FACTORS INFLUENCING FEED EFFICIENCY AND BACKFAT THICKNESS IN STATION TESTED BEEF BULLS

Records taken on 372 young beef bulls tested at the Ellerslie Bull Test Station, Alberta, Canada from November 1981 to April 1987 were analyzed to quantify the effects of age of dam, on-test age, on-test liveweight and herd of origin of bull on feed efficiency (feed/gain, kg/kg) in the test period (n = 231) and ultrasonic measurement of bakcfat thickness (mm) at the end of the test (n = 372). The reduction in $R^2$ due to each influencing factor (i.e. the variation accounted for by the factor) was used to indicate the importance of the influencing factor. Age of dam and on-test age of bull were not important factors on feed/gain and ultrasonic backfat thickness, as they accounted for less than 0.5% of the variation in feed/gain and ultrasonic backfat thickness, respectively (p > 0.1). On-test liveweight had some influence on feed/gain and ultrasonic backfat thickness, accounting for 3.5% (p < 0.01) and 0.4% (p < 0.05) of the total variation, respectively. The regression coefficients of feed/gain and ultrasonic backfat thickness on on-test liveweight were 0.016 (kg/kg)/kg and .013 mm/kg, respectively, both being significant (p < 0.05), indicating that lighter bulls entering the test were generally more efficient in feed utilization in the test period and had less backfat at the end of the test than heavier entering bulls. Herd of origin of bull accounted for a substantial amount of the total variation (> 16%) in feed/gain and ultrasonic backfat thickness (p = 0.08), indicating that a prolonged aqjustment period was needed to reduce the influence of herd of origin when assessing aggregate genetic merit of beef bulls for growth rate, feed efficiency and lean meat production using a central station performance testing program.