Heritabilities and Genetic, Environmental and Phenotypic Correlations of Carcass Traits in a Population of Diverse Biological Types and their Implications in Selection Programs

Genetic, environmental and phenotypic relationships among four growth and 12 carcass traits of 2,453 crossbred steers representing 16 different sire breeds were analyzed. Heritabilities were: birth weight .43; gain to weaning .07; feedlot gain .57; retail product (kg) .58, (percentage) .63; fat trim (kg) .47, (percentage) .57; bone (kg) .57, (percentage) .53; kidney fat (kg) .77, (percentage) .82; fat thickness .41; rib eye area .56; marbling .40 and Warner-Bratzler shear .31. The genetic and environmental correlations differed greatly in magnitude and sign among different trait combinations. Genetic regressions of component tissues on side weight showed that genetic aspects of higher growth rate led to increased growth of lean relative to fat. In contrast, environmental increases in growth rate led to relatively more fat deposition. Expected response was evaluated for six selection criteria at age constant and weight constant bases. When selection for growth rate was evaluated at a constant age, retail product and bone percentages declined and fat trim percentage increased because of increased carcass weight. The trends were reversed when adjusted to a constant slaughter weight. This suggests that selection for increased growth rate results in later maturing lean types. Selection response in fat thickness followed the pattern of response in fat trim percentage and response in rib eye area followed the response in retail product percentage, as expected from the high genetic correlations between these traits. Selection criteria, except gain to weaning, that increased retail product percentage also decreased marbling, but the decline generally was small. Mass selection for rate of gain and against external fat in record of performance programs will improve lean yield of carcasses when evaluated at a constant weight, removing the need for expensive and time consuming progeny tests, unless changes in other carcass traits are desired.