Direct and Correlated Responses to Selection for Increased Weaning or Yearling Weight in Angus Cattle. I. Measurement of Selection Applied 1,2

Four 50-cow, C-sire lines of Angus cattle were established as part of a long-term selection project. Performance data through yearling age were collected on 2,749 calves during the 16-yr period, 1964 to 1979. Lines were selected for individual weaning weight, (WWL), individual yearling weight (YWL) and a combination of individual and progeny weaning weights (PTL). The fourth line was maintained as an unselected control line (CL) to monitor yearly environmental changes. Criteria in the CL were zero selection differentials for both weaning and yearling weights. Traits analyzed were birth weight (BW), preweaning daily gain (WDG), weaning weight (WW), weaning conformation grade (WG), weaning condition score (WC), weaning to yearling daily gain (YDG), yearling weight (YW), yearling conformation grade (YG) and yearling condition score (YC). Over the 16-yr period, 3.87 and 3.72 generations of selection had occured in the WWL and YWL, respectively. The PTL was terminated in 1978, and 2.68 generations of selection had occured to that point. Mean selection differentials in standard measure per generation for WWL, YWL and CL, respectively, were: BW, .43, .46, .00; WDG, .96, .70, .08; WW, .97, .74, .08; WG, .48, .37, .09; WC, .53, .30, .00; YDG, .11, .86, .37; YW, .67, 1.00, .25; YG, .43, .85, .33; and YC, .22, .48, .23. Selection indexes in retrospect were also calculated. (