Evaluation of Carcass, Live, and Real-Time Ultrasound Measures in Feedlot Cattle: II. Effects of Different Age End Points on the Accuracy of Predicting the Percentage of Retail Product, Retail Product Weight, and Hot Carcass Weight1,2

Data from 970 feedlot steers and bulls were used to evaluate effects of different age end points on the accuracy of prediction models for percentage of retail product, retail product weight, and hot carcass weight. Cattle were ultrasonically scanned three to five times for fat thickness, longissimus muscle area, and percentage of intramuscular fat. Live animal measures of body weight and hip height were also taken during some of the scan sessions. Before development of prediction equations, live and ultrasound data were adjusted to four age end points using individual animal regressions. Age end points represented mean age at slaughter (448 d), mean age at the second-to-last scan before slaughter (414 d), mean age at the third-to-last scan before slaughter (382 d), and an age end point of 365 d. Ultrasound and live animal measures accounted for a large proportion of the variation in the dependent variables regardless of the age end point considered. For all three traits, final models based on independent variables adjusted to earlier ages of 365 and 382 d showed better or at least similar model R2 and root mean square errors than those based on independent variables adjusted to a mean slaughter age of 448 d. Validation of the models using independent data from 282 steers resulted in a mean across-age rank correlation coefficient of .78, .88, and .83 between actual and predicted values of the percentage of retail product, hot carcass weight, and retail product weight, respectively. Mean across-age rank correlation of breeding values for the corresponding traits were .92, .89, and .82. The results of this study suggest that live and ultrasound traits measured as early as 365 d could be used to predict end product traits as accurately as similar measures made before slaughter at age 448 d.