Effects of age, size and season on cryotolerance of dog semen - A retrospective analysis.

In a retrospective analysis of 508 ejaculates from 297 dogs, efficiency of semen cryopreservation and effects of age, season and dog size on characteristics of fresh and cryopreserved semen were evaluated. Volume of the sperm-rich ejaculate fraction increased to 10 years of age, decreased in 10 and 11-year old dogs (P < 0.001) but did not differ among seasons. Total sperm count was less in 10 and 11-year-old dogs (P < 0.001). The percentage of progressively motile, membrane-intact and morphologically normal spermatozoa before cryopreservation was least in 10 and 11-year-old dogs (P < 0.001). Cryopreservation resulted in less progressively motile spermatozoa (P < 0.001) with this being most pronounced in 10 and 11-year-old dogs (cryopreservation x age P = 0.004). The cryopreservation-induced decrease in morphologically normal spermatozoa (P < 0.001) was not affected by dog body weight. Number of cryopreserved AI doses differed among age groups (P < 0.001) and was less in 10 and 11-year-old dogs (median 1.5) compared with younger dogs (6-7 years, median 4.3). When ejaculates were grouped by a threshold of ≥ 35% progressively motile spermatozoa after freezing-thawing, 86.5% of all ejaculates were greater the threshold but this percentage decreased to 66% in 10 and 11-year-old dogs (P < 0.001). In conclusion, sperm cryotolerance is consistent for much of a dog´s lifespan but decreases after a certain age. Dog semen can be cryopreserved successfully throughout the year. Post-thaw semen characteristics were not different among cryopreserved ejaculates obtained from dogs differing in body weight.