GENETIC AND OF PHENOTYPIC PARAMETERS FOR CARCASS TRAITS AMERICAN SHORTHORN BEEF-CATTLE

Carcass measurements were taken on 1,292 steers and collected by the A merican Shorthorn Association. The sires were purebred or appendix-reg istered Shorthorn. Because all dams were not Shorthorn, genetic fracti ons 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 an alyzed jointly with a multivariate REML algorithm to estimate heritabi lities of and genetic and phenotypic correlations among the traits. Th e sire model chosen as best fit of the data included fixed effects of herd of origin (377 classes), slaughter group (118 classes), year of b irth (1979-1995), and covariates for linear effects of genetic fractio ns of breeds (13) of dam and slaughter age, with sire (n = 409) as a r andom effect. Estimates of heritability were .60 +/- .19, .49 +/- .19, .46 +/- .19, .97 +/- .21, .45 +/- .19, .88 +/- .21, and .54 +/- .19 f or previous order of traits, respectively. Most genetic correlations w ere 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 genetical ly correlated with ribeye area (-.85 +/- .10) and positively genetical ly correlated with fat thickness (.67 +/- .15). Most phenotypic correl ations 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 ribey e area (-.16 +/- .04) and ribeye area with yield grade (-.61 +/- .03). Results seem to indicate that genetic antagonisms between quantity an d quality traits were small to moderate. Thus, the opportunity seems t o exist for breeding plans to improve carcass quality without having a ny 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 selectio n within breed.