Factors influencing ultrasound estimates of carcass traits and their relationships with body weight, gain, scrotal circumference and hip height in feedlot bulls

Body weight (WT), scrotal circumference (SC), hip height (HHT) and ultrasou nd estimates of backfat thickness (BF), lean meat yield (LMY) and rib eye a rea (REA) were recorded in 64 bulls from two synthetic breed groups. The bu lls were fed either a high concentrate (85% grain) or low concentrate (55% grain) diet. WT, SC, HHT, BF, LMY and REA were recorded on days 100, 128 an d 156 of feedlot test. The data were analyzed by general linear model analy sis of variance to study the effects of energy level, breed group and their interaction on ultrasound estimates of BF, REA and LMY and their changes i n feedlot test. Stepwise regression analysis considering the separate effec ts of each diet and breed group was applied to study the relative influence s of final WT, SC and HHT on variation in the ultrasound estimates. The res ults indicated that HHT did not have any significant association with the c arcass traits. Breed group and level of energy did not have significant eff ects on the relationships between WT and SC with BF which were significant and positive. The regression coefficient of BF on average daily gain (ADG) was significant only on high concentrate diet. There was significant differ ence in the regression coefficient of REA on WT between the two breed group s. The regression of REA on BW was greater in DSX breed. The regression coe fficients of LMY on WT and SC were negative and significant. The regression of LMY on ADG was significant only at high level of concentrate in the die t. The level of energy in feedlot diet was a significant (P<0.01) source of variation for all the ultrasound carcass estimates. The bulls on high ener gy diet were significantly fatter (8.1 vs 4.9 mm BF) and had larger rib eye area (79.4 vs 64.3 cm(2)) compared to those on ow energy diet. Breed group did not have any significant effects on the ultrasound carcass measurement s. The results indicated that in general WT, SC and ADG had strong influenc es on the carcass traits and therefore, these measurements could be used to improve the prediction of carcass composition. However, these relationship s could be influenced by the genotype of animals and the level of energy in the feedlot diet in young beef bulls.