Repeatability of number of progeny born to bulls used in group mating of cows

The group mating of bulls in pasture situations is a management practice that might be more efficient if an individual bull’s ability to sire calves could be predicted. Retrospective data on numbers of progeny born to bulls from 4 populations (Angus and 3 composite breeds) in 4 consecutive years of spring calving were used to estimate repeatability of number of calves born per bull per season. Data only included bulls and cows 2 yr of age and older. Bulls were mated in cohort groups of 5 to 11 (median = 9) bulls assigned to 17 groups of 65 to 267 (median = 214) cows. The median ratio of cows to bulls was 22.8 (range = 10.8 to 26.0). The breeding season was 60 to 63 d for 3 yr and 49 d for the last year. Only bulls that remained in breeding pastures for the entire season were used for data analysis. Of these, 19 bulls were used in each of 2 populations and 20 were used in each of the other 2. Thirty-eight bulls were used for a single season, 23 were used for 2 consecutive seasons, 12 were used for 3 seasons, and 5 were used all 4 years. Parentage was determined by SNP genotyping. Ten bulls sired 0, 1, or 2 calves in a breeding season, 11 sired 42 to 56 calves in a season, and the median number was 17. Variances for number of calves and square root of number of calves sired by a bull in a season were estimated from a mixed model with bull as the random effect and mating group (confounded with year and population) as fixed effects. Repeatability was the ratio of bull variance to bull plus residual variance. Repeatability estimates were 0.62 for number and 0.60 for square root of number using all populations in a single analysis. Estimates ranged from 0.39 to 0.86 for number and 0.36 to 0.86 for square root of number when estimated within population. Number of calves sired by bulls used in groups of up to 11 bulls in one breeding season is a good indicator of the number sired in a subsequent season if bull health and fitness are not factors.