Relationship between the characteristics of frozen-thawed ram spermatozoa and in vitro embryo production

To select rams suitable for ovine in vitro embryo production (IVP), the predictive values of the screening tests used to identify unsuitable rams need to be established. The present study examined some characteristics of frozen-thawed ram spermatozoa that might be evaluated routinely in a commercial breeding programme. These included sperm motility, plasma membrane integrity, morphology, and acrosome and capacitation status of the sperm population. Cryopreserved spermatozoa from four Dorset rams, which had previously satisfied the selection criteria for inclusion in a commercial breeding programme, were used for IVP. The overall contribution of the four rams and the ejaculates within each ram to the variability (R2) in the production of blastocysts was very small (2.1% and 2.5% respectively). The analysis of the sperm characteristics by logistic regression revealed a significant and positive association between total post-thaw sperm motility, viability and longevity with in vitro blastocyst production. However, there was no association between the other surface characteristics of the spermatozoa measured in this study with embryo production. Despite the absence of differences between the rams in the low incidence of polyspermic fertilization, the significant and detrimental effects of polyspermic fertilization on in vitro blastocyst production rates were quantified by logistic regression analysis. A large proportion of the variability within the IVP system was unaccounted for by the analysis of sperm and oocyte characteristics evaluated in this study. Thus, the identification of other factors contributing to the variability in the production of embryos in vitro warrants further investigation. No single sperm characteristic was sufficient to predict the ultimate outcome of blastocyst production. Rather, assessments of multiple characteristics within the IVP system are required to make accurate predictions.