This study evaluated the effect of different dietary levels of metabolizable energy on the body composition and net protein requirements of 24 Santa Inês non-castrated growing sheep at average age of 50 days and average body weight of 13.00 ± 0.56 kg. After a ten-day adaptation period, four animals were slaughtered to be used as a reference for estimates of empty body weight (EBW) and initial body composition of the other animals. The remaining animals were assigned into a randomized block design with diets containg different levels of metabolizable energy (2.08; 2.28; 2.47 and 2.69 Mcal/kg of DM) and crude protein (12.74; 14.36, 15.97 and 17.65% CP of DM) with five replicates. The N daily excretion was estimated by extrapolating regression equations of N intake (g/kg BW0.75/day) in function of retention (g/kg BW0.75/day) for zero intake. The derivative regression of the logarithm of protein content in function of the logarithm of EBW enabled the estimation of net protein requirement for empty body weight gain (EBWG). Daily nitrogen excretion was 277 ± 0.05 mg/kg BW0.75/day and net protein requirement for maintenance was 1.73 g/kg BW0.75/day. Protein content of EBW of animals decreased from 157.83 to 144.33 g/kg of EBW when the BW of the animals increased from 15 to 30 kg, respectively. The amount of protein deposited in the gain decreased from 137.47 to 125.71 g/kg of EBWG with the increase of body weight of the animals from 15 to 30 kg, respectively. Nitrogen fecal excretion and protein net requirement for maintenance of Santa Inês growing lambs was lower than the values commonly cited by the main systems of feed evaluation and nutrient requirements for small ruminants.
maintenance; metabolizable protein; weight gain
