Horse Breeding

Mares Matter

July 12, 2013

The success of horse breeding with frozen-thawed semen depends a lot on the mare.

Dr. Robert O. Gilbert

Dr. Robert O. Gilbert explains the importance of mares and fertility when breeding with frozen-thawed semen. Photo courtesy of Synbiotics.

By Dr. Robert O. Gilbert for AQHA Corporate Partner Zoetis

Much attention is paid to the selection of stallions for frozen semen, considering genetic value, semen quality and frozen semen fertility. But the stallion is only half of the equine team. Equivalent attention should be given to selection of mares for insemination with frozen-thawed semen to ensure optimal results.

No matter which breeding strategy is used, the single factor influencing mare fertility most is age. After about 10 years old, a mare’s fertility decreases steadily. The ideal mare for frozen semen use is a young maiden mare up to about 8 years old. In general, foaling mares have better fertility than barren mares, although in some studies, previously fertile mares who have not been bred for a season or two have better fertility than those with foals at foot. Mares who have experienced fertility problems in the past usually have the worst likelihood of carrying a foal to term.

Results with frozen-thawed semen will mirror those depicted above – young maiden mares best, with foaling or previously fertile but unbred mares next, followed by older maidens and mares with known breeding problems.

So, you’re ready to breed your mare. What steps do you need to take to breed her with shipped semen? In AQHA’s free Mare Care: Breeding Tips report, Dr. Racquel Rodeheaver explains the process of preparing your mare, targeting a breeding date, ordering semen, inducing a follicle to ovulate, receiving and evaluating semen and much more.

Of course, genetic considerations that drive breeding decisions often do not coincide with mares of the ideal age group or fertility history, and owners need more information. A transient uterine inflammatory response follows natural mating or insemination. However, some mares are prone to a more exaggerated postbreeding endometritis, which requires intensive management and is associated with poorer fertility. Added to this, frozen-thawed semen tends to provoke a more outspoken inflammation than natural mating or insemination with fresh extended semen. This is thought to be because seminal plasma (removed during processing for freezing sperm) exerts a modulating effect, and because the freezing and thawing process causes some sperm cells to become fractured, spilling content that is usually intracellular and does not usually come into contact with the uterine lining (endometrium). Mares with a history of postbreeding endometritis are likely to react more strongly to frozen-thawed semen, making them less than ideal candidates for frozen semen insemination. Those who accumulate a considerable amount of intrauterine fluid before ovulation (independent of mating) are also less likely to be bred successfully with frozen semen.

Having said all this, many mares become pregnant every year to frozen semen who do not fit the profile of the ideal mare for this purpose. Much depends on careful management of the mare, appropriate timing of the insemination, and correct, gentle and clean placement of the semen.

The Mare Care: Breeding Tips free report is a perfect resource for beginning breeders wanting to breed their first mare. Learn the ropes from Dr. Racquel Rodeheaver as she walks you through the process of prepping and breeding your mare. Download your free copy today.

If you wish to use a genetically superior mare who is no longer young or who has had some previous issues with fertility, consult your veterinarian. Ask for a comprehensive breeding soundness examination, possibly including endometrial culture, cytology and biopsy, as well as diligent physical examination and rectal palpation and ultrasonography, all of which will allow a realistic appraisal of the chances of success. Then, knowing whether you face good or bad odds for pregnancy, proceed using the best possible management and technique.

Dr. Robert O. Gilbert is a professor of reproductive medicine at the Cornell University College of Veterinary Medicine.