REAL-TIME ULTRASONIC MEASUREMENT OF FAT THICKNESS AND LONGISSIMUS MUSCLE AREA .2. RELATIONSHIP BETWEEN REAL-TIME ULTRASOUND MEASURES AND CARCASS RETAIL YIELD

Feedlot steers (n = 180) representing 11 sire-breed groups were ultras onically measured for fat thickness (FTU) and longissimus muscle area (LMU) at two 60-d intervals during the feeding period and four 21-d in tervals corresponding to serial slaughter dates to predict carcass ret ail yield parameters. Two fat trim levels, 8 and 0 mm, were used to ca lculate percentage of trimmable fat (FAT8P and FAT0P) and retail produ ct percentage (RPD8P and RPD0P) for each carcass. Regression technique s were used to evaluate best-fit equations that explained variation in retail product components. When FAT8P, FATOP, RPD8P, and RPDOP were r egressed on USDA yield grade (YG), R(2) values ranged from 75 to 76% ( P < .001). Comparatively, when live animal predictors of YG components (FTU, LIMU, and final live weight) were used as the independent varia bles, equations predicting retail yield had R(2) values of 61 to 65% ( P < .01). Equations using final FTU as the independent variable explai ned 58 to 64% (P < .001) of the variation in FAT8P, FAT0P, RPD8P, and RPD0P. Equations with FTU, LMU, and either WT, AGE, marbling, or quali ty grade resulted in R(2) values similar to those with only FTU, indic ating the strong influence of fat on retail yields. These results indi cate that ultrasonic predictors explained about 10% less variation in retail product percentage than did carcass measures.