Sperm morphology and fertility of progeny-tested AI Swedish dairy bulls

Use of bull semen with high levels of sperm abnormalities, reflecting genital dysfunction, is not recommendable to use for AI since it most likely leads to sub-fertility. Sperm morphology deteriorates with age, a source of concern when using ageing progeny-tested AI-bull sires. However, whether a relationship between sperm morphology and fertility after AI in progeny-tested bull sires is present remains to be proven and constituted the reason for a retrospective study of progeny-tested bull sires of the Swedish Red and White (SRB) and Swedish Holstein (SLB) breeds performed with data of 8 SRB and 4 SLB sires analysed in an University specialized andrological laboratory. Particular attention was paid to the influence of age and breed as well as the variation between- and within-bull for presence and level of sperm abnormalities. Sperm morphology differed between sires, ejaculates and breed, with 6/12 sires having ejaculates with more than 10% of morphologically deviating sperm heads, a commonly used threshold for young At bulls. However, with the exception of pear-shaped or narrow at the base anomalies, individual defect mean values were always within what is expected for a bull sire and considered within acceptable limits. Breed and age affected some semen (ejaculate volume) and sperm morphology variables (nuclear pouches, acrosome defects and head contour). Estimated breeding values for male fertility output differed significantly among bulls, with the percentage of morphologically normal spermatozoa in the overall ejaculate being positively related to fertility. Few individual sperm abnormalities correlated negatively to fertility, as pear-shaped sperm heads, proximal droplets or loose heads (SRB), probably owing to the reduced sire material examined. In conclusion, the presence of relationships between sperm morphology and fertility after AI calls for routine, frequently done assessments of sperm morphology in AI-stud bull sires during their entire active life.