 e ach forage:concentrate ratio were given one of two crude protein (CP) intakes in a 3 x 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 D M. Concentrates were based on barley and soyabean meal and were formul ated to equalize CP intakes for the three forage:concentrate ratios. F or diets containing 75, 50 and 30% silage, metabolizable energy intake s 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 j oints) 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 a ffect performance, but increased carcass fat concentration at all thre e forage:concentrate ratios. It was concluded that increasing the prop ortion of concentrates in grass silage-based diets increased carcass w eight gain per MJ of metabolizable energy consumed and tended to reduc e carcass fatness. Contrary to previous findings with young growing ca ttle, increasing protein intake did not alleviate the problem of great er fat deposition in finishing cattle given high-silage diets, but rat her significantly increased carcass fat concentration.