The energy and protein requirements for gain of Brangus and Nellore bulls were estimated using the comparative slaughter technique, in which the empty body as well as the gain composition were estimated through the 9th -10th -11th rib cut. Twenty-eight animals from each breed were used in this research. At the beginning of the trial four animals from each breed were slaughtered to determine the initial body composition while the remaining bulls had free access to diets containing 20, 40, 60 or 80% of concentrate (dry matter basis) during 145 days. The average initial and final shrunk body weights were 238 and 402 kg for Brangus and 230.7 and 361.9 kg for Nellore, respectively. Animals averaged eight months of age in the beginning of the trial. The final average shrunk body gain, empty body gain, and hot carcass gain differed between breeds and concentrate levels with greater values found for Brangus and 60% of dietary concentrate, respectively. Fat and energy contents in the empty body were greater for Brangus while water, protein, and ash were higher for Nellore bulls. Protein and energy requirements for empty weight gain differed between breeds averaging 0.161 and 0.188 kg of protein and 3.02 and 2.46 Mcal of energy per kg of empty weight gain for Brangus and Nellore, respectively. Although it was observed greater energy requirement for Brangus and greater protein requirement for Nellore, no effect of concentrate level was found probably because the age of the bulls prevented animals to reach a stage of greater fat deposition.
beef cattle; body composition; feedlot; growth; maturity
