Forty Nellore bulls, with average initial live weight (LW) of 240 kg, were used. Four reference bulls and four bulls fed for maintenance were used, and the remaining was assigned to eight treatments, with four different concentrate levels in the diets (20, 40, 60 and 80%) and two (15 and 18%) levels of crude protein (CP). The protein, fat and energy contents retained in the body were estimated by regression equations of the logarithm of protein, fat or energy body content, as a function of the logarithm of empty body weight (EBW). By deriving the prediction equations of body content of protein, fat, or energy, as a function of the EBW logarithm, the net requirements of protein and energy, for gains of 1 kg EBW, were determined. The deriving equation was Y = b. 10ª. Xb-1, being a and b intercept and regression coefficient, respectively, of the prediction equations of protein or energy body contents. Net energy requirement for maintenance (NEm) was estimated as the intercept anti-log of the equation obtained by linear regression among the logarithm of heat production and the metabolizable energy intake. The net energy requirement for weight gain of zebu bulls can be obtained by the equation: NEg = 0.0413 x EBW0.75 x gain of EBW0.978. The NEm for these animals was 83.70 kcal/EBW0.75. The following equation was obtained to estimate the retained protein (RP), as a function of EBW gain and retained energy (RE): RP = - 39.0169 + 200.638 gain of EBW + 0.4166 RE. The net protein requirement of Nellore bulls decreased as the LW increased, being 183.22 and 173.72 g/kg of EBW gain for animals with 200 and 400 kg of LW, respectively.
energy; Nellore; protein; requirements
