Abstract
Carcass measurements were taken on 1,292 steers and collected by the American Shorthorn Association. The sires were purebred or appendix-registered Shorthorn. Because all dams were not Shorthorn, genetic fractions of breeds of origin were determined for each dam. Measurements for hot carcass weight; dressing percentage; fat thickness; ribeye area; kidney, pelvic, and heart fat (KPH); marbling; and yield grade were analyzed jointly with a multivariate REML algorithm to estimate heritabilities of and genetic and phenotypic correlations among the traits. The sire model chosen as best fit of the data included fixed effects of herd of origin (377 classes), slaughter group (118 classes), year of birth (1979-1995), and covariates for linear effects of genetic fractions of breeds (13) of dam and slaughter age, with sire (n=409) as a random effect. Estimates of heritability were .60+/-.19, .49+/-.19, .46+/-.19, .97+/-.21, .45+/-.19, .88+/-.21, and .54+/-.19 for previous order of traits, respectively. Most genetic correlations were not significantly different from zero. Genetic correlations of hot carcass weight were significant and positive with dressing percentage (.65+/-.19) and with ribeye area (.70+/-.14). Dressing percentage was significantly positively genetically correlated with ribeye area (.79+/-.16) and negatively genetically correlated with yield grade (-.56+/-.29). Yield grade was also significantly negatively genetically correlated with ribeye area (-.85+/-.10) and positively genetically correlated with fat thickness (.67+/-.15). Most phenotypic correlations were significant and positive. Only the phenotypic correlations of dressing percentage with marbling and with yield grade, and ribeye area with KPH and with marbling were not significantly different from zero. Significant negative correlations were fat thickness with ribeye area (-.16+/-.04) and ribeye area with yield grade (-.61+/-.03). Results seem to indicate that genetic antagonisms between quantity and quality traits were small to moderate. Thus, the opportunity seems to exist for breeding plans to improve carcass quality without having any adverse genetic effect on hot carcass weight, dressing percentage, or ribeye area. The high heritability observed for marbling indicates that a low genetic potential for marbling can be remedied by selection within breed.
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