RESUMO
One hundred ninety-one Angus steers (age = 546 d ± 33.5; BW = 36.4 kg ± 4.2), sired by bulls divergently selected for feed efficiency over a 3-yr period, were used to compare growth, efficiency, body composition, and carcass characteristics. Selected Angus sires were either high (Hi) or low (Lo) for residual average daily gain (RADG) expected progeny differences (EPDs) and either high (Hi) or low (AVG; breed average) for marbling (MARB) EPDs. Steer weight and body composition, via ultrasound, were measured at weaning (205 d) and at 1 yr (365 d) of age. Steers entered the feedlot at 454 d of age and completed a 70-d GROWSAFE BEEF System evaluation to determine DMI, ADG, and residual feed intake (RFI). Steers were then slaughtered as they reached a backfat thickness of 1.3 cm. Carcasses were chilled for 48 h at 2 °C, ribbed, and USDA yield and quality grade data were collected. The right side of the carcass was fabricated and primal and subprimal weights were collected. A 2.5-cm longissimus steak was removed, vacuum-packaged, aged for 14 d, and frozen for slice shear force determination. Additionally, a 1.3-cm longissimus steak was removed from year 3 steers for proximate analysis. The GLM procedure of SAS was used and the main effects of RADG and MARB and their interaction were tested by the error term, SIRE(RADG*MARB). Year was evaluated as a replicate. Weight, ultrasound backfat, and REA were increased (P ≤ 0.05) at weaning in the Hi compared with AVG MARB steers. Feed efficiency, measured by RFI, was improved (P = 0.05) in the Hi RADG steers compared with Lo RADG steers. Slaughter weight and HCW were heavier (P ≤ 0.03) in the Hi RADG steers compared with Lo RADG steers. An interaction (P = 0.05) between RADG and MARB selection was found for marbling score, steers selected for Lo RADG and Hi MARB had greater marbling scores than all other groups. Longissimus proximate composition from year 3 showed that lipid content was greater (P < 0.01) in the Hi MARB and Lo RADG groups compared with the AVG MARB and Hi RADG groups, respectively. These findings suggest that selection using RADG or MARB EPDs has minimal impact on carcass yield. However, positive selection pressure placed on these breeding values can potentially improve efficiency and carcass quality. Lastly, it appears that improvements in feed efficiency can be obtained without negatively affecting beef carcass merit, especially USDA quality grade.
