Genetic improvement of carcass composition and fat partitioning in steers using yearling bull measurements

This research evaluated relationships between growth performance and yearling ultrasound measurements collected on yearling beef bulls with carcass measurements, carcass lean content, and fat partitioning of crossbred beef steers. Particular focus placed on relationships between ultrasound longissimus depth and width measurements collected in yearling bulls with steer carcass data adjusted to alternative slaughter endpoints. Yearling bull carcass lean meat yield was predicted as accurately and precisely by equations involving ultrasound fat depth and ultrasound longissimus muscle depth and width measurements or ultrasound fat depth and longissimus muscle depth measurements as by equations involving ultrasound fat depth and ultrasound longissimus muscle area. An equation based on carcass weight, fat depth and longissimus muscle area and modified to include live growth and ultrasound fat depth and longissimus muscle area tended to predict whole side carcass lean content less accurately and precisely than equations based on ultrasound measurements. The 7-bone wholesale rib section was a reasonable predictor of whole-side and 10-11-12th rib section lean, as well as subcutaneous and intermuscular fat content. Post-weaning gain, ultrasound fat depth, longissimus muscle depth, width and area were heritable and were genetically correlated to the corresponding carcass traits in steers. Scan age-constant yearling bull ultrasound lean meat yield estimates had stronger genetic correlations with steer lean meat yield (based on dissection of the 10-11-12th rib section) at slaughter age and carcass weight constant endpoints than at carcass fat depth and marbling score constant endpoints. Regardless of the slaughter endpoint chosen, dissected steer carcass lean meat yield can be improved more rapidly using selection indexes based on individual yearling bull traits than by selection based on pre-adjusted ultrasound lean meat yield estimates. Post-weaning yearling bull growth and ultrasound measurements were genetically correlated with subcutaneous, intermuscular, and body cavity fat content of the 10-11-12th rib section in steers. Results indicate that opportunities exist to conduct genetic evaluations for steer carcass composition at commercially relevant (i.e. carcass weight, fat depth or marbling score constant) endpoints. Ultrasound longissimus muscle depth and width measurements may enhance selection for lean content and fat partitioning in steer carcasses.