Sources of Variance Affecting Dairy Herd Fertility and Delayed Returns to Service

Fertility and delayed return data were obtained for 20 or more cows bred in 23 herds in each of 22 locals of a Wisconsin cooperative for four consecutive years. It was found that 19.8% of the variation in annual herd fertility of 10-cow herds was due to differences among years, locals, and herds while 80.2% was random. Repeatability of herd fertility, as measured by the number of cows conceiving per 10-cow herd sample, was 0.17 within years and 0.10 for successive years. The herds averaged 33.1 first services each year with 18.1 total returns from all services. Cows returning for service after 26 days or more were considered delayed. Differences among herds within locals accounted for 29.0% of the variation in percentage of returns that was delayed. Variances due to years, locals, years locals, and years herds were rather negligible (1.6% or less). The repeatability for percentage of delayed returns in herds on successive years was about 0.39 whereas for individual cows, the repeatability of number of days between services was only about 0.15.