EVALUATION OF CARCASS TRAITS IN YEARLING BEEF BULLS USING REAL-TIME ULTRASOUND
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Two experiments were undertaken to assess the potential application of real-time ultrasound (RTUS) to evaluate live animal carcass traits in yearling bulls. In experiment 1, RTUS was used to examine breed variation for subcutaneous fat depth (USFAT) and longissimus dorsi et lumborum area (USREA); study USFAT and USREA growth during a 112 day performance test; determine if end of test (EOT) RTUS traits could be predicted from measurements taken earlier in the test; and to determine the effect of age (YFAT, YREA) and weight adjustments (WREA) on RTUS measurements. Angus (AA), Charolais (CH), Hereford (HH), Shorthorn (88) and Simmental (8M) bulls (n= 886) were studied over two years at two stations. Bulls were weighed and scanned at the 12/13th rib site using an Aloka SOOV RTUS machine (3.SMHz, 17cm transducer) at 28 day intervals. The CH and 8M breeds had less (P < 0.05) USFAT and YFAT than all other breeds, but did not differ from each other. Breed differences were also noted for EOT USREA (CH= SM > SS = AA > HH, P < 0.05), YREA (CH SM > SS = AA > HH, P < 0.05) and WREA (CH > SM > 8S = AA = HH, P < 0.05). Coefficients of variation ranged from 29.8% to 48.4% for USFAT and from 9.0% to 10.8% for USREA. Low correlations were noted between EOT RTUS measurements and measurements taken prior to the end of test. The associations were too low to be of predictive value. Regression of RTUS parameters on days on test indicated that USFAT and USREA growth were predominantly linear (p < 0.05). Breed differences were noted in USFAT (SS > AA > HH > SM= CHI P > 0.05) and USREA (CH= SM > SS = AA= HHI P < 0.05) growth rates. However, the low R2 (0.28 to 0.49 for USFAT and 0.47 to 0.55 for USREA) and high RSD (1.02 mm to 1.25 mm for USFAT and 6.37 to 8.15 cm2 for USREA) values confirm that it is difficult to predict EOT RTUS measurements from measurements taken earlier in the test. Across breeds, the correlation between USFAT and YFAT at EOT was 0.92 (P < 0.05), and correlations between USREA with YREA and WREA were 0.88 and 0.501 respectively (P < 0.05) at EOT. The low association between USREA and WREA indicates that body weight should be taken into consideration when USREA is compared among animals that differ in body weight. Experiment 2 assessed the repeatability and accuracy of preslaughter USFAT and USREA measurements, and the value of these measurements as predictors of carcass lean meat yield (CARLEAN) in yearling AA (n= 41) and CH (n= 41) bulls (Group 1). Carcass lean yield (BTLEAN) was predicted from carcass measurements using the cutability equation developed for the Agriculture Canada Blue Tag program. Actual carcass lean yield (CARLEAN) was estimated by physical dissection of a seven-bone rib section from each carcass. Stepwise regression was used to develop equations to predict CARLEAN from both live (USFAT and USREA; USLEAN) and carcass measurements (GFAT and GREA; GLEAN). Three additional data sets were used in this study. Group 2 (n=135) was used to determine if the growth and ultrasound traits of the 82 dissected bulls were representative of purebred ROP bulls of the same breeds. Group 3 (n= 52) consisted of slaughtered ROP bulls for which both RTUS and carcass data were available. Group 4 consisted of AA (n= 25), CH (n= 22), HH (n= 23), SM (n= 19) and Limousin (n= 22) bulls that were ultrasonically scanned, slaughtered and dissected (7- bone rib). No significant differences in growth or RTUS characteristics were detected between the Group 1 and 2 bulls (P > 0.05). The standard error of repeated measures (SER) was 0.65 mm and 4.00 cm2 for repeated USFAT and USREA measurements, respectively (Group 1). The standard error of prediction (SEP) between RTUS and carcass measurements was 1.07 mm and 5.64 cnf for fat depth and 1. dorsi area, respectively (Group 1). The live animal lean yield prediction equation (USLEAN) had higher R2 (0.73) and lower RSD (1.79 g 100g-1) than the prediction equation based on the corresponding carcass measurements (GLEAN; R2= 0.69, RSD = 1.94 g 100g-1). Results indicated that the USLEAN equation predicted CARLEAN better (SEP= 1.80 g 100g-1) than BTLEAN (SEP= 2.26 g 100g-1) or GLEAN (SEP = 1.91 g 100g-1) in the Group 1 bulls. The correlation between USLEAN and BTLEAN was 0.81 (P < 0.05) for Group 2 bulls used to test the prediction equation. Results from the Group 4 bulls used to test the USLEAN equation verified that live measurements can predict CARLEAN as effectively as carcass measurements (SEP= 2.28 g 100g-1, 2.26 g 100g-1 and 2.57 g 100g-1 for USLEAN, BTLEAN and GLEAN). These results confirm that variation exists both within and between breeds for RTUS traits and their development rates. Based on the results of this study, the prediction of yearling RTUS traits from measurements taken prior to EOT is difficult. Adjustment of EOT RTUS traits for age differences does not appear to be necessary when animals are similar in age, but USREA should be adjusted for differences in weight. RTUS measurements taken on yearling bulls can be highly repeatable and accurate. Live animal USFAT and USREA measurements can predict carcass lean content as accurately as the corresponding carcass measurements in yearling bulls.