Gluteus medius and rump fat depths as additional live animal ultrasound measurements for predicting retail product and trimmable fat in beef carcasses

This study was conducted to determine the ability of additional ultrasound measures to enhance the prediction accuracy of retail product and trimmable fat yields based on weight and percentage. Thirty-two Hereford-sired steer s were ultrasonically measured for 12th-rib fat thickness, longissimus musc le area, rump fat thickness, and gluteus medius depth immediately before sl aughter. Chilled carcasses were evaluated for USDA yield grade factors and then fabricated into closely trimmed, boneless subprimals with 0.32 cm s.c. fat. The kilogram weight of end-point product included the weight of trimm ed, boneless subprimals plus lean trim weights, chemically adjusted to 20% fat, whereas the fat included the weight of trimmed fat plus the weight of fat in the lean trim. Prediction equations for carcass yield end points wer e developed using live animal or carcass measurements, and live animal equa tions were developed. including ultrasound ribeye area or using only linear measurements. Multiple regression equations, with and without ultrasound r ump fat thickness and gluteus medius depth, had similar R-2 values when pre dicting kilograms of product and percentages of product, suggesting that th ese alternative variables explained little additional variation. Final unsh runk weight and ultrasound 12th-rib fat thickness explained most of the var iation when predicting kilograms of fat. Rump fat and gluteus medius depth accounted for an additional 10% of the variation in kilograms of fat, compa red with the equation containing final weight, ultrasound ribeye area, and ultrasound 12th-rib fat thickness; however, the two equations were not sign ificantly different: Prediction equations for the cutability end points had similar R-2 values whether live animal ultrasound measurements or actual c arcass measurements were used. However, when ultrasound ribeye area was exc luded from live animal predictions, lower R-2 values were obtained for kilo grams of product (0.81 vs 0.67) and percentages of product (0.41 vs 0.17). Conversely, the exclusion of ultrasound ribeye area had little effect on th e prediction accuracy for kilograms of fat (0.75 vs 0.74) and percentage fa t (0.50 vs 0.40). These data substantiate the ability of live animal ultras ound measures to accurately assess beef carcass composition and suggest tha t the alternative ultrasound measures, rump fat and gluteus medius depth, i mprove the accuracy of predicting fat-based carcass yields.