Sperm vitrification has been recently developed, but fertility trials have not been performed yet in equine species. In this study, a new warming technique for vitrified donkey semen was developed and the uterine inflammatory response and fertility were compared to conventional freezing. In Experiment 1, sperm was vitrified in straws and warmed in 3 ml of extender or in a water bath at: 37 °C/30 s; 43 °C/10 s; and 60 °C/5 s. Sperm motility, plasma and acrosome membranes and DNA integrity were compared between treatments. In Experiment 2, jennies were inseminated twice (500 × 10 sperm) in the uterine body either with vitrified or frozen semen (2 cycles/jenny). Pregnancy rates and the uterine inflammatory response (polymorphonuclear neutrophil concentration; PMN) were evaluated after artificial insemination (AI). No differences between warming in extender/water bath were found and 43 °C/10 s was better than lower temperatures in terms of total (53.8 ± 13.2%) and progressive sperm motility (41.4 ± 11.4%). No differences in PMN concentration (×10 PMN/ml) were found between vitrified (276.8 ± 171.6) or frozen (309.7 ± 250.7) semen after AI. However, PMN decreased faster (P < 0.05) using vitrified semen. Pregnancy rates were greater for vitrified (22%) than frozen semen (10%) but not statistically different. In conclusion, donkey sperm vitrified in straws could be directly warmed in a water bath at 43 °C/10 s, reducing the uterine inflammatory response obtained after AI and promoting positive pregnancy outcomes. These findings confirm the possibility to use vitrified semen as an alternative for AI in jennies.
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