Sperm chromatin structure and sperm morphology: Their association with fertility in AI-dairy Ayrshire sires

Prognostic relations between sperm variables and sire fertility are yet elusive. A retrospective analysis of sperm morphology and chromatin stability (studied using sperm chromatin structure assay [SCSA]) and their relation to fertility after AI (as proportions of 60 days of nonreturn to estrous [NRR], corrected NRR, or calving rate) was studied with preselected frozen semen doses from a group (N = 43) of AI-sires of the Finnish Ayrshire breed composed of 50% subfertile bulls (<55% NRR) and 50% fertile sires (>55% NRR). Fertility, indicated by all three parameters, correlated significantly only with the percentage of morphologically normal spermatozoa, a variable which negatively correlated with the percentage of DNA fragmentation at the time of SCSA, thus confirming the value of always having high numbers of morphologically normal spermatozoa in AI-doses. Proportions of major sperm defects also related to fertility but only when considering corrected NRR, not with calving rate, indicating that proportions of normal spermatozoa, a value surpassing differences between sperm laboratory screening methods, might be valuable and could be easily made routine by the industry. Though SCSA as a method is being contested for DNA- and chromatin analyses in the light of epigenetic changes, a particular parameter, the High Green fluorescence, showed the highest values for sperm doses collected from bulls having meiotic problems and containing a high proportion of diploid spermatozoa (approximately 20%) and also in bulls having a reciprocal chromosomal translocation, thus suggesting such a parameter might be useable to discriminate which bulls ought to be studied in more detail, including cytogenetic analyses.