Effect of Multiple Freezing of Stallion Semen on Sperm Quality and Fertility

• The effect of multiple freezing on sperm quality and fertility was evaluated in good and poor freezer stallions.
• Previously frozen stallion semen may be thawed, diluted, and refrozen with stepwise decreasing sperm motility, membrane, and acrosome integrity after each freeze/thaw cycle.
• Sperm chromatin integrity was unaffected after two and three freeze/thaw cycles.
• Pregnancy rates after insemination of mares with refrozen semen are drastically reduced.

To increase the efficiency of equine semen, it could be useful to split the artificial insemination dose and refreeze the redundant spermatozoa. In experiment I, semen of 10 sires of the Hanoverian breed, with poor and good semen freezability, was collected by artificial vagina, centrifuged, extended in INRA82 at 400 × 106 sperm/mL, and automatically frozen. After this first routinely applied freezing program, semen from each stallion was thawed, resuspended in INRA82 at 40 × 106 sperm/mL, filled in 0.5-mL straws, and refrozen. These steps were repeated, and sperm concentration was adjusted to 20 × 106 sperm/mL after a third freezing cycle. Regardless of stallion freezability group, sperm motility and sperm membrane integrity (FITC/PNA-Syto-PI-stain) decreased stepwise after first, second, and third freezing (62.3% ± 9.35, 24.0% ± 15.4, 3.3% ± 4.34, P ≤ .05; 29.6% ± 8.64, 14.9% ± 6.38, 8.3% ± 3.24, P ≤ .05), whereas the percentage of acrosome-reacted cells increased (19.5% ± 7.59, 23.9% ± 8.51, 29.2% ± 6.58, P ≤ .05). Sperm chromatin integrity was unaffected after multiple freeze/thaw cycles (DFI value: 18.6% ± 6.6, 17.2% ± 6.84, 17.1% ± 7.21, P > .05). In experiment II estrous, Hanoverian warmblood mares were inseminated with a total of 200 × 106 spermatozoa of two stallions with either good or poor semen freezability originating from the first, second, and third freeze/thaw cycle. First-cycle pregnancy rates were 4/10, 40%; 1/10, 10%; and 0/10, 0%. In conclusion, as expected, sperm viability of stallion spermatozoa significantly decreases as a consequence of multiple freezing. However, sperm chromatin integrity was not affected. Pregnancy rates after insemination of mares with refrozen semen are reduced.