More efficient steers on good diets are not the most efficient ones on limiting diets

Nutrition represents the major operative cost of any beef cattle production. Improvements on feed efficiency (FE) can lead to significant economic benefits and, parallelly, reduce its environmental footprint. Usually, animals selected for FE on high-protein (HP) diets are expected to perform as efficiently on low-protein (LP) diets. This experiment used 30 Bos indicus steers (398 ± 24 kg BW) to determine the agreement between the FE of beef cattle fed a LP and a HP diet. As hypothesis, it was suggested that the agreement would be high. A completely randomized block design was used, where 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 rumen degradable protein requirements. Average daily gain (ADG) and dry matter intake (DMI) were measured, while residual feed intake (RFI) and residual gain (RG) were calculated. Kappa analysis was used to determine the agreement between FE of both diets. In the LP diet, ADG was 0.93 kg/d (0.38 to 1.47), DMI averaged 9.67 kg/d (7.9 to 12.1), RFI varied between -1.55 and 1.84, and RG from -0.61 to 0.53. In the HP diet, ADG was 1.16 kg/d (0.77 to 1.57) and DMI averaged 9.87 kg/d (4.79 to 11.87). RFI varied between -2.53 and 1.61 and RG from -0.34 to 0.33. Kappa analysis showed no agreement for RFI (5.56%) nor RG (9.09%) between diets. These results suggest that different physiological mechanisms are responsible for FE regulation in both diets; thus, appropriate diets targeting each scenario must be used when selecting animals for feed efficiency.