Feed efficiency and nitrogen use rankings of Bos indicus steers differ on low and high protein diets

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. © 2020 Elsevier B.V.