The effects of selection for increased pre-weaning growth rate on the post-weaning performance of composite breed steers

Abstract

Proc. A wt. Sot. Anim. Prod. 1994 Vol. 20 THE EFFECTS OF SELECTION FOR INCREASED PRE-WEANING GROWTH RATE ON THE POST-WEANING PERFORMANCE OF COMPOSITE BREED STEERS A.J. LYMBERY and G.D. TUDOR Dept of Agriculture, PO Box 1231, Bunbury, W.A. 6230 Selection for increased growth rate in beef cattle increases liveweight at all ages (Mrode 1988). The effect on other important traits, such as efficiency of feed use, has, however, been less well documented. Our aim was to examine the effects of selection for increased preweaning growth rate in composite breed cattle on postweaning gain and feed intake traits in yearling steers. The cattle were 5-breed composites, comprising approximately l/4 Brahman, Charolais and Friesian, and l/8 Angus and Hereford. Fourteen years of selection for increased preweaning growth rate at Wokalup Research Station in the south west of Western Australia has resulted in a genetic improvement of between 0.1 kg/day, as measured by changes in estimated breeding values, to 0.3 kg/day, as measured by difference from a contra! line (unpublished data). The growth rates, intakes, and feed conversion efficiency of 10 steers from this selection line were compared with 10 control steers, the progeny of semen stored from foundation bulls and cows randomly chosen from the selection line. The steers were individually fed to appetite in pens (100 m' on hay (in vitro dry matter digestibility (DMD) 628 g/kg and ) protein content (CP) 103 g/kg DM) for 77 days, and then grain (47% wheat, 30% lupin, 20% chaff, 1% urea; in vitro DMD 801 and CP 142 g/kg DM) for 63 days. Both diets were supplemented with a proprietary mineral mix. The growth rates of individual animals were calculated by regressing liveweight measured weekly against time, and all data were analysed by analysis of variance. Selected steers were 10% heavier on entering the feedlot (Table 1, P < 0.01). Growth rates were 6780% greater, and feed intakes (measured as kg DM/hd.day) 32-47% more on grain than on hay diets (P c 0.001 by repeated measures analysis of variance), although the effect of diet is confounded with age and stage of maturity in such an analysis. There were no significant differences for these variables between steers from selected and control lines, and no significant interactions between line and diet, but feed intake measured as g DM/kg liveweight.day was greater (P c 0.05) for control than for selected steers. Table 1. (2 se) liveweight, growth rate, feed intake and feed efficiency of control lines when fed hay and grain diets from selected and The reduced feed intake/kg liveweight in selected steers suggests that selection for increased growth rate has improved the biological efficiency of feed use in these animals. Herd et al. (1991) found Angus 0a steers selected for high yearlin, orowth rate to be more efficient than steers selected for low growth rate in feed use for maintenance, but not for liveweight gain. Our experimental design could not distinguish between maintenance and growth components of feed use. Gross feed efficiency, measured as the ratio 0 of liveweight gain to feed intake (on a per head, rather than per k, liveweight basis) did not differ between selected and control lines. HERD, R-M., SPECK, P.A. and WYNN, P.C. (1991). Aust. J. Exp. Agric. 31: 591-5. MRODE, R.A. (1988). A rzim. Breed. Abstr. 56: 155-67. 347