Fifty F1 Limousin x Nellore bulls were allotted to ten treatments, with five concentrate levels (25, 37.5, 50, 62.5, and 75%) and two diet protein balance methods (one isoprotein and the other changing protein as diet energy changed). The intake of dry matter (DM), organic matter (OM), crude protein (CP), neutral detergent fiber (NDF) and total digestible nutrients (TDN) were determined. After the slaughter, all animal body parts were weighed, sampled and DM, total nitrogen and ether extract concentrations were determined. Protein, fat and energy contents retained in the body were estimated by regression equations of logarithm of protein, fat or energy body content, as a function of logarithm of empty body weight (EBW). By deriving the prediction equations of body content of protein, fat, or energy, as a function of the logarithm of EBW, the net requirements of protein and energy, for gain of 1 kg EBW, were determined. The deriving equation was Y = b. 10ª. Xb-1, where a and b were the 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 the linear regression of the logarithm of heat production and the metabolizable energy intake. The diet balance methods did not influence the nutrients intakes. The DM intake (DMI) was not affected by the concentrate level (CL), with means of 7.39 kg/day. Dietary CL did not affect OM intake (7.08 kg/day). Increasing CL and NDF intake showed a linear decrease and TDN intake showed a linear increase. In diets with variable protein levels, CP intake increased linearly. Isoprotein diets were not affected by the CL (0.89 kg/day). Net energy requirement for weight gain of F1 Limousin x Nellore bulls, in Mcal/kg, can be obtained by the equation: NEg = 0.038 x EBW0.75 x EBW gain0.9896. The NEm for these animals was 76.36 kcal/EBW0.75. The following equation was obtained to estimate the retained protein (RP), in g/day, as a function of EBW gain, in kg/day: RP = 174.14524 x EBW gain.
concentrate levels; crossbred; energy; intake; nutritional requirements; protein
