Use of real-time ultrasound to evaluate live animal carcass traits in young performance-tested beef bulls.

Young bulls were ultrasonically scanned to 1) study breed differences for 12th rib fat depth (USFAT) and longissimus muscle area (USREA), 2) evaluate the nature and predictability of USFAT and USREA development, and 3) determine the effect of age (YFAT and YREA, respectively) or weight (WREA) adjustments on USFAT and USREA. Angus (AA), Charolais (CH), Hereford (HH), Shorthorn (SS), and Simmental (SM) bulls (n = 886) were studied in 2 yr at two stations. Breed differences were observed for end of test USFAT (AA = SS > HH > CH = SM, P < .05) and USREA (CH = SM > SS = AA > HH, P < .05). Within-breed coefficients of variation were 29.8 to 48.4% for end of test USFAT and from 9.0 to 10.8% for end of test USREA. In most bulls, USFAT development was neither linear nor quadratic (P > .05), but USREA development was linear (P < .05). Prediction equations were characterized by low r2 and high residual standard deviation (RSD) values, although those for d-84 USREA had r2 values from .63 to .70 and RSD values from 4.09 to 5.80 cm2. High associations were obtained between end of test USFAT with YFAT (r = .83 to .91, P < .05) and USREA with YREA (r = .74 to .84, P < .05) but not for end of test USREA with WREA (r = .31 to .56, P < .05). These results indicate that end of test ultrasound measurements may be a useful addition to performance testing programs.