Nutrition represents the major operating cost of beef cattle production. Improvements on feed efficiency (FE) can lead to significant economic benefits and reduce the environmental footprint of red meat production. Usually, livestock selected for FE on high-protein diets are expected to perform as efficiently on low-protein diets. This experiment used 55 Brahman steers (346 ± 8 kg BW) to determine the agreement in FE rankings between a diet in which rumen degradable protein (RDP) was limiting and a protein abundant diet. It was suggested that the agreement would be low and FE in low-protein diets would be related to nitrogen (N) preservation mechanisms. A completely randomized block design was used. Each steer represented an experimental unit. Steers were fed in individual pens for two periods of 70 days, including an adaptation of 10 days, with diets supplying either 70 % or 100 % of their RDP requirements. Residual feed intake (RFI), residual gain (RG), and residual feed intake and gain (RIG) were determined based on average daily gain (ADG), dry matter intake (DMI), and body weight. Kappa analysis was used to determine the agreement in FE rankings between both diets. In the low-protein diet, ADG was 0.99 kg/d (0.38–1.53 kg/d), DMI averaged 1.9 kg/100 kg BW d−1 (1.6–2.3 kg/100 kg BW d−1), RFI varied between −1.22 and 1.58, and RG from -0.62 to 0.53. In the high protein diet, ADG was 1.21 kg/d (0.64–1.74) and DMI averaged 1.8 kg/100 kg BW d−1 (1.0–2.3 kg/100 kg BW d−1). RFI varied between −1.52 and 1.58 and RG from -0.36 to 0.41. Kappa analysis showed no agreement (P > 0.10) for RFI (−7 %), RG (2 %), nor RIG (−1 %) between diets. More efficient steers in the low-protein diets, measured as RG, excreted less (P = 0.02) N in urine as a proportion of BW and as a proportion of N intake, resulting in higher N use efficiency. This relationship was not present when steers were fed the high-protein diet (P = 0.55). These results suggest that different physiological mechanisms are responsible for FE regulation in both diets; thus, appropriate diets must be used when selecting animals for FE.

中文翻译：

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Bos indicus 阉牛的饲料效率和氮使用排名在低蛋白和高蛋白日粮上有所不同

营养是肉牛生产的主要运营成本。提高饲料效率 (FE) 可以带来显着的经济效益并减少红肉生产的环境足迹。通常，选择高蛋白饮食进行 FE 的牲畜在低蛋白饮食中表现同样有效。该实验使用 55 头婆罗门公牛 (346 ± 8 kg BW) 来确定瘤胃可降解蛋白 (RDP) 有限的饮食和蛋白质丰富的饮食之间 FE 等级的一致性。有人建议，该协议将是低的，低蛋白饮食中的 FE 将与氮 (N) 保存机制有关。使用完全随机区组设计。每头牛代表一个实验单元。公牛在单独的围栏内饲喂两个 70 天的时期，包括 10 天的适应期，日粮满足其 RDP 需求的 70% 或 100%。根据平均日增重 (ADG)、干物质摄入量 (DMI) 和体重确定残余采食量 (RFI)、残余增重 (RG) 以及残余采食量和增重 (RIG)。Kappa 分析用于确定两种饮食之间 FE 等级的一致性。在低蛋白饮食中，ADG 为 0.99 kg/d（0.38-1.53​​ kg/d），DMI 平均为 1.9 kg/100 kg BW d-1（1.6-2.3 kg/100 kg BW d-1），RFI 在-1.22 和 1.58，以及从 -0.62 到 0.53 的 RG。在高蛋白饮食中，ADG 为 1.21 kg/d (0.64-1.74)，DMI 平均为 1.8 kg/100 kg BW d-1（1.0-2.3 kg/100 kg BW d-1）。RFI 在 -1.52 和 1.58 之间变化，RG 在 -0.36 到 0.41 之间变化。Kappa 分析显示饮食之间的 RFI (-7 %)、RG (2 %) 和 RIG (-1 %) 没有一致性（P > 0.10）。在低蛋白饮食中更有效的阉牛，以 RG 测量，在尿液中排泄较少 (P = 0.02) N 作为 BW 的比例和 N 摄入的比例，导致更高的 N 使用效率。当公牛饲喂高蛋白饮食时，这种关系不存在（P = 0.55）。这些结果表明，不同的生理机制负责两种饮食中的 FE 调节；因此，在为 FE 选择动物时必须使用适当的饮食。