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Expanding Options for Equine Breeding Management

The process of breeding can be complicated even before adding in the option of embryo transfer. Learn more about what options are available for breeding your mare.

By Karen A. Von Dollen, DVM, MS, DACT - Hagyard Equine Medical Institute

Kentucky Equestrian Directory 2021 Issue


The decision of whether or not to breed your mare is difficult in and of itself, but this process can be complicated when considering the options which exist for how to produce a foal from your mare. First and foremost, you must decide whether you intend for your mare to carry her own foal, or whether you would prefer to utilize a recipient mare. Transfer of an embryo into a recipient mare allows a donor mare to continue a performance career unencumbered by the demands of gestation, provides a means to produce more than one foal per mare per year, and in some cases can circumvent subfertility limitations of a donor mare. If you are planning to breed your mare to carry her own foal, your main decisions will be the method of semen delivery (natural mating or artificial insemination, AI) and type of semen if you are opting for AI (fresh, cooled, or frozen). Artificial insemination affords the ability to incorporate genetics into your breeding program from far flung geographic locations, both domestic and international. Semen can be shipped cooled, for short term use following collection and processing, or frozen. Frozen semen can be stored indefinitely and used months, years, or decades after a stallion is deceased. If you are planning for embryo transfer, the above decisions regarding semen still apply, but more choices must be made regarding embryo generation and handling.

Immature oocytes collected during transvaginal aspiration

While transfer of in vivo produced equine embryos (those flushed from a donor mare about a week after breeding) has been practiced for many years, advancements in laboratory techniques more recently have made it possible to produce equine embryos outside of a mare’s uterus by fertilizing an oocyte with a single sperm. Once an embryo is produced (either by a mare or a lab), it can either be transferred directly into a synchronized recipient mare or cryopreserved through vitrification for transfer at a later date. The latter option can be attractive for owners who hope to produce embryos outside of the traditional breeding season and have them available to transfer in the future to have a foal born at their desired time of the year. Embryo vitrification also decreases the pressure to synchronize a recipient mare, as the embryo is safely stored in liquid nitrogen and can remain there until the ideal recipient uterus is available.

The above process of in vitro embryo production is known as Intracytoplasmic Sperm Injection (ICSI). In order to perform ICSI to produce embryos, oocytes must be collected from a donor mare. The most common way that this is achieved currently is by Transvaginal Oocyte Aspiration (TVA, also known as Ovum Pickup or OPU). As a macabre but practical consideration, oocytes can also be obtained from ovaries following the death of a mare.

embryo transfer
An embryo flushed from a donor mare, prior to transfer into recipient mare.

The process of TVA involves transrectally guiding the mare’s ovary close to the vaginal wall, while a rigid ultrasound probe is simultaneously placed in the vagina and the ovary held in close apposition to the ultrasound. A long needle traveling immediately adjacent to the ultrasound is used to puncture the ovary and aspirate the fluid from the mare’s follicles while fresh fluid is repeatedly flushed through the follicle and the needle manipulated to scrape the oocyte from the follicular wall. As you might expect from visualizing this description, this procedure is more invasive than many other reproductive techniques such as AI or embryo flushing, and is associated with unavoidable risks. Mare oocytes are more tenaciously adhered to the follicular wall than those of some other species, which presents a challenge for recovery yield. Once all accessible follicles are aspirated and flushed, the fluid is filtered and searched for oocytes. The oocytes can then be shipped to an ICSI laboratory or handled onsite, depending on the laboratory setup of the clinic performing TVA.

ICSI represents the most efficient way to use semen, as only a fraction of a breeding dose is used for the injection procedure. For this reason, it is an attractive method for semen which is in very limited supply (such as a stallion who is deceased but still has frozen semen banked). Maximizing efficiency in stretching the last reserves of precious stallion genetics is just one reason to consider ICSI. Other reasons can be broadly categorized as stallion-driven or mare-driven. Some subfertile stallions benefit from ICSI when other methods of breeding have been unsuccessful. On the mare side, ICSI offers an opportunity to attempt to salvage the reproductive career of a mare who has otherwise proved difficult to produce pregnancies or foals. Examples include mares affected by chronic uterine infections, those with reproductive tract trauma (such as adhesions or cervical damage sustained during dystocia), or those that repeatedly fail to produce embryos despite optimized management. It can also offer more scheduling flexibility than breeding, as it can be performed at any stage of the estrous cycle and does not necessitate follow up appointments such as those required for an embryo flush or pregnancy checks. The ideal time to perform aspiration of immature oocytes is when the mare has a maximal number of small follicles, as this will increase anticipated oocyte yield.

equine pregnancy
Transrectal ultrasound of equine pregnancy at 14 days

Intercepting the reproductive process at the oocyte stage requires precision to nurture this delicate gamete from the moment it is aspirated from the mare to maximize the chance that it will realize its potential as an embryo, then foal. Even the best laboratory efforts are imperfect in this endeavor to mimic Mother Nature, and owners should embark on TVA/ICSI with realistic expectations that multiple rounds may be necessary in order to reach a desired outcome, and that even then your perfect foal may not materialize. Success rates are variable and affected by a multitude of factors on both the stallion and mare sides of the equation.

Navigating the ever-broadening landscape of equine reproduction can be daunting, and making choices for a breeding program are not one-size-fits-all. Each choice in the decision tree has pros and cons, with each decision you make often budding into more branches of options. The scope of this article precludes an exhaustive discussion of considerations when weighing these decisions, but it is the intent that it may serve as a starting point for consultation with your veterinarian as we approach the 2021 breeding season.

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