Efficiency of metabolizable energy utilization for maintenance and weight gain and metabolizable energy and total digestible nutrients requirements of F1 Limousin x Nellore bulls

Fifty F1 Limousin x Nellore bulls were allocated in 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). Dry matter intake (DMI) required to maintain the energy balance was calculated dividing the metabolizable energy intake (MEI) required for maintenance (112.89 kcal/ka EBW0.75), by diet ME concentration (kcal/ka of DM). in each treatment. The net energy concentration for maintenance (NEm) of each ration was obtained by dividing heat production in fasting (76.36 kcal/kg EBW0.75) by DMI to maintain energy balance, expressed in of DM/kg EBW0.75. NEg concentration was calculated by dividing the energy retained per day (kcal/kg EBW0.75) by the DMI above the maintenance requirements, expressed in g DM/kg EBW0.75. The efficiencies of utilization of ME (k) for maintenance (k(m)) and for weight gain (k(f)) were estimated from the relationship between the net energy concentration, for maintenance or gain, respectively, as function of the ME of the diet, and kf was also calculated as the linear regression coefficient between retained energy and MEI, together with the data of the maintenance animals. The k value for maintenance (k(m)) was 0,68 and for gain (k(f)) were 0.52; 0.50; 0.55; 0.60 and 0.50, respectively, for diets with 1.93; 2.09; 2.28; 2.42 and 2.70 Mcal of ME/kg of DM, obtained for concentrate levels of 25; 37.5; 50; 62.5 and 75%. ME, DE and TDN maintenance requirements were 112.3 and 137.0 kcal/kg EBW0.75 and 31.1 g/kg EBW0.75 respectively.