Sperm characteristics and zona pellucida binding in relation to field fertility of frozen-thawed semen from dairy AI bulls.

The present study examined the relationship between bull sperm characteristics immediately post-thaw and some characteristics registered after swim-up, including the ability of spermatozoa to bind to homologous zona pellucidae (ZP) in vitro, and fertility after artificial insemination (AI) of 9426 females. Frozen-thawed semen from 22 AI bulls of the Swedish Red and White Breed, represented by 43 different frozen batches (1-4 batches/bull, 2 consecutive ejaculates/batch), was examined with the aim of determining concentration, motility patterns, morphology and membrane integrity. In addition, the frozen-thawed spermatozoa were subjected to a swim-up procedure and those separated in this way were tested with two assays of sperm-binding to the ZP of homologous oocytes in vitro (ZBA), using either a relative ZBA index against a control bull of proven high fertility or absolute binding (Absolute ZBA). The correlations of the various sperm traits and 56-day non-return rates (NRR) after field AI were retrospectively examined as single traits and as combinations of traits (combined measures), including regression analysis of significant traits. Among the sperm characteristics, positively significant (p < 0.01) correlations with NRR were found for linear motility post-thawing (r = 0.45-0.59) and the concentration of motile spermatozoa after swim-up (r = 0.43-0.63). Results obtained with the absolute ZBA approach were significantly (p < 0.05) correlated with NRR (r = 0.50), whereas the correlation between NRR and the ZBA index was not significant. The use of combined measures of sperm traits, including the ability to bind to ZP, showed a stronger predictive correlation with NRR (r = 0.68-0.75), compared with single traits. The results suggest that the combined analysis of sperm linear-motility patterns, swim-up separated sperm motility and absolute ZBA can provide a valuable assessment of the fertilizing capacity of AI bull semen.