Breeding

A Horse-Breeding Headache?

Why does my stallion's semen not freeze well?

Cryopreservation, or freezing, of stallion semen can be a useful tool to preserve valuable genetics for use when a stallion is unavailable, injured or dies. Unfortunately, some stallions who produce acceptable pregnancy rates with conventional breeding management produce semen that does not tolerate the freeze-thaw process well enough to be used in a breeding program. When this happens, it can be a source of frustration for owners, managers and veterinarians. In this article, we will briefly discuss a few of the many factors affecting fertility and post-thaw motility of frozen-thawed semen. It is important to start with good-quality semen. Excellent semen quality prior to cryopreservation often yields useable semen after cryopreservation, and it follows that semen of suboptimal quality will not improve with freezing. The cryopreservation and eventual thawing for use are stressful to the spermatozoa, and many cells are lost in the process. In some freezing stations, ejaculates with a pre-freeze progressive motility of less than 50 percent are rejected as unsuitable for freezing. Even so, all stallions are different, and not all semen tolerates cryopreservation equally. Occasionally, even stallions who produce good-quality ejaculates and are fertile under normal breeding conditions produce sperm that do not withstand the freezing process well.

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About 20 percent of stallions are “good freezers” and produce spermatozoa that survive the freezing process well and have post-thaw motility greater than 40 percent. An additional 60 percent are “adequate freezers” and will have post-thaw motility of 20-40 percent. The remaining 20 percent are classified as “poor freezers,” with less than 20 percent motility post-thaw. Appropriate management may improve the freezing quality of the ejaculate in some stallions and allow successful freezing for long-term storage. Unfortunately, in many cases, those classified as “poor freezers” will remain in that category. Proper preparation of the stallion before submitting an ejaculate for freezing can be valuable to ensure that the best possible sperm are available. Sperm are stored in the stallion’s reproductive tract for many days after they are produced, and with conventional breeding management, retain their fertility.  However, as they age, these sperm may undergo changes that make them less tolerant to the stresses of the cryopreservation process and that may lower the post-thaw motility of the ejaculate. An excellent practice prior to freezing a stallion’s ejaculate is to make a series of “clean out” collections to deplete the stored sperm so fresh new sperm are available for freezing. Typically, a minimum of three to five collections should be performed, but more may be necessary depending on the previous collection schedule and individual stallion factors. Collections should continue until the best semen quality for that stallion is achieved. There are a number of unexplained variables between ejaculates of individual stallions, and in some stallions, different freezing protocols may yield different results. No one protocol is best, and all stallions should be treated as individuals when processing sperm for cryopreservation. Freezing trials using different protocols may be worthwhile to find the method that produces the best results for a particular stallion. Different cryoprotectants, nutrient sources, cooling rates, freezing rates and other variables have all been investigated. Finding the right combination may improve the performance of a particular stallion’s semen. Most freezing centers have several protocols to choose from and will work to find a suitable technique for each stallion.

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Dietary supplements may sometimes have positive effects on semen quality. In some reports, stallions that were categorized as “poor freezers” had improvement in post-thaw motility after dietary supplementation with docosahexaenoic acid (DHA).  Other reports indicate that dietary antioxidants (such as selenium, Vitamin E and zinc) and polyunsaturated fatty acids have shown some potential to improve semen quality in fresh semen. Although this research is promising, more controlled studies are necessary to evaluate supplementation on post-thaw motility and fertility of stallions. Caution should be used when choosing a dietary supplement, as many are not proven to be effective and may actually contain substances that are harmful to sperm quality. Semen cryopreservation can be an excellent way to preserve a stallion’s genetics. The process can be frustrating in those stallions that do not freeze well, and stallion owners should be aware that every stallion is different. If you are thinking about having semen cryopreserved, it is best to schedule this early in the stallion’s career, when semen quality is optimal and age-related changes are not an additional complication. Dr. Aime K. Johnson is a Diplomate of the American College of Theriogenologists from Auburn University College of Veterinary Medicine.