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 levels of crude protein (CP) (15 and 18%). The dry matter (DM) intake enough to maintain the energy balance was calculated dividing the metabolizable energy (ME) consumption enough for maintenance, of 131.92 kcal/kg EBW0.75, by the ME concentration (kcal/kg of DM) of the diet, in each treatment. The net energy concentration of each ration for maintenance (NEm) was obtained by dividing the heat production in fasting, 83.70 kcal/kg EBW0.75, by the DM consumption to maintain the energy balance, expressed in g of DM/kg EBW0.75. The metabolizable energy (ME) consumption enough for maintenance and the heat production in fasting were obtained experimentally with the same animals of this experiment. The NEg concentration was calculated by dividing the energy retained per day, in kcal/kg EBW0.75, by the DM consumption above the maintenance needs, expressed in g DM/kg EBW0.75. The ME efficiencies of utilization (MEEU) for maintenance (km) and for weight gain (kf) were calculated from the relationship among the net energy concentration, for maintenance or gain, respectively, in function of the ME of the diet and the kf was also calculated as the linear regression coefficient between retained energy and the ME intake, together with the data of the maintenance animals. The ME requirements for maintenance and gain were obtained by the relationship between the net demands and the respective MEEU. The total digestible nutrients (TDN) requirements were calculated dividing the ME requirements by 0.82, obtaining the digestible energy (DE) demands and, later, dividing the DE requirements by 4.409. The MEEU for maintenance (km) was 0.63 and the MEEU for gain (kf) were 0.30, 0.35, 0.38, and 0.43, respectively, for diet ME concentrations of 2.65, 2.84, 2.92, and 2.90 Mcal/kg of DM, obtained for the concentrate levels of 20, 40, 60 and 80%. ME, DE and TDN maintenance requirements were 132.9 and 162.0 kcal/kg EBW0.75, and 36.7 g/kg EBW0.75, respectively.
efficiency of utilization; metabolizable energy; Nellore
