This study aimed to evaluate the performance and the blood biochemical and physiological parameters of Dorper × Santa Ines lambs receiving three levels of metabolizable energy (ME). A completely randomized design with three treatments (ME levels) and eight replicates (lambs) was used. Twenty-four non-castrated male lambs of four months of age and 21.07 ± 3.91 kg body weight (BW) received a diet composed of 40% Tifton 85 hay and 60% concentrate feed, on a dry matter basis. The amount of feed supplied was adjusted to meet 115, 201, and 288 kcal ME per kg of metabolic weight (MW = BW0.75) daily. Average daily gain and the intake of dry matter (g/day and % BW0.75/day), nutrients, and energy increased linearly (P < 0.001) with ME level. The dry matter intake relative to BW (% BW/day) and feed conversion ratio showed quadratic responses (P < 0.05) to ME level. Serum creatinine decreased linearly (P = 0.038), whereas plasma glucose increased linearly (P = 0.045) with ME level. Serum urea presented a high variation (standard error range = ±1.98-6.07 mg/dL) in response to 288 kcal/kg0.75/day of ME supply, and did not stabilize during the trial. Respiratory rate increased linearly (P < 0.001) with ME level at each time of day (morning/afternoon), but with higher intensity in the afternoon. Heart rate and rectal temperature increased linearly (P < 0.001) with ME level. Given that the range of ME level simulated a restricted-to-ad libitum ME intake, the best response in terms of feed efficiency can be obtained with 252 kcal/kg0.75/day of ME supply, which probably reflects the best balance of metabolic and physiological parameters. Thus, in order to achieve high productive performance in tropical environments, feedlot lambs should be fed with high-concentrate diets.
Keywords: Blood metabolite; Feed efficiency; Fiber intake; Heart rate; Maintenance requirement; Sheep nutrition.