The Effects of Dietary Crude Protein Level on Rate, Efficiency and Composition of Gain of Growing Beef Bulls

Two experiments were conducted to evaluate the effects of dietary CP level on rate, efficiency and composition of gain of growing beef bulls. In Exp. 1, 59 bulls (333 ± 15.8 kg) were used. Eleven bulls were slaughtered on d 0 to provide an estimate of initial carcass composition (9-10-11 rib section chemical analyses), and remaining bulls were assigned to treatment diets containing 10, 12 or 14% dietary CP. Bulls fed the 10% CP diet grew slower (P< .05) than bulls fed the 12 or 14% CP diets, although dry matter intake and feed-to-gain ratio did not differ. Bulls fed the 12% CP diet had fatter carcasses (P< .05) than bulls fed the 10 or 14% CP diets and had greater daily fat accretion than bulls fed the 10% CP diet. In Exp. 2, 60 bulls (318 ± 9.0 kg) were used. Bulls were assigned to initial slaughter (n = 6) or to one of three dietary treatments, 10, 12 or 14% CP, and were slaughtered after feeding for 66, 136 or 202 d (n = 6 · treatment−1· slaughter time−1). Bulls fed 10% CP diets had lower (P< .05) rates of carcass protein accretion during d 0 to 136 and d 0 to 202. Carcass fat gain was similar among treatments over the entire experiment, although bulls fed the 14% CP diet gained more fat during d 0 to 136 than bulls fed the other treatments. These data indicate that 10% dietary CP is not adequate to support maximal growth of young bulls and suggest that limiting CP may decrease fatness.