Horse-Breeding Mathematics

Why are there variations in frozen-thawed semen insemination doses?

The goal of any breeding program is to produce a live, healthy foal in a cost-efficient manner. Obtaining a foal utilizing frozen-thawed semen does not alter this goal; it just adds aspects to the equation that typically do not exist when utilizing fresh or chilled semen. Historically, frozen semen is sold by the dose, with no live-foal guarantee. The supply of frozen semen for a particular stallion is also not infinite. Therefore, breeding with frozen-thawed semen has the added pressure of minimizing the amount of frozen-thawed semen needed to produce a live, healthy foal while maximizing the number of foals produced with that stallion’s frozen-thawed semen. If the goal is to use the minimum amount of frozen-thawed semen possible to achieve a pregnancy, why then is the insemination dose eight straws for one stallion and two straws for another stallion? The number of straws in an insemination dose is a function of multiple variables. These variables include the number of total and progressively motile sperm (those sperm that are forward-moving with reasonable velocity) within the insemination dose and within each straw, the site of semen deposition with the mare’s uterus, and the per-cycle pregnancy rates of the semen.

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Unlike fresh or chilled semen, no standard insemination dose exists in the United States for frozen-thawed semen. In fact, some U.S. laboratories base insemination dose recommendations on total sperm numbers, whereas others base it on progressively motile sperm. For the laboratories that base recommendations on total sperm number, their typical recommended dose is 600 to 800 million total sperm (range = 400 million to 1 billion total sperm). When based on progressively motile sperm, the typical recommended insemination dose is a minimum of 200 or 250 million progressively motile sperm. Regardless of what the insemination dose recommendation is based, most laboratories agree that a post-thaw progressive motility of at least 30 percent or 35 percent is preferential for optimizing per-cycle pregnancy rates. When determining insemination doses for international exportation, specific minimum requirements may exist and should be confirmed prior to freezing semen for these purposes. The aforementioned insemination doses are based on conventional artificial insemination, meaning that the semen is deposited in the uterine body. Research and clinical data have demonstrated that depositing semen at the tip of the uterine horn adjacent to the pre-ovulatory follicle or recent ovulation (a technique referred to as deep-horn insemination) may result in acceptable pregnancy rates while requiring fewer sperm to be inseminated compared to uterine body semen deposition. Deep-horn insemination can also be utilized at existing insemination dose recommendations if acceptable pregnancy rates are not being obtained utilizing uterine body insemination techniques. I utilize a transrectally guided deep-horn insemination technique for all frozen-thawed semen inseminations. Frozen-thawed per-cycle pregnancy rates average 30 percent to 55 percent but, can range from zero to more than 70 percent.  Regardless of the insemination technique utilized, the insemination dose should be adjusted (decreased or increased) as needed based on per-cycle pregnancy rates. Frozen semen can be packaged a number of ways:  plastic straws (0.5-5.0 ml capacity), flat aluminum packets (10-15 ml capacity), glass vials (1-10 ml capacity), plastic bags (10-25 ml capacity), and pellets (0.1-0.2 ml capacity). Currently in the United States, the preferred method of frozen semen packaging is the 0.5 ml plastic straw. In general, the larger the capacity of the package, the more likely it contains the entire insemination dose. The 0.5 ml straw, however, can contain anywhere from 50 to 800 million total sperm, depending on the laboratory that processed and cryopreserved (froze) the semen, meaning that one to 10 or more straws may be required per insemination dose. The number of insemination doses required per mare cycle is one to three, depending on the method of breeding management utilized by the veterinarian. Frozen-thawed semen must be inseminated into the mare within zero to 12 hours prior to ovulation or within six to eight hours post-ovulation to obtain the best pregnancy rates for most stallions. If the mare is to be inseminated only once, the insemination is performed within zero to six hours post-ovulation. This technique requires that the mare be examined every six hours starting zero to 24 hours after administration of an ovulatory induction agent. The other option to ensure proper timing of insemination is “timed” insemination. This management method requires a minimum of two inseminations per cycle, and potentially three, if the mare does not ovulate in the predicted time frame. A total of one to three insemination doses are utilized per cycle with this management method, depending on whether one orone-half of an insemination dose was utilized at each insemination. Both breeding management methods are reported to result in similar pregnancy rates but the “timed” insemination method, while requiring more doses of frozen semen, requires less intensive monitoring of the mare, which may result in decreased veterinary expenses.

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Insemination doses are not created equal, nor are they “set in stone.” They should be adjusted as needed based on the per-cycle pregnancy rates obtained for that stallion and/or for a specific “batch” of semen from that stallion. Deep-horn insemination should be considered whenever possible to improve per-cycle pregnancy rates or when reductions in the insemination dose are desired. Discussing the pros and cons of the methods of breeding management with your veterinarian should occur as soon as the decision is made to breed with frozen-thawed semen to determine the best option for your scenario. Dr. Shelby Hayden is a clinical assistant professor of theriogenology in the Department of Veterinary Clinical Sciences at the Center of Veterinary Health Sciences at Oklahoma State University.