EFFECTS OF FEEDER CATTLE FRAME SIZE, MUSCLE THICKNESS, AND AGE CLASS ON DAYS FED, WEIGHT, AND CARCASS COMPOSITION

Feeder steers (n = 189) representing nine frame size x muscle thicknes s subclasses were started on a high-concentrate diet as calves, yearli ngs, or long yearlings and slaughtered at a constant s.c. fat thicknes s (13.5 mm) to determine the effects of feeder cattle frame size (larg e, medium, small), muscle thickness (No. 1, No. 2, No. 3), and age cla ss on carcass traits and composition. After slaughter, one side of eac h carcass was physically separated into muscle, fat, and bone. Subcuta neous, intermuscular, and internal fat depot percentages were calculat ed as proportions of total fat weight. Effects of age class, frame siz e, and muscle thickness were significant for time-on-feed, slaughter w eight, and carcass weight. Among age classes, long yearling steers req uired the fewest (P < .05) days on feed and heaviest (P < .05) live we ights to reach the fat thickness end point. Increased frame size and d ecreased muscle thickness were associated with greater time-on-feed an d heavier weights at slaughter. However, the effects of muscle thickne ss on slaughter traits were not consistent among frame and age subclas ses; effects were greatest among large-framed steers and smallest amon g 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:b one ratio among muscle thickness groups; no (P > .05) compositional di fferences 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 differen ces in muscle thickness. Differences in fat partitioning were noted am ong 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) perce ntage of internal fat.