Selection in Beef Cattle. I. Selection Applied and Generation Interval2

Selection applied and generation interval were studied in three 150-cow-6-sire lines selected for (1) weaning weight (WWL), (2) yearling weight (YWL), or (3) an index of yearling weight and muscling score (IXL). Data were birth weights, weaning weights, yearling weights and muscle scores of 2,956 calves raised at the Fort Robinson Beef Cattle Research Station, Crawford, Nebraska during the years 1960 to 1970. Formulas were presented for calculating average cumulative selection differentials and generation coefficients in populations with overlapping generations. Generation interval based on actual age of parents for the 2,956 calves weaned was 4.6 years. Sire age was 4.1 years and dam age was 5.1 years. By 1970 an average of 2.0, 1.8 and 1.9 generations of selection had been practiced in the weaning weight, yearling weight and index lines, respectively. Average sire selection differentials, per generation, in WWL, YWL and IXL were, respectively: weaning weight 1.51, 1.18, 0.86; yearling weight 1.42,1.79, 1.22; muscling score 0.51, 1.11, 2.13 standard deviations.