The ABCs of Artificial Insemination

The ABCs of Artificial Insemination

The stallion is encouraged to ejaculate into the AV by the presence of a mare in estrus or a dummy (padded barrel-like stand on which the stallion can mount, seen here).

Photo: Anne M. Eberhardt/The Horse

There are many reasons why you might decide to breed your mare by artificial insemination (AI). The most common is to allow you to use a stallion that is a distance away--even abroad--and so increase the scope of stallions available and reduce the risks and expense of travel for your mare. However, AI might also be chosen for reasons such as: To minimize disease transfer by using antibiotic extenders; to increase the chance of conception in mares susceptible to uterine endometritis; to reduce the risk of physical injury to humans and stock, especially if the mare or stallion is particularly nervous; or to breed difficult mares and stallions, i.e., those with physical abnormalities caused by accidents, infection, poor perineal conformation, psychological problems, etc. (Care must always be taken, however, to ensure that such problems are not heritable.) From the stallion owner's point of view, AI might increase the number of mares covered per stallion per season, and hence increase financial return.

Before embarking on AI for the first time, it is important that you contact the relevant breed society in time to ascertain their regulations regarding the use of AI and obtain the necessary paperwork. Armed with this information, you can choose your stallion and discuss your requirements with your veterinarian.

As a mare owner, you are likely to only witness the insemination of your mare. However, it is good to have an understanding of what happens behind the scenes--how semen is collected, evaluated, and stored--so you can make educated decisions when arranging semen delivery and appreciate why things do not always go according to plan.

AI is based upon the principle of collecting semen from a stallion, adding a specifically formulated extender that allows for varying lengths of storage and/or transport, then inseminating the semen into a mare.

Semen Collection

The most common method of semen collection is via an artificial vagina (AV). The AV mimics the mare's natural vagina by providing a warm, sterile lumen (cavity) surrounded by a water jacket, under some pressure, plus a collecting vessel. Warm water is introduced into the water jacket to ensure a lumen temperature of 111-118°F (44-48ºC). The lumen pressure can be adjusted by increasing the volume of water or by pumping in air. The collecting vessel should also be warm, 100-111°F (38-44°C), as cold shock to sperm at any time is fatal.

The stallion is encouraged to ejaculate into the AV by the presence of a mare in estrus or a dummy (padded barrel-like stand on which the stallion can mount). If a mare is used, covering occurs normally except the stallion's penis is diverted into the AV rather than entering the mare's vagina. A dummy provides a safer alternative for well-trained stallions.

Once the semen has been collected, it must be kept at body temperature (100°F; 38ºC) until it has been evaluated. It is then used immediately or extended prior to longer-term storage.

Semen Extension and Storage

Immediately after collection and before storage, the semen must be evaluated to determine its quality. Assessment should include volume, sperm concentration, and progressive motility as indicators of the number of live sperm present and the insemination doses that can be obtained. Morphology (a study of the configuration or structure of sperm), live:dead sperm ratio, cytology (the study of cells), pH (acidity), longevity, and bacteriology also can be assessed, especially if a problem is suspected. The semen sample arriving for your mare should have been assessed at collection and an evaluation report included, giving a quality guarantee.

There are three types of semen that might be available for your mare: Fresh, chilled/cooled, or frozen.

Fresh (raw) semen is inseminated exactly as it was collected from the stallion, with no extender added. It must be kept at body temperature and used immediately; therefore, invariably the stallion and mare will be at the same farm.

Chilled semen is extended and stored at 39-41°F (4-5ºC). It remains viable under these conditions for up to 48 hours, allowing transport over reasonable distances. A variety of extenders are used; most are based on non-fat dried skimmed milk solids, skimmed milk, or occasionally egg yolk. Chilled semen can be transported via a postal or carrier service in specially designed insulated containers, such as the Equitainer. If you use chilled semen, it is likely to arrive in such a container and should be left in the container in a cool, safe place until its use, which should be as soon as possible.

Frozen semen is treated similarly to chilled semen, although the extenders also contain a cryoprotectant (ingredients that protect against "freezer burn") such as glycerol, an anti-freeze that protects sperm from rapid freezing. Frozen semen contains less extender, so it is smaller in volume and more concentrated, allowing it to be stored in 0.5-5.0-mL straws, reducing the storage capacity required. Freezing in liquid nitrogen allows indefinite storage and transport around the world. If you are importing semen for your mare, it is very likely to be frozen and should arrive in a liquid nitrogen flask.

Semen Insemination

In order to synchronize the arrival of the semen sample, ovulation in your mare, and the availability of your veterinarian or inseminator, the mare's estrous cycle will be manipulated. Your vet will advise you on the best method, but normally a period of progesterone treatment, followed by prostaglandin F2 alpha (PGF2 alpha) and maybe human chorionic gonadotropin (hCG) or gonadotropin-releasing hormone (GnRH), or perhaps just PGF2 alpha alone, will be used. This will allow ovulation time to be targeted to within five days.

During this period, your vet will scan your mare regularly to determine the exact timing of ovulation. For chilled semen, this will allow you enough warning to call the chosen stud farm, order the semen, and allow time for delivery. The semen will normally arrive within 24 hours of collection, and if kept cool, will allow another 24-36 hours during which (hopefully) the mare will ovulate and be inseminated within the life span of the semen.

For frozen semen there is no problem with limited life span. However, frozen semen has a short post-thaw lifespan (six to 12 hours), so inseminating as close to ovulation as possible is vital. Hence, such mares are often scanned every six to 12 hours when ovulation is imminent.

When the semen arrives, it is very important to make sure that it is accompanied by the correct paperwork, such as health certification, import license, evaluation report, and confirmation of stallion identity. You must make sure that you have the correct paperwork from your breed registry to verify that AI is being used to cover your mare.

Before or at the time of insemination, a small sample of the semen should be evaluated by your veterinarian. Motility is the parameter usually assessed, giving an indication of sperm viability. This can easily be done under a standard light microscope. A poor sample might not be worth inseminating, or at least you will not be too surprised when conception fails. If you receive a poor sample, inform the stallion's farm so they can review their handling procedures. You might get a refund or free semen.

Insemination Process

Insemination is relatively simple, and regardless of whether the semen is frozen, chilled, or fresh, the technique is the same. Your mare should be restrained in stocks while her perineal area is washed and her tail is bandaged (see photo above). The inseminator will insert an insemination pipette or gun (with the aid of their lubricated hand and forearm) into the vagina, through the cervix, and into the uterus using a forefinger. The semen is deposited into the uterus (see photo on page 39). As an alternative, the inseminator might guide the insemination pipette through the cervix by feeling through the rectum wall, via rectal palpation. This method is reported by some to reduce infection risk as only the fine insemination pipette breaches the outer defenses of the mare's reproductive tract.

For fresh and chilled semen, the volume inseminated might be up to 100 mL and will contain 500 million progressively motile sperm per insemination dose. Prior to insemination, the sample is drawn up into a syringe that is attached to the end of the inseminating pipette. Once the pipette is in place within the uterus, the syringe plunger is slowly depressed and the sample expelled.

For frozen semen, the sample is held in straws (0.5-5.0 mL). Larger volume straws are thawed in a water bath and the semen drawn up into a syringe and inseminated as described previously. For smaller volume straws (0.5-1.0 mL), specially designed insemination guns allow the straw to be held within the gun and the semen directly expelled when the plunger is depressed.

Whatever the volume used for frozen semen, normally 800 million sperm are inseminated.


AI is a relatively easy, safe, and convenient method of covering mares with stallions from all over the world. As with all techniques, it takes skill to make sure all the proper steps are taken to provide the best chance for pregnancy.

About the Author

Mina C.G. Davies Morel, BSc, PhD

Mina Davies Morel, PhD, is head of the equine group at the Institute of Biological, Environmental and Rural Sciences at Aberystwyth University in the United Kingdom. She has particular interest in equine reproductive physiology and its application to stud management, and she is the author of a number of scientific papers and text books on the subject. She is a leisure rider and owner of Welsh Cob Section Ds.

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