ABSTRACT The objective of this experimental study was to evaluate the effects of different dietary energy sources on the carcass and meat traits of feedlot Nellore bulls. A total of 48 Nellore bulls at 32 months of age with 354.35±19.56 kg body weight were studied for 85 days. The bulls were allotted to four groups of 12 individuals: Whole corn grain - diet based on whole corn grain; Ground corn - diet based on ground corn grain; Citrus pulp - diet based on citrus pulp; and Steam-flaked corn - diet based on steam-flaked corn. Diets did not influence carcass quality or meat quality. Average values for fat depth, rib eye area, shear force, total cooking losses, and carcass yield were 4.97 mm, 74.71 cm2, 5.85 kgf, 26.82%, and 52.0%, respectively. Based on the quantitative and qualitative analysis of this study, there is no restriction for the adoption of any of the tested diets in feedlots with Nellore bulls and the evaluated energy sources may be used to replace ground corn grain.

Resumo: O objetivo deste trabalho foi avaliar o efeito da substituição da proteína bruta do farelo de soja pela ureia de liberação lenta sobre o desempenho, as características de carcaça e o custo alimentar de bovinos Nelore terminados em confinamento. Foram utilizados 48 machos inteiros da raça Nelore, com idade média de 22 meses e peso inicial de 367,95±18,52 kg. Utilizou-se o delineamento inteiramente casualizado com quatro tratamentos: FS, 8,91% de farelo de soja e 0% de ureia de liberação lenta (ULL); OP33, 6,01% de farelo de soja e 0,46% de ULL; OP67, 2,94% de farelo de soja e 0,94% de ULL; e OP100, 0% de farelo de soja e 1,41% de ULL. Não houve diferença entre os tratamentos para as variáveis peso inicial, peso final, ganho médio diário, ganho de carcaça, consumo de matéria seca, conversão alimentar, eficiência biológica, peso de carcaça quente, rendimento de carcaça e custo alimentar da arroba produzida. A ureia de liberação lenta, usada em substituição parcial ou total à proteína do farelo de soja na dieta, não altera o desempenho, as características de carcaça e a eficiência econômica de bovinos de corte confinados.

Abstract: The objective of this work was to evaluate the effect of replacing crude protein of soybean meal by slow-release urea on the performance, carcass traits, and feeding costs of feedlot Nelore steers. Forty-eight Nelore steers were used, with an average age of 22 months and weight of 367.95±18.52 kg. A completely randomized design was used with four treatments: SM, 8.91% soybean meal and 0% slow-release urea (SRU); OP33, 6.01% soybean meal and 0.46% SRU; OP67, 2.94% soybean meal and 0.94% SRU; and OP100, 0% soybean meal and 1.41% SRU. No difference was observed between treatments for the variables initial weight, final weight, average daily gain, carcass gain, dry matter intake, feed conversion, biological efficiency, hot carcass weight, carcass yield, and feed cost of produced arroba. Slow-release urea, used to partially or totally replace protein of soybean meal in the diet, does not modify the performance, carcass traits, and economic efficiency of feedlot cattle.

The objective of this study was to determine the nutritional requirements of energy and protein and estimate the efficiencies of metabolizable energy utilization for fat and protein deposition, as well as for maintenance (k m) and growth (k g). An experiment of comparative slaughter was carried out with thirty-seven 14-month-old (±1 month) Nellore bulls with 259±24.9 kg. Animals were divided as follows: five to reference, four to maintenance level and twenty-eight bulls feeding ad libitum. Bulls were also grouped in 4 different feedlot periods (42, 84, 126 and 168 days) for slaughter. The diet was composed of corn silage and concentrate, at a 55:45 ratio. After the slaughter, the left half carcasses were totally dissected for determination of body composition. The energy requirements for maintenance were obtained by exponentially relating the heat production and the metabolizable energy intake, while the energy requirements for gain (NEg) were obtained according to empty body weight (EBW) and EBW gain (EBG). The net protein requirements for gain (NPg) were estimated according to EBG and retained energy (RE). The net (NEm) and metabolizable (MEm) energy requirements for maintenance were 76.5 and 113.84 kcal/EBW0.75/day, respectively. The k m was 0.67. The equations for NEg and NPg were: NEg (Mcal/day) = 0.0555 × EBW0.75 × EBG1.095 and NPg (g/day) = 263.37 × EBG - 23.21 × RE. The k g was 0.33. The efficiencies to deposition of energy as protein and fat were 0.18 and 0.71, respectively. The model obtained for the percentage of retained energy as protein (%REp) was %REp = 2.4221 × (RE/EBG)-1.6472. The net and metabolizable energy requirements for maintenance of Nellore bulls were 76.5 and 113.84 kcal/EBW0.75/day. The energy and protein requirements for gain could be obtained by the respective equations: NEg (Mcal/day) = 0.0555 × EBW0.75 × EBG1.095 and NPg (g/day) = 263.37 × EBG - 23.21 × RE.