HETEROGENEITY OF VARIANCES FOR CARCASS TRAITS BY PERCENTAGE BRAHMAN INHERITANCE

Heterogeneity of carcass trait variances due to level of Brahman inher itance was investigated using records from straightbred and crossbred steers produced from 1970 to 1988 (n = 1,530). Angus, Brahman, Charola is, and Hereford sires were mated to straightbred and crossbred cows t o produce straightbred, F-1, back-cross, three-breed cross, and two-, three-, and four-breed rotational crossbred steers in four non-overlap ping generations. At weaning (mean age = 220 d), steers were randomly assigned within breed group directly to the feedlot for 200 d, or to a backgrounding and stocker phase before feeding. Stocker steers were f ed from 70 to 100 d in generations 1 and 2 and from 60 to 120 d in gen erations 3 and 4. Carcass traits included hot carcass weight, subcutan eous fat thickness and longissimus muscle area at the 12-13th rib inte rface, carcass weight-adjusted longissimus muscle area, USDA yield gra de, estimated total lean yield, marbling score, and Warner-Bratzler sh ear force. Steers were classified as either high Brahman (50 to 100% B rahman), moderate Brahman (25 to 49% Brahman), or low Brahman (0 to 24 % Brahman) inheritance. Two types of animal models were fit with regar d to level of Brahman inheritance. One model assumed similar variances between pairs of Brahman inheritance groups, and the second model ass umed different variances between pairs of Brahman inheritance groups. Fixed sources of variation in both models included direct and maternal additive and nonadditive breed effects, year of birth, and slaughter age. Variances were estimated using derivative free REML procedures. L ikelihood ratio tests were used to compare models. The model accountin g for heterogenous variances had a greater likelihood (P < .001) than the model assuming homogeneous variances for hot carcass weight, longi ssimus muscle area, weight-adjusted longissimus muscle area, total lea n yield, and Warner-Bratzler shear force, indicating improved fit with percentage Brahman inheritance considered as a source of heterogeneit y of variance. Genetic covariances estimated from the model accounting for heterogeneous variances resulted in genetic correlations of or ne ar unity. These results suggest that different genetic values be consi dered for genetic evaluation of carcass yield and shear force traits f rom steers with different degrees of Brahman inheritance.