The impact of sperm motility on non-return rate in preselected dairy bulls.

The objective of this study was to investigate whether variation in sperm motility, after preselection of bulls for this trait, has any impact on the non-return rate (NRR). The material consisted of pre- and post-freezing motility records and NRR records from 117 young Al bulls with at least 100 services each. Data representing 2 breeds at 2 different Al studs were analysed within studs. Both quadratic and linear regression analyses were performed to describe the relationship when the fixed effects of herd-year, month and order number of the insemination, parity and breed of the female, and the effects of service bull were considered simultaneously. The overall correlation with NRR was low. Motility before or after freezing never accounted for more than 1% of the variation in any of the adjusted NRR. These results showed that the remaining variation in motility was a poor predictor of NRR, which still showed a substantial variation between bulls. The significant b-values varied between 0.13 and 0.24. A motility threshold level of 50%, which is often stipulated for approval of the semen after freezing, does not seem to be the optimal choice. In order to avoid unnecessary culling, it is therefore advisable to reduce the level below which ejaculates should be rejected. The examination and selection for fertility should be completed by direct recording and evaluation of NRR. How far the motility level should be lowered to allow bulls to be tested for fertility in the field warrants investigation by utilizing semen with different degrees of reduced motility.