GENETIC PARAMETER ESTIMATES FOR CARCASS AND YEARLING ULTRASOUND MEASUREMENTS IN BRANGUS CATTLE

Carcass measurements of 12th-rib fat thickness (CARCFAT), longissimus muscle area (CARCLMA), and weight (CARCWT) on 2,028 Brangus and Brangu s-sired fed steers and heifers, as well as yearling weights (YWT) and ultrasound measures of 12th-rib fat thickness (USFAT) and longissimus muscle area (USLMA) on 3,583 Brangus bulls and heifers were analyzed t o estimate genetic parameters. Data were analyzed using a six-trait an imal model and an average information REML algorithm. The model includ ed fixed effects for contemporary group and breed of dam, covariates f or age at slaughter or measurement, and random animal and residual eff ects. Heritabilities for CARCFAT, CARCLMA, CARCWT, USFAT, USLMA, and Y WT were .27 +/- .05, .39 +/- .05, .59 +/- .06, .11 +/- .03, .29 +/- .0 4, and .40 +/- .04, respectively. Genetic correlations between CARCFAT and USFAT, CARCLMA and USLMA, and CARCWT and YWT were .69 +/- .18, .6 6 +/- .14, and .61 +/- .11, respectively. The favorable and moderately strong genetic correlations between carcass measurements and similar yearling breeding-animal ultrasound measurements indicate that such me asurements of 12th-rib fat and longissimus muscle area are useful in p redicting genetic values for carcass leanness and longissimus muscle a rea. Selection using yearling ultrasound measurements of breeding catt le should result in predictable genetic improvement for carcass charac teristics. Inclusion of yearling ultrasound measurements for fat thick ness and longissimus muscle area should enhance national cattle evalua tion programs.