Effect of Season on the Characteristics of Warmblood Stallion Spermatozoa Stored in a Liquid State at 5 °C

The aim of this study was to investigate the effect of season (breeding vs. non-breeding) on the characteristics of warmblood stallion spermatozoa during liquid storage. Ejaculates were collected from eight stallions during the breeding and non-breeding seasons (March-July and September-December, respectively) and were diluted in an EquiPro extender. Semen was stored for up to 96 h at 5 °C. Analysis of stored sperm included the assessment of motility (CASA system), acrosomes with normal apical ridges (NAR), plasma membrane integrity (PMI), mitochondrial membrane potential (MMP), and adenosine triphosphate (ATP) content. In sperm stored for up to 24 h, the values of the following parameters were higher (<i>p</i> ≤ 0.05) in March-July than in September-December: progressive motility (PMOT), PMI, MMP, beat cross frequency (BCF), and linearity (LIN). The values of the studied parameters decreased during storage, and the observed changes were affected by season. The values of NAR, PMI and MMP, average path velocity (VAP), straight line velocity (VSL), and curvilinear velocity (VCL) decreased (<i>p</i> ≤ 0.05) after 24 h of storage during the breeding season, whereas a significant decrease in these parameters was observed after prolonged storage (48 h or longer) during the non-breeding seasons. Cooled sperm of warmblood stallions collected during the breeding season was characterized by higher initial quality than the sperm collected during the non-breeding season. However, sperm sampled during the non-breeding season appeared to be more resistant to cold shock, which increases their suitability for cold storage at 5 °C. Stallion sperm sampled in both seasons were characterized by similar suitability for liquid storage. Further research is needed to assess the fertilizing capacity of stored sperm.