A comparison between freezing methods for the cryopreservation of stallion spermatozoa

The effects of sperm freezing concentration (40 × 106 mL−1 vs. 400 × 106 mL−1), straw size (0.25 mL vs. 0.5 mL) and freezing method (liquid nitrogen vapour in a Styrofoam® box vs. programmable freezing machine) were evaluated in a 2 × 2 × 2 factorial experimental design using 3 split ejaculates from each of 4 stallions. Immediately after thawing, the total motility and forward progressive motility of spermatozoa frozen at a concentration of 40 × 106 mL−1 was higher than for spermatozoa frozen at 400 × 106 mL−1. No significant differences were observed in the semen parameters assessed after cryopreservation in either 0.25 or 0.5 mL straws. However, the programmable freezer provided a more consistent and reliable freezing rate than liquid nitrogen vapour. We conclude that an effective protocol for the cryopreservation of stallion spermatozoa at low concentrations would include concentrations of 40 × 106 mL−1 in 0.25 mL straws using a programmable freezer. This freezing protocol would be suitable for emerging sperm technologies such as sex-preselection of stallion spermatozoa as the sorting process yields only low numbers of spermatozoa in a small volume available for either immediate insemination or cryopreservation.