Effects of two simulated semen culling programs on predicted fertility in an artificially inseminated cow population.

Two semen culling programs designed to improve fertility of semen used in an artificial insemination program were compared. First inseminations and return service data on cows inseminated with semen from 21 Holstein bulls during 1 yr were studied. The culling of less fertile ejaculates within bulls, based on any available semen quality test or combination of tests, would bring about less than a 1% increase in nonreturn rate if 40 to 50% of ejaculates were called. If all semen from the 19% low fertility bulls were culled, this would entail culling 22% of the semen used to inseminate the population and would achieve an improvement of 1% in overall fertility. This could be done with no loss in predicted difference for milk yield of bulls used. Culling of bulls is a functionally simpler and less expensive program than is testing and culling ejaculates within bulls, but this requires that fertility of bulls be known and that it will not change substantially from predicted.