RESULTS OF SELECTION FOR YEARLING OR 18-MONTH WEIGHT IN ANGUS AND HEREFORD CATTLE

A selection experiment was carried out in Angus cattle to compare the effectiveness of selection on 13-month weight (with first mating at 14 months of age - AS1 line) or on 18-month weight (with first mating as 2-year-olds - AS2 line), alongside a control Angus herd (AC0 line). A yearling mated Hereford herd, selected on 13-month weight (HS1 line) was also established. Data from calves born in 1972-1986 were analysed for birth weight (BW), weaning weight (WW), 9-month weight (9W), yearling weight (YW) and 18-month weight (18W). By 1986 there had been an average of 3.4, 3.8, 4.9 and 4.6 generations of selection in the AC0, AS2, AS1 and HS1 lines, respectively. Corresponding generation intervals were 4.4, 4.4, 3.2 and 3.3 years, respectively, and selection differentials (standard deviations per generation) averaged 1.04 and 0.86 for YW in the AS1 and HS1 lines and 1.02 for 18W in the AS2 line. The AS1 annual response was about twice the AS2 response, for all traits analysed. Realised heritabilities when responses were obtained as deviations from control were 0.31 +/- 0.04 (YW: AS1 line), 0.33 +/- 0.06 (YW: HS1 line), and 0.23 +/- 0.05 (18W: AS2 line). Corresponding values after adjusting for genetic trend in the control were 0.40 +/- 0.05, 0.45 +/- 0.06 and 0.35 +/- 0.05, respectively. The realised genetic correlation between YW and 18W, derived from the direct and correlated responses in the AS1 and AS2 lines, was 0.96 +/- 0.12.