The effect of antioxidant agents' addition and freezing method on quality parameters of frozen thawed ram semen

The aim of this study was to evaluate the effect of antioxidant agents and freezing methods on the ability of ram sperm to preserve its post-thaw quality characteristics. Six Chios rams were subjected to 52 weekly semen collections. Each ram was used as semen donor for freezing experiments once every 2 weeks. Equal number of good quality spermatozoa from each ejaculate (concentration ≥1 × 10⁹ spermatozoa/ml, motility ≥70%, motility score ≥3.5) were pooled. Three equal aliquots of the pooled sample were diluted using three different fractions of a milk-based and glycerol extender (control, quercetin-enriched, α-tocopherol-enriched). Three freezing methods were applied (slow and fast freezing rate in a programmable freezer, vapors of liquid nitrogen) in every aliquot. Sperm aliquots were tested before freezing, immediately after thawing and after 3 h of incubation at 37 °C. Sperm motility (%) was evaluated microscopically. The percentage of membrane and acrosome-intact spermatozoa (IL%) as well as the percentage of membrane-intact and acrosome-reacted spermatozoa (ARL%) were determined by eosin-nigrosin stain. Furthermore, the percentage of hypo-osmotic swelling (HOS) test-positive spermatozoa was estimated. The results revealed no beneficial effect of the antioxidant treatment on the parameters of post-thaw semen (P > 0.05). However, the slow freezing rate method was more beneficial regarding motility, IL, ARL and HOS-positive spermatozoa compared to the other methods. In conclusion, the antioxidant agents used in this study failed to protect sperm against cryopreservation stress; however, the choice of the appropriate freezing method could contribute to the improvement of post-thaw ram sperm quality.