GENETIC PARAMETER ESTIMATES OF ULTRASOUND-MEASURED LONGISSIMUS MUSCLEAREA AND 12TH RIB FAT THICKNESS IN BRANGUS CATTLE

Data from 2,101 Brangus calves born from 1986 to 1990 were analyzed wi th a REML procedure using a derivative-free algorithm in a mixed linea r animal model to obtain variance component estimates of ultrasound-me asured longissimus muscle area and fat thickness. Direct additive heri tabilities (h(a)2) of .39 and .40 were obtained for age-constant weani ng and yearling longissimus muscle area (WLMA and YLMA, respectively), with a genetic correlation (r(g)) of .66 between them. The r(g) of YL MA with birth weight (BWT), weaning weight (WWT), postweaning gain (PW G), yearling weight (YWT), frame score (FS), and scrotal circumference (SC) were .17, .29, .43, .38, .01, and .19, respectively. The h(a)2 o f age-constant yearling 12th rib fat thickness (FAT) was .14, and catt le averaged .44 cm (SD .19). Positive r(g) were obtained between FAT a nd WLMA (.19) and YLMA (.12). Negative r(g) of FAT with WWT, YWT, and SC were -.17, -.53, and -.33, respectively. Positive r(g) were obtaine d between FAT and BWT (.52), PWG (.44), and FS (.14). Maternal heritab ilities ( h(m)2 )of WLMA, YLMA, and FAT were .01, .01, and .10, respec tively. Weight-constant WLMA, YLMA, and FAT h(a)2 were .36, .39, and . 11, respectively. Selection based on either age-constant YLMA or FAT c ould potentially result in 1.06 cm2 or .005 cm change per year, respec tively, which would be slightly greater than change from selection bas ed on weight-constant YLMA or FAT. Selection based on WLMA or YLMA sho uld be effective, and changes in these traits, growth, and SC should b e possible in tandem. Genetic differences in ability to deposit fat pr obably were not expressed by these young cattle at the nutritional lev els provided. Response to selection based on FAT should be minimal, an d the effect of such selection on other traits is uncertain. There was no maternal influence on longissimus muscle areas; however, similar h (m)2 and h(a)2 for FAT indicate that variability in preweaning materna l environment has an effect on FAT equal to that of variability in an individual's own fattening ability.