Evaluation of carcass, live, and real-time ultrasound feedlot cattle: I. Assessment of sex and breed measures in effects

Carcass and live-animal measures from 1,029 cattle were collected at the Io wa State University Rhodes and McNay research farms over a 6-yr period. Dat a were from bull, heifer, and steer progeny of composite, Angus, and Simmen tal sires mated to three composite lines of dams. The objectives of this st udy were to estimate genetic parameters for carcass traits, to evaluate eff ects of sex and breed of sire on growth models (curves), and to suggest a s trategy to adjust serially measured data to a constant age end point. Estim ation of genetic parameters using a three-trait mixed model showed differen ces between bulls and steers in estimates of h(2) and genetic correlations. Heritability for carcass weight, percentage of retail product, retail prod uct weight, fat thickness, and longissimus muscle area from bull data were .43, .04, .46, .05, and .21, respectively. The corresponding values for ste er data were in order of .32, .24, .40, .42, and .07, respectively. Analysi s of serially measured fat thickness, longissimus muscle area, body weight, hip height, and ultrasound percentage of intramuscular fat using a repeate d measures model showed a limitation in the use of growth models based on p ooled data. In further evaluation of regression parameters using a linear m ixed model analysis, sex and breed of sire showed an important (P < .05) ef fect on intercept and slope values. Regression of serially measured traits on age within animal showed a relatively larger R-2 (62 to 98%) and a small er root mean square error (RMSE, .09 to 8.85) as compared with R-2 (0 to 58 %) and RMSE (.31 to 67.9) values when the same model was used on pooled dat a. We concluded that regression parameters from a within-animal regression of a serially measured trait on age, averaged by sex and breed, are the bes t choice in describing growth and adjusting data to a constant age end poin t.