(The following text is taken from several papers written by Dr. Corey Miller, DVM, MS, Dip ACT. The references
have been removed from this text, and can be furnished upon request.)
Breeding with frozen semen can present tremendous advantages to the mare and stallion owner, but there are
also considerable challenges for the successful use of frozen semen in the breeding management of mares.
Frozen semen adds a new dimension to the horse breeding industry by allowing the long-term preservation of
spermatozoa from superior stallions and permitting distribution of this semen to breeding establishments
worldwide. Geographical constraints are abolished so semen can be selected from a larger pool of stallions,
including deceased stallions. Additionally, the semen can be used to breed mares even when the stallion is at
performance events, ill, or recovering from an injury. Frozen semen can also be stored as insurance against
possible injury to the stallion or his death.
Undesirable aspects of cryopreserved semen are also apparent. For instance, pregnancy rates are likely to be
reduced for a high percentage of stallions, given our existing technology in this area. Opportunities for errors and
corruption also increases, but could be held in check by quality control during processing and insemination
techniques, as well as improved techniques for documenting parentage. The biggest challenge when using frozen
semen is that the pregnancy rates are usually lower than those obtained from fresh or cooled-transported semen
from the same stallions despite more intensified mare management. In other words, more time, effort, and money
are expended when breeding mares with frozen semen with less satisfactory results as compared to those
obtained when using fresh or cooled-transported semen. Consequently, breeding mares with frozen semen is not
a field procedure; the breeding management of mares with frozen semen is most efficiently managed in a clinic
setting because special equipment is necessary for the storage, handling, and insemination of frozen semen.
Additionally, a veterinarian should be chosen who is experienced in breeding mares with frozen semen and whose
schedule permits frequent palpation of the mare when she comes into heat.
When breeding with frozen semen, the mare needs to be managed very intensely. Frozen-thawed semen has a
limited life in the reproductive tract when compared to fresh or cooled-transported semen. Therefore, when one
dose of frozen semen is provided, the frozen-thawed semen should be inseminated into the mare
within 6 hours of ovulation. This requires dedication on the part of the veterinarian to check the mare
frequently when she is in heat, as well as expertise to be able to accurately predict when the mare is going to
ovulate. The time required is lessened somewhat when an Deslorelin (or an Ovuplant® implant) or hCG are
administered to the mare after she comes into heat to help ensure that ovulation will occur in a predictable
Factors Influencing Success
Numerous factors influence pregnancy rates achieved in mares bred with frozen stallion semen and include the
viability of processed stallion semen, the number of spermatozoa inseminated, the frequency of insemination, the
concentration of spermatozoa in extender, the type of extender utilized (including antibiotics), the cooling rate of
extended semen, the storage time and storage temperature prior to breeding, the response of spermatozoa to
freezing, and the inherent fertility of the stallion and mare. If the quality of the stallion's fresh semen is poor or
pregnancy rates achieved by breeding with fresh semen are low, it is highly unlikely that breeding with frozen
semen will be successful.
Spermatozoa are very sensitive to many environmental factors, including temperature, light, physical trauma, and
a variety of chemicals. Therefore, any factor that negatively impacts the ability of spermatozoa to resist
environmentally induced damage will adversely affect fertility achieved when using processed semen for breeding.
In general, semen from stallions does not respond favorably to the normal freeze-thaw cycle. Pregnancy rates
with frozen stallion semen can vary from 0% to 70% per cycle. Although the upper level of this range is
preferable, the pregnancy rate per cycle is in the 25%–40% range for many stallions. Thus, before investing in
frozen semen, the success rate with the semen that you are considering purchasing should be investigated. You
should question the post-thaw progressive motility rate of the frozen semen, as well as the pregnancy rate
achieved with the semen. You should be certain that an adequate breeding dose is provided with a minimum of
200 million progressively motile sperm, based on post-thaw motility. The freezability of semen varies greatly
among individual stallions, as well as among different ejaculates from a given stallion. Generally, the minimum
acceptable post-thaw motility is 30%. The only way to accurately assess the fertility of frozen semen is to breed
mares and determine subsequent pregnancy rates.
The selection of the veterinarian is critical to a successful outcome. The veterinarian should be
experienced with the breeding management of mares with frozen semen, the handling and storage of frozen
semen, and offer a clinic setting where the mare can be palpated very frequently when she is close to ovulation
(i.e., every 6 hours).
Mare Management Considerations
The importance of the management of the mare cannot be over-emphasized. If your mare is not
managed properly, she will not conceive, no matter the quality of the semen. Before the mare is to be
bred, she should undergo a complete breeding soundness examination. The past reproductive history of a mare
and her body condition play vital roles in determining her inherent fertility and are often overlooked. Additionally,
the mare’s ovaries, uterus, and cervix should be examined by rectal palpation/ultrasonography; the vagina should
be examined by speculum and digital palpation; and, depending upon the status of the mare, an endometrial
culture and biopsy may also be warranted. Any abnormalities detected should be corrected as deemed
necessary by your veterinarian, such as “windsucking,” urine pooling, or cervical lacerations before the breeding
season to allow enough time for the uterus to be treated for any possible infection. Studies have shown that first
cycle pregnancy rates with frozen semen are highest in maiden mares, then foaling mares, and finally barren
After breeding according to the above guidelines, the mare has to be evaluated 6–24 hours after insemination.
Some degree of inflammatory response will occur in virtually every mare after insemination. Therefore,
it is recommended that the mare be checked for uterine fluid after insemination, and, if fluid is present, that a
uterine lavage be performed to remove this fluid. This is especially true when breeding with frozen semen due to
certain components of the semen extender (e.g., glycerol and egg yolk) that incite temporary uterine
inflammation. Many veterinarians routinely perform a uterine lavage and infuse the uterus with antibiotics after
breeding with frozen semen, with the theory that this will help ensure that the uterus will be in the best possible
condition for conception to occur.