Artificial insemination (AI) is an increasingly popular method of breeding mares, as it reduces many of the risks of natural service while providing several advantages. Benefits include accessing broader genetics, using stored or transported semen, and eliminating the need to transport mares to breeding sites.

Several types of artificial insemination are available, and the best method to use primarily depends on semen quality and mare fertility. Typically, during artificial insemination, the veterinarian deposits the semen into the uterine body.

For mares with suboptimal fertility or when using poor-quality semen, is also possible to do deep horn insemination or hysteroscopic insemination to increase the chances of pregnancy.

Fresh semen, fresh-cooled semen, or frozen semen can all be used depending on availability, breeder preference, and mare and stallion fertility. Each type of semen has pros and cons, which must be considered when making a breeding plan.

Consult with your veterinarian to determine the optimal type of semen for your mare’s situation to ensure a successful pregnancy.

Artificial Insemination

Artificial insemination (AI) refers to insemination by a veterinarian, rather than natural service by the stallion. AI has become extremely popular in parts of the world, with some countries reporting up to 90% of foals born annually are the result of AI. [1][2]

Major advantages of artificial insemination over natural service include: [3]

  • Reduced risk of traumatic injuries from breeding
  • Reduced spread of infectious diseases between horses
  • Ability to test the semen for infectious disease prior to breeding
  • Ability to evaluate semen quality prior to breeding
  • Ability to split a single ejaculate into multiple breeding doses to service more mares

It is important to note that some breed organizations, such as the Jockey Club for Thoroughbreds, do not allow foals resulting from artificial insemination to be registered. [3] Research your breed’s policies on assisted reproductive technologies before committing to a breeding protocol.

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Artificial Insemination Techniques

All artificial insemination techniques aim to deposit semen into the mare’s reproductive tract in a sterile fashion to prevent infections in the uterus.

Before breeding, mares must have their tails wrapped and perineal area cleaned thoroughly to reduce the risk of contamination. All of the equipment used in artificial insemination must be sterilized prior to use. [4]

Traditional Artificial Insemination

Traditional AI technique involves introducing a long insemination pipette through the cervix of the mare, to deposit the semen into the uterus. This technique is suitable for most mares, and can be used with fresh, fresh-cooled and frozen semen. [5]

Deep Horn Insemination

The goal of deep horn insemination is to deliver the semen into the tip of the uterine horn, so that the maximum amount of sperm can enter the oviduct for fertilization. Veterinarians typically use this technique for frozen semen, or if the semen volume or quality is low. [5][6]

To perform this procedure, the veterinarian introduces a flexible insemination pipette through the cervix, then advances the pipette to the tip of the uterine horn by manipulating the pipette via the rectum. Once the pipette tip is in the correct location, semen is deposited. [5]

Deep horn insemination has approximately 30% better success rates over uterine body insemination for low concentration semen doses. [7]

Hysteroscopic Insemination

Hysteroscopic insemination, using a camera to direct deposition of semen within the reproductive tract, is an uncommon procedure that is only used if the breeding dose has an extremely low sperm concentration. [5][6]

Timing is extremely important for this procedure, as the already low number of sperm rapidly die after entering the reproductive tract. Most veterinarians aim to complete this procedure immediately before ovulation, or immediately after ovulation. [5]

In this procedure, the veterinarian introduces the endoscope through the cervix until the endoscope reaches the junction between the uterus and oviduct. Direct visualization of the oviduct ensures proper placement of the semen sample into the oviduct to maximize the chances of fertilization. [5]

Pregnancy rates are around 50 – 75% using this technique with low sperm concentration semen. [5]

Types of Semen

The type of semen used in these procedures depends on the location of the stallion, breeder preference, quality of the semen, and the mare’s fertility.

Fresh Semen

Fresh semen is semen collected up to 6 hours prior to performing artificial insemination on the mare, requiring both horses to be in a similar geographical location.

Many horse breeders prefer fresh semen AI to natural service, as it reduces the risks of injury or infection for both horses.

Fresh semen also allows for additional flexibility regarding the timing of breeding compared to fresh-cooled or frozen semen. Aceptable pregnancy rates can be achieved up to 72 hours after ovulation if the semen quality is excellent. [3]

Timing of Insemination

Most AI programs involve insemination of mares every other day, starting from the second or third day after they come into heat and continuing until ovulation is identified by ultrasound. [3]

There is no additional benefit to inseminating mares daily or multiple times a day, unless the semen quality is poor. Doing so introduces additional risk of uterine infection. [3]

For mares that have a higher risk of uterine infections, veterinarians may choose to monitor their estrus cycle closely, and only perform one AI procedure at approximately 48 hours prior to ovulation. [3]

Pregnancy Rates

Pregnancy rates from artificial insemination vary significantly depending on the study. Several studies show increased pregnancy rates using AI when compared to natural service, particularly for subfertile mares. [8]

Fresh semen also has a higher pregnancy rate compared to fresh-cooled or frozen semen. Previous studies have shown pregnancy rates for fresh semen between 76 – 84% per breeding cycle. [8]

Fresh-Cooled Semen

Fresh-cooled semen refers to semen cooled to around 4-8oC (39-46oF), which allows for transport and short-term storage of between 12 – 36 hours. [4]

Using fresh-cooled semen enables breeding to stallions located across the country, so long as transport methods are able to deliver the semen within a short timeframe.

Advantages and Disadvantages

The major reason breeders choose fresh-cooled semen over fresh semen is the ability to transport the semen to the mare, rather than bringing the mare to the stallion. This offers benefits such as: [4]

  • Lower cost for transportation
  • Reduced stress for the mare
  • Increased access to superior stallions that may be outside the breeder’s immediate geographical location

For stallion owners, fresh-cooled semen allows the stallion to serve more mares per breeding season than fresh or natural service. A single ejaculate can be split into multiple breeding doses. [4]

The major disadvantage of fresh-cooled semen is the amount of additional expertise, time, and management breeding requires compared to fresh semen or natural service. [4][9] Mares in artificial insemination programs often require repeated ultrasounds to track their estrus cycles or hormonal management to induce ovulation.

Additionally, use of fresh-cooled semen typically involves extensive coordination between the veterinarians for the stallion and mare. They must work together to ensure that stallion collection and transport of the semen are completed before the mare ovulates. Delays in collection or transport can lead to a missed breeding cycle and wasted semen. [4][9]

Timing of Insemination

The timing and number of inseminations for fresh-cooled semen is somewhat controversial, but primarily depends on the quality of the semen, fertility of the mare, and preferences and experience of the veterinarian.

For a standard situation with good quality semen and a fertile mare, many veterinarians will breed once or twice during her heat cycle shortly prior to ovulation. [4] With poor quality semen or a subfertile mare, usually one dose is administered 24 hours prior to ovulation or as close to ovulation as possible. [4][10]

The ideal breeding window is between 24 hours prior to ovulation and up to 16 hours after ovulation. [10] Veterinarians breeding prior to ovulation often give mares ovulation-inducing medications to ensure that the time between insemination and ovulation is as short as possible. [4]

These medications also ensure that individual variation in the length of the mare’s estrus cycle does not impact the breeding cycle. [4]

Pregnancy Rates

Fresh-cooled semen has lower pregnancy rates than fresh semen, but higher rates than frozen semen. Research indicates that the pregnancy rates for fresh-cooled semen is around 40 – 65% per breeding cycle. [8]

Frozen Semen

Frozen semen is typically prepared with an appropriate semen extender after collection and then frozen using liquid nitrogen.

Not all stallions are good candidates for frozen semen, as the freezing process can markedly reduce sperm count and semen quality. [4]

Estimates suggest that around 20% of stallions produce good quality frozen semen, and around 60% produce acceptable quality semen. The reasons for variation in frozen semen quality is unknown, but may be genetic. [4]

Frozen semen is growing in popularity as advances in breeding and freezing techniques have increased pregnancy rates associated with this type of semen. [1]

Frozen Semen Technique

Frozen semen must be thawed prior to artificial insemination, requiring an additional step compared to using fresh or fresh-cooled semen. Typically, frozen semen is rapidly thawed in a body temperature water bath.

After thawing, the veterinarian loads each semen straw into a specialized pipette with a stylet that pushes the semen out of the straw when the veterinarian is ready. [5]

Advantages and Disadvantages

The major benefit of frozen semen is the ability to store the semen almost indefinitely, so long as the liquid nitrogen stores are properly maintained. [4]

The increased storage time afforded by using frozen semen provides benefits such as: [4][9]

  • Transport of semen long distances, including between continents, allowing for access to genetics from overseas
  • Storage of semen from stallions with competitive careers, allowing access to their genetics despite a busy competition schedule
  • Stockpiling of semen from aged stallions prior to their passing, or stockpiling of semen before castration
  • Ensure the selected semen is rapidly available when the mare is at the optimum point in her estrus cycle.

Frozen semen has similar transport issues to fresh-cooled semen, but transport delays are less likely to affect semen quality as long as the liquid nitrogen is maintained. The cost of shipping frozen semen, however, is higher than fresh-cooled. [9]

The most significant disadvantage of frozen semen is a decreased conception rate compared to other types of semen. This is attributed to the effects of freezing on sperm function. [4]

Furthermore, frozen semen use requires more intensive management on the part of the veterinarian, resulting in increased time and costs for the mare owners. [4][6][11]

Timing of Insemination

Since frozen semen is already lower quality than other types of semen, most veterinarians aim to perform AI as close to ovulation as possible to maximize the chance of success.

The timing of insemination also depends on the amount of semen available. With more available semen, the timing of breeding doses to exactly when ovulation occurs becomes less critical. [5]

If there is only one breeding dose available, most veterinarians choose to administer ovulation-inducing medication and closely track the mare’s ovaries for the following 40 hours.

Once ovulation is identified, the mare is bred with the single breeding dose. [4][5][6] Mares must be bred within 6 hours of ovulation with frozen semen, otherwise pregnancy rates decline significantly. [4]

If two or more doses are available, the mare can be bred prior to her expected ovulation time, as well as after ovulation, to maximize the chances of fertilization. [5][6][11]

Studies show that there is no significant difference in pregnancy rates between single-dose or multiple-dose breedings. However, the added flexibility of multiple doses makes it a preferred strategy for many breeders. [4][11]

Pregnancy Rates

Frozen semen is generally considered to have a lower pregnancy rate than fresh-cooled semen, but some studies show similar pregnancy rates or even improved rates. [4][12]

Reported pregnancy rates range from 38 – 73% per breeding cycle, with 76 – 82% of mares becoming pregnant by the end of the breeding season. [4][6][9]

Summary

  • Artificial insemination reduces the risks associated with natural service, while providing additional benefits such as increased access to genetics and the ability to store semen
  • The type of artificial insemination performed depends on semen quality, type of semen, and mare fertility
  • Fresh, fresh-cooled, and frozen semen are all options for artificial insemination, however each have their own advantages and disadvantages
  • Artificial insemination protocols require you to work closely with your veterinarian for the best chances of a successful pregnancy

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References

  1. Kowalczyk. A. et al. Factors Influencing the Popularity of Artificial Insemination of Mares in Europe. Animals. 2019. View Summary
  2. Aurich. J. E. Artificial Insemination in Horses—More than a Century of Practice and Research. Journal of Equine Veterinary Science. 2012.
  3. Brinsko. S. P. and Blanchard. T. L., Eds.Manual of Equine Reproduction, 3rd edition. St. Louis, Mo: Mosby/Elsevier. 2011.
  4. McKinnon, A. O. et al. Equine Reproduction. Wiley-Blackwell, 2010.
  5. Dascanio. J. J. et al., Eds. Equine Reproductive Procedures, 2nd edition. Wiley-Blackwell. 2021.
  6. Metcalf. E. S. The Efficient Use of Equine Cryopreserved Semen. Theriogenology. 2007. View Summary
  7. Govaere. J. et al. Effect of Artificial Insemination Protocol and Dose of Frozen/Thawed Stallion Semen on Pregnancy Results in Mares. Reproduction in Domestic Animals. 2014. View Summary
  8. Haadem. C. S. et al. A Retrospective Cohort Study on Fertility in the Norwegian Coldblooded Trotter after Artificial Insemination with Cooled, Shipped versus Fresh Extended Semen. Acta Vet Scand. 2015.View Summary
  9. Loomis. P. R. The Equine Frozen Semen Industry. Animal Reproduction Science. 2001. View Summary
  10. Newcombe. J. and Cuervo-Arango. J. The Effect of Time of Insemination With Fresh Cooled Transported Semen and Natural Mating Relative to Ovulation on Pregnancy and Embryo Loss Rates in the Mare. Reproduction in Domestic Animals. 2011. View Summary
  11. Loomis. P. R. and Squires. E. L. Frozen Semen Management in Equine Breeding Programs. Theriogenology. 2005. View Summary
  12. Crowe. C. a. M. et al. A Retrospective Study of Artificial Insemination of 251 Mares Using Chilled and Fixed Time Frozen-Thawed Semen. Equine Veterinary Journal. 2008.View Summary