This study evaluatedthe effect of different dietary levels of metabolizable energy on the body compositionand net protein requirements of 24 Santa Inês non-castrated growing sheepat average age of 50 days and average body weight of 13.00 ± 0.56 kg. Aftera ten-day adaptation period, four animals were slaughtered to be used as a referencefor estimates of empty body weight (EBW) and initial body composition of theother animals. The remaining animals were assigned into a randomized block designwith diets containg different levels of metabolizable energy (2.08; 2.28; 2.47and 2.69 Mcal/kg of DM) and crude protein (12.74; 14.36, 15.97 and 17.65% CPof DM) with five replicates. The N daily excretion was estimated by extrapolatingregression equations of N intake (g/kg BW0.75/day) in function ofretention (g/kg BW0.75/day) for zero intake. The derivative regressionof the logarithm of protein content in function of the logarithm of EBW enabledthe estimation of net protein requirement for empty body weight gain (EBWG).Daily nitrogen excretion was 277 ± 0.05 mg/kg BW0.75/day andnet 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 EBWwhen the BW of the animals increased from 15 to 30 kg, respectively. The amountof protein deposited in the gain decreased from 137.47 to 125.71 g/kg of EBWGwith the increase of body weight of the animals from 15 to 30 kg, respectively.Nitrogen fecal excretion and protein net requirement for maintenance of SantaInês growing lambs was lower than the values commonly cited by the mainsystems of feed evaluation and nutrient requirements for small ruminants.
