Effects of Residual Feed Intake Phenotype and Dietary Glycerin on Growth and Carcass Composition of Feedlot Nellore Bulls.

The objective of the study was to determine the effects of residual feed intake (RFI) phenotype and addition of crude glycerin as a partial substitute for dietary corn on feedlot performance and carcass traits of Nellore bulls. The experiment was conducted as a completely randomized design in a 2 × 2 factorial arrangement, in which treatment groups were initially balanced for body weight (BW) of animals. The main treatment factors were RFI phenotype (Low or High) and diet (corn-based or corn-based with glycerin substituted for 20% of the corn in the diet). Twenty-five Nellore bulls (15 ± 0.6 months of age; 311 ± 36 kg) were individually fed ad libitum once daily at 0900 and weighed every 32 d until slaughter at 96 d. Residual feed intake was calculated at the end of the experiment as the difference between observed and expected dry matter intake (DMI) based on a linear regression of mid-test metabolic BW and average daily gain for the trial. Then, RFI phenotypes were retrospectively assigned to each experimental unit for the specific treatment factor. Residual feed intake was significantly different between RFI phenotypes (-0.92 vs. 0.52 ± 0.21 kg/d for Low and High RFI groups, respectively; P < 0.001). Final BW and BW gain were not different between RFI phenotype groups. However, DMI and DMI as a percentage of BW were lower for Low compared to High RFI groups (6.3 vs 7.8 ± 0.3 kg/d for DMI, and 1.4 vs. 1.8 ± 0.05% for DMI as percentage of BW, respectively; P ≤ 0.003). High RFI bulls had lower feed efficiency than Low RFI bulls (0.22 vs 0.18 ± 0.01 kg/kg, respectively; P = 0.02). Hot carcass weight and carcass yield did not differ between RFI phenotypes. Next, substitution of 20% of dietary corn with glycerin did not affect (P ≥ 0.11) performance or carcass traits. Interaction of RFI phenotype and diet was not significant. In conclusion, feedlot Nellore bulls of negative RFI phenotype had lower DMI but maintained growth and carcass traits compared to positive RFI phenotype bulls. Furthermore, performance and carcass traits of yearling Nellore bulls in feedlot for 96 days was maintained when crude glycerin was substituted for 20% of dietary corn. This project was funded by São Paulo Research Foundation (FAPESP), São Paulo, Brazil.