Relationships between reproductive success of young bulls and weight at selection and joining time

Abstract

A trial was designed to investigate the responses to selection within 3 closed herds. Six weight-selected yearling bulls were used each year for single sire paddock straightbred joinings in an Angus and Hereford selection herd (coded AS1 and HS1) from 1971 to 1980. Five weight-selected bulls were used each year as 2-year-o1ds, in another selection herd (AS2) during the same years. Heifers and cows within each herd were allocated to bulls by balancing for cow age, calving date, and avoiding close relatives. 167. bulls were joined with 25–30 females (including 25% virgin heifers), in balanced numbers per bull per herd in any 1 year, for periods averaging 70 days (1971–1975), and 58 days (1976–1980). Reproduction data within each herd were examined. Effects of year on selection and joining weights of bulls were highly significant in 2 of the herds, but not in the third (AS1) herd. The within-herd residual standard deviation was 22 kg for joining weight. There were few significant year effects on pregnancy rate or per cent calves born, but year effects on herd mean calving dates were significant. Average residual standard deviations were 14.5%, 15.5%, and 5.2 days respectively. Bull weight within-year was not a significant covariate. There was no difference between the mean joining weight of fertile or subfertile bulls where the classification was based on pregnancy rates achieved.

Keywords:

• cattle
• selection
• reproduction
• fertility
• breeds
• Hereford
• Angus
• bull weight