“Comparison of the quality of frozen-thawed and cooled-rewarmed dog semen”

Semen may be preserved for short-term storage either by cooling and rewarming or by freezing and thawing. The freeze-thaw process causes significant sperm damage, and it may be preferable to cool and rewarm samples, although cooled spermatozoa has a limited lifespan. Ejaculates were collected from 6 adult male beagle dogs. The second fraction was divided into 2 aliquants, one of which was diluted with a Tris-egg yolk extender, placed into 0.5-ml straws and frozen in liquid nitrogen vapor, before being stored in liquid nitrogen. The second aliquant was diluted with a nonfat dried milk-glucose extender, cooled to and stored at 5 °C. Portions of the cooled semen were removed on Days 0, 1, 2, 4, 6, 8 and 10, and the the semen quality was evaluated after rewarming. The semen quality of the frozen semen was assessed after thawing. A variety of assessments were made, including sperm motility, sperm morphology, acrosome status, hypo-osmotic swelling, and longevity at 39 °C. Comparisons within ejaculates were made between the semen quality of the frozen samples and the samples that had been cooled and stored for different time periods. Semen quality of the samples that had been cooled and stored deteriorated on a daily basis; however, in the first 2 d after collection, semen quality was always superior to that of the samples that had been frozen and thawed. The mean time taken for semen quality of samples that had been cooled and rewarmed to become equal to samples that had been frozen and thawed varied for each parameter of semen evaluation, but overall it was 118.7 ± 25.9 h. Should short-term cryopreservation of dog semen be contemplated to allow sample transportation, it is clear that using conventional methodology dilution and cooling is the most suitable technique, provided that the sample is used within approximately 4.9 d of collection. Should the required length of storage exceed this time, it would be prudent for the sample to be frozen and then thawed.