GENETIC-VARIATION AND PREDICTION OF ADDITIVE AND NONADDITIVE GENETIC-EFFECTS FOR 6 CARCASS TRAITS IN AN ANGUS-BRAHMAN MULTIBREED HERD

Estimates of covariances and sire expected progeny differences of addi tive and nonadditive genetic effects for six carcass traits were obtai ned using records from 486 straightbred and crossbred steers from 121 sires born between 1989 and 1995 in the Angus-Brahman multibreed herd of the University of Florida. Steers were slaughtered at a similar car cass composition end point. Covariances were estimated by REML procedu res, using a generalized expectation-maximization algorithm applied to multibreed populations. Straightbred and crossbred estimates of herit abilities and additive genetic correlations were within ranges found i n the literature for steers slaughtered on an age- or weight-constant basis for hot carcass weight, longissimus muscle area, and shear force but equal to or less than the lower bound of these ranges for fat-rel ated traits. Maximum values of interactibilities (i.e., ratios of nona dditive variances to phenotypic variances in the F-1) and nonadditive genetic correlations were smaller than heritabilities and additive gen etic correlations in straightbreds and crossbred groups. Sire additive and total direct genetic predictions for longissimus muscle area, mar bling, and shear force tended to decrease with the fraction of Brahman alleles, whereas those for hot carcass weight and fat thickness over the longissimus were higher, and those for kidney fat were lower in st raightbreds and F-1 than in other crossbred groups. Nonadditive geneti c predictions were similar across sire groups of all Angus and Brahman fractions. These results suggest that slaughtering steers on a simila r carcass composition basis reduces variability of fat-related traits while retaining variability for non-fat-related traits comparable to s laughtering steers on a similar age or weight basis. Selection for car cass traits within desirable (narrow) ranges and slaughter of steers a t similar compositional end point seems to be a good combination to he lp produce meat products of consistent quality.