Commercial semen freezing: Individual male variation in cryosurvival and the response of stallion sperm to customized freezing protocols

One of the challenges for those attempting to cryopreserve stallion spermatozoa is dealing with the stallion to stallion variability in the cryosurvival of their semen. In the dairy industry, each bull stud, essentially utilizes a single cryopreservation technique, and bulls that produce sperm that do not cryopreserve well using that technique are replaced by other bulls. However, replacing stallions is unlikely to prove acceptable to the equine industry, where specific genotypes are desired. Instead, to increase the number of stallions that can be effectively utilized for cryopreserved semen production, it is likely that more than one method for cryopreserving sperm will be necessary. This manuscript reviews some of the processes involved in cryopreservation, how individual sperm physiology affects the ability to survive freezing and thawing, and how cryopreservation protocols can be customized to maximize sperm cryosurvival on an individual stallion basis.