The Horse Breeding "Gold Standard"

How veterinarians can evaluate frozen semen.

The techniques for freezing, or cryopreserving, equine semen have made great strides in the past 15 years. The use of frozen equine semen for breeding has become more popular due to these techniques and better mare management techniques. Still, the use of frozen semen has not taken off as many have expected. Reasons include more extensive palpation and management of the mare and decreased pregnancy rates as compared to fresh or cooled semen. Additionally, horse owners, breeding managers and veterinarians may not be comfortable with procedures required when using frozen semen.

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The most common packaging for equine frozen semen is the 0.5-mL capacity straw because it provides the most uniform freeze and thaw rates for the package contents. The number of straws required for one breeding dose of semen can vary from one to 10 or more straws. The mare being bred may require more than one breeding dose depending on the ability to pinpoint ovulation. Frozen equine semen does not live in the mare’s reproductive tract as long as fresh semen. Therefore, the mare has to be bred as close as possible to the release of the ovum/egg (called ovulation). The most common window is within six hours of ovulation to achieve maximal pregnancy rates. There are many laboratory tests that can be performed to evaluate the quality of frozen semen. Progressive motility evaluates how fast and straight the cells are swimming. Morphology looks at whether the cells are perfectly made or have physical defects. Sperm Chromatin Assay evaluates the quality of the DNA within the spermatozoa. The “gold standard” is fertility testing the semen on normal fertile mares to determine the least number of straws per breeding dose that will give acceptable pregnancy rates per breeding cycle. Using recipient or sham mares, the number of straws per breeding dose is decreased until pregnancy rates drop below an acceptable percentage. This allows the stallion-breeding manager to sell the semen and assure the client that an acceptable percentage of breedings will result in a pregnancy for the purchaser. Additionally, it allows the stallion owner to use the least number of straws per pregnancy, thereby increasing the number of years his frozen semen stock will last.

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As much as veterinarians want to examine semen, fresh, cooled or cryopreserved, the ultimate and only reliable test of all semen is the number of mares that achieve pregnancy per number of mares bred. The reason it is the “gold standard” is due to the fact that it takes time, patience, skill and, last but not least, money (gold). Dr. Joe Noble is an AQHA member who, with Dr. Ginger Noble, owns Noble Equine Veterinary Service in Purcell, Oklahoma. They specialize in embryo transfer and equine reproduction.