Relationship of visual assessments of feeder lamb muscularity to differences in carcass yield traits

We examined the relationship between visual differences in muscle thickness among feeder lambs and subsequent differences in carcass composition. Medium-framed, crossbred feeder lambs (n = 120) were selected at two commercial feedlots to exhibit distinct phenotypic differences in muscularity. The lambs were assigned scores (ranging from 1 to 9; 1 = extremely thin, 5 = average, 9 = extremely thick) for muscle thickness and were sampled serially on d 0, 14, 28, and 42 of the trial. After recording yield grades, one side of each carcass was deboned, and the soft tissues from the entire side were ground, sampled, and analyzed for lipid and moisture content. The opposite side was fabricated into boneless, closely trimmed (.25 cm maximum fat depth) subprimal cuts. When lambs of the same frame size were compared at the same live weight, greater muscle thickness was associated with greater (P < .05) fat-free muscle mass. Correspondingly, thickly muscled lambs produced carcasses of a given weight that had a higher (P < .05) composite yield of lean meat and a lower (P < .05) proportion of trimmable fat compared with carcasses of thinly muscled lambs. When comparisons were made at the same percentage of extractable fat in the carcass, greater muscle thickness was associated with heavier (P < .05) live and carcass weights, increased (P < .05) fat-free muscle mass, and increased (P < .05) weights of trimmed, boneless subprimal cuts. Results suggest that visual assessments of muscle thickness in feeder lambs, as applied in this study, are indicative of commercially important differences in carcass yields of lean meat.