REAL-TIME ULTRASONIC MEASUREMENT OF FAT THICKNESS AND LONGISSIMUS MUSCLE AREA .1. DESCRIPTION OF AGE AND WEIGHT EFFECTS

Serial ultrasonic measures of fat thickness (FTU) and longissimus musc le area (LMU) were taken on 180 feedlot steers representing 11 sire-br eed groups at two 60-d intervals. Cattle then were slaughtered in four groups of 45 at 21-d intervals with ultrasonic measures taken at each interval. Analyses of variance indicated weight (WT) effects (P < .00 1) for all FTU measures and many of the LMU measures. Age was a source of variation (P < .05) for FTU and LMU over the first five measuremen t dates. Sire-breed (SBD) effects (P < .001) were prevalent for all FT U estimates and early LMU predictions. Some dam line and SBD x dam lin e interactions were detected (P < .1) for the FTU and LMU estimates. A ge effects were described quadratically (P < .001) when FTU and LMU we re regressed on age (R(2) = 47 and 68%, respectively). When FTU and LM U were regressed on WT, effects were described quadratically (P < .001 ) with R(2) values of 46 and 72%, respectively. Animals then were assi gned to one of four biological types (BT) based on percentage of retai l product (0-mm fat trim) using discriminant cluster analysis. Within BT, ultrasonic variables were regressed on age. Quadratic effects (P < .01) were detected for both FTU and LMU. Weight also contributed to v ariation observed in both ultrasonically measured variables for all BT (P < .001). These data indicate that changes in fat and muscle size a s affected by age and weight are detectable using serially collected u ltrasound measures. Furthermore, the age and weight relationships of p redicted FTA and LMA varied across the four biological types. This sug gests a need for derivation of biological type-specific age and(or) we ight adjustment equations for cattle evaluation purposes.