CRUDE PROTEIN-REQUIREMENTS OF LARGE FRAME CATTLE FED 2 LEVELS OF ENERGY AS WEANED CALVES OR AS BACKGROUNDED YEARLINGS

Two trials were conducted to determine the crude protein (CP) requirem ents of large-frame (LF) cattle managed under two regimes and fed one of two energy levels. Trial 1 involved 216 recently wearied LF calves, while trial 2 involved 192 LF backgrounded yearling steers. In each t rial, the cattle were fed diets formulated to one of two energy levels (11.17 and 11.88 MJ ME kg-1 DM) and one of six CP levels (11, 13, 15, 16, 17 and 19% DM basis). High-energy feeding increased (P < 0.05), a verage daily gain (ADG), average daily carcass gain (ADCG) and average daily fat gain (ADFG) of weaned calves but did not influence the perf ormance of the backgrounded yearlings. Weaned calves fed 16, 17 or 19% CP had higher (P < 0.05) ADG than those fed 11%, while those fed 16% CP exhibited increased (P < 0.05) ADFG. ADG and ADCG of weaned calves and backgrounded yearlings and ADFG of weaned calves fed low-energy di ets increased (P < 0.05) linearly with increasing dietary CP concentra tion. Daily carcass lean gain of weaned calves fed low-energy diets de creased in a quadratic manner (P < 0.05) as dietary CP level increased . No influence of dietary CP concentration was observed on carcass tis sue gain of the backgrounded yearlings. It was concluded that energy b alance of animals fed low-energy diets improved with CP supplementatio n and that current NAS-NRC (1984) CP requirements accurately predict p erformance of LF cattle.