Cattle and Beef Quality CRC 2 WHEN PASTURES LIMIT GROWTH RATE OF STEERS THOSE BRED FOR LOW RESIDUAL FEED INTAKE GROW FASTER

Growth on pasture and body composition before feedlot entry was measured on Angus and Angus-cross steers born over five years from parents selected for low postweaning residual feed intake (RFI, high efficiency, 271 steers from 31 sires) or selected for high RFI (low efficiency, 250 steers from 28 sires), and 40 steers from an intermediate unselected line created in the last 2 years. After 1.5 generations of divergent selection, least-squares means for initial weight for low-RFI line and high-RFI line steers did not differ (252 v 250kg; P>0.05), but the low-RFI line steers grew 4.4 % faster (P<0.05) than high-RFI line steers (0.66 v 0.64kg/day) to be 2.1 % heavier (P=0.07) at the end of backgrounding (418 v 409kg). At that time the low-RFI line steers had less (P<0.05) subcutaneous fat over the rib and rump (3.2 v 4.2mm; 4.4 v 5.3 mm) and a trend toward slightly smaller eye-muscle area (52.1 v 52.6cm2; P=0.06). Significant (P<0.05) regressions for daily growth rate, final weight and the two fat depth measurements, with midparent estimated breeding value for RFI, provided further evidence for genetic associations with postweaning RFI