Effects of Breed and Age Classification on Live Weight and Carcass Traits of Steers Shown at International Quality Beef Show

Live weight and carcass data were collected on 1,419 Angus, 627 Hereford and 457 Shorthorn steers shown at the International Quality Beef Show from 1956 through 1967. These steers were grouped into three age classes. Herefords were the heaviest and Angus the lightest for live weight. Differences among breeds for carcass weight were small and non-significant. Hereford steers had the lowest unadjusted and adjusted fat thickness values and the lowest marbling scores of the three breeds. Angus had the largest area of longissimus and the highest marbling score while Shorthorn had the smallest area of longissimus. The two traits, for which breed effects were the largest, were area of longissimus and marbling score. Of the traits included in this study, these are the two traits that probably would be least influenced by environment. Although all comparisons among breeds for area of longissiums were significantly different (P<.05), the larger differences were due to the smaller longissimus area of the Shorthorn steers, (68.3 for Shorthorn vs. 75.5 and 74.3 cm2, respectively, for Angus and Hereford). Least square means for marbling score were 8.0, 7.4 and 6.8, respectively, for Angus, Shorthorn and Hereford. Live weight, carcass weight, area of longissimus, fat thickness, and marbling score significantly increased with increasing age as measured by age classification. Adjustment of longissimus area by dividing by carcass weight (a common adjustment in beef carcass evaluation) would not be recommended since areas of longissimus were larger and longissimus/100 kg of carcass were smaller for the older age classes. The covariate, carcass weight, greatly reduced differences among age classes for longissimus area and especially for fat thickness.