Feeder steers (n = 189) representing nine frame size x muscle thickness subclasses were started on a high-concentrate diet as calves, yearlings, or long yearlings and slaughtered at a constant s.c. fat thickness (13.5 mm) to determine the effects of feeder cattle frame size (large, medium, small), muscle thickness (No. 1, No. 2, No. 3), and age class on carcass traits and composition. After slaughter, one side of each carcass was physically separated into muscle, fat, and bone. Subcutaneous, intermuscular, and internal fat depot percentages were calculated as proportions of total fat weight. Effects of age class, frame size, and muscle thickness were significant for time-on-feed, slaughter weight, and carcass weight. Among age classes, long yearling steers required the fewest (P < .05) days on feed and heaviest (P < .05) live weights to reach the fat thickness end point. Increased frame size and decreased muscle thickness were associated with greater time-on-feed and heavier weights at slaughter. However, the effects of muscle thickness on slaughter traits were not consistent among frame and age subclasses; effects were greatest among large-framed steers and smallest among long yearling steers. Compositional differences (P < .05) were noted among age and muscle subclasses. Long yearling steers had the lowest (P < .05) percentage of bone among all age classes and a higher (P < .05) percentage of fat than steers fed as calves. No. 2 steers had the highest (P < .05) percentage of bone and the lowest (P < .05) muscle:bone ratio among muscle thickness groups; no (P > .05) compositional differences were observed between No. 1 and No. 3 steers. Differences in muscle:bone ratio, though statistically significant, were relatively small in magnitude and were not directionally consistent with differences in muscle thickness. Differences in fat partitioning were noted among frame and muscle subclasses. Large-framed steers had the lowest (P < .05) percentage of s.c. fat and the highest (P < .05) percentage of internal fat among frame sizes. No. 3 steers deposited the lowest (P < .05) percentage of intermuscular fat and the highest (P < .05) percentage of internal fat.
