Direct response to divergent selection for yearling growth rate in Angus cattle

An experiment to evaluate the effects of selection for yearling growth rate (average daily gain from birth to yearling age) on the major components of beef herd profitability was started in 1974. This paper describes the design, population structure, selection applied and the realised direct response to selection. Two divergent selection lines (high line and low line) and an unselected control line were created in a closed Angus herd. Generation intervals after 17 years of selection were 3.2 yr, 3.3 yr and 3.3 yr for high, control and low lines, respectively, resulting in a corresponding 5.5, 5.0 and 5.1 generations of selection. The high effective population sizes of 67, 106 and 71 per generation for high, control and low selection lines, respectively, resulted in a low rate of inbreeding per generation for each selection line (0.74% for high line, 0.53% for control line and 0.88% for low line). Average selection differentials per year were 0.016 kg/day and −0.018 kg/day for the high and low selection lines, respectively, resulting in a corresponding direct selection response per year of 0.006 kg/day and −0.007 kg/day. Using animals recorded prior to 1964 as the base, the mean estimated breeding value (EBV) for yearling growth rate at the start of the experiment (1974) was calculated to be 0.015 kg/day. After 17 years (1991) of selection average yearling growth rate EBV for high line calves was 0.115 kg/day, compared to 0.030 kg/day for the control line and −0.060 kg/day for the low line calves. Realised heritabilities were 0.37 ± 0.09 for the high line and 0.38 ± 0.09 for the low line.