January 2021 - Making Wise Choices
Written by CRM
Friday, 01 January 2021 19:50
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Judgment comes into play when using frozen semen for horse breeding.

Breeding with frozen semen is now well established in the equine industry. Its use represents a huge advantage: semen can be stored indefinitely and will stay viable as long as it is stored under the required conditions. However, there are also significant downsides to its use, resulting in an overall lower pregnancy rate of around 48-58 percent1 after breeding with frozen semen. The average cycle pregnancy rate for a mare bred using frozen semen is between 30 and 60 percent.

Because frozen semen remains viable in a mare’s uterus for only about 12 hours after thawing, an intense breeding management program is essential. In addition to careful attention to detail and appropriate breeding management protocols, mare selection is an important step in deciding when to use frozen semen.

Not all mares are good candidates for a frozen semen program. Several factors should be taken into consideration, including the age of the mare, the mare’s reproductive history (previous gestation/ barren), reproductive status and results of a breeding soundness examination.

Young mares (less than 6 years old) with a normal conformation are more likely to be successfully bred with frozen semen. However, young mares under heavy exercise, or who have received medication to either enhance performance or suppress normal reproductive function may need time (up to year after leaving the program) to recover and become good candidates. Within this group, maiden and foaling mares have a higher pregnancy rate compared to barren mares,1 with the highest pregnancy rate for maiden mares compared to foaling mares.

Clearly, mares with a history of subfertility or infertility will likely experience the same difficulty with frozen semen, but it is easy to forget the potential risks of breeding a maiden mare with frozen semen, which is often expensive and may be less fertile due to the freezing process. Thus, even when a maiden is selected for breeding with frozen semen, we strongly recommend a full breeding soundness exam to verify the reproductive health of the mare.

Success rates for older mares (more than 15 years old) have been quite variable in the literature. Primary determinants of fertility in these mares appear to be a sound reproductive history and good anatomic conformation.

Older mares who became pregnant and foaled without complications in the past three years are better candidates than either older maiden mares or mares with a history of reproductive complications. Interestingly, in one study, older barren mares were more likely to become pregnant with frozen semen than older maiden mares6. A breeding soundness exam by a veterinarian is particularly critical in both these groups to detect anatomic or other conditions that could prevent normal pregnancy.
Older, maiden mares have a higher incidence of poor uterine clearance, while frozen semen is likely to result in an exacerbated and prolonged inflammatory response in these mares. Likewise, older multiparous mares are more likely to have poor vulvar or vaginal conformation, resulting in pneumovagina (wind-sucking) or urovagina (urine-pooling), cervical trauma or uterine fibrosis. These conditions will compromise fertility and may result in poor fertility if not managed carefully or corrected prior to breeding.

Even in the absence of a veterinary exam, visual inspection of breeding mares at least each year can lead to important information. Mares that have good or excellent vulvar conformation are more likely to be good candidates for breeding with frozen semen than those with questionable conformation. To identify a good candidate, a few “rules of thumb” can be helpful: Ideally, a mare’s vulva should be vertical, with the anus protruding slightly beyond the vulva. Most or all of the vulva should be positioned below the level of the pelvis. Gentle separation of the vulvar lips should not result in “windsucking,” and there should be no discharge or discoloration below the vulva after urination.

Carefully collecting and recording a full breeding-history can save prospective breeders a lot of heartache and money, regardless of which breeding method they have selected. Because mares often fall into patterns during consecutive breeding seasons, past performance can be a predictor of future performance.

If the reproductive history is known and infectious or trauma-related causes of infertility are determined to be unlikely, older mares with a history of at least one previous foaling are more likely to become pregnant and also less likely to experience difficulty delivering or caring for a foal than older maiden mares. Conversely, a reproductive history of a difficult foaling and subsequent infertility or subfertility may lead to a diagnostic work-up to identify cervical tears or other problems that can be repaired surgically before the mare is bred with frozen semen.

Many mares are selected for a breeding program with frozen semen due to their genetic merit and “match” to a stallion, and clearly these factors are as important as the ability of the mare to quickly become pregnant and carry a foal. However, the cost associated with frozen-semen breedings, and the prospect of using valuable semen in a mare that has an inherently low fertility also must be considered. To have the greatest chance of ending up with a new foal by their chosen stud, breeders should evaluate their mares’ fitness to be enrolled in a frozen-semen program each year by reviewing the mares’ records, observing the mares’ general health and anatomic conformation and performing targeted breeding soundness examinations before each breeding season.