What You Need to Know

By Dr. Lindsay Rogers, DVM

From bloodlines to athletic career to temperament, there are many different reasons to breed your mare. While the mare owner may have no trouble making the decision to breed their mare and choosing the perfect stallion, sorting through the myriad details involved in the actual breeding process can be challenging. Being informed about mares’ cycles and the different options for breeding can help make the breeding process go more smoothly, especially for first-time breeders.

Mares are seasonally polyestrus. This means they have multiple heat cycles (estrus cycles) throughout the breeding season, but go through a period of not cycling (anestrus) during the winter, with transition periods in the spring and fall. A normal, cycling mare has an average 21-day cycle when she is out of heat (diestrus) for about 14 days and in heat (estrus) for about seven days. The lengths of diestrus and estrus can vary among mares, but an individual mare is usually consistent within her own cycles. During the estrus portion of the cycle, a follicle, which contains the unfertilized egg, increases in size on the ovary. In the average mare, a follicle reaches 35-65 millimetres in diameter before ovulating. The variation in follicle size is typically due to breed, but each individual mare will have a fairly consistent follicle size at which she ovulates. She will ovulate and release an egg at the end of her heat cycle. For best results, the mare should be bred as close to ovulation as possible, as this is the most fertile part of the cycle and hormones can induce ovulation within a certain time period. The egg is fertilized in the oviduct (which connects the ovary to the uterus) and then the fertilized egg drops down into the uterus about five days after fertilization. A normal mare will continue to cycle this way throughout most of her lifetime, but fertility does decline with age. Mares generally have good fertility until they are about 15 years old and then fertility begins to slowly decline between 15-20 years of age. After 20 years of age, fertility declines rapidly and mares will often no longer cycle once they reach their mid-20s.

In order to determine where a mare is in her cycle, two different methods can be employed. In more intensive breeding management programs, these methods are often used in conjunction with each other. The simpler method, usually employed by breeders who are hand-breeding, is to tease the mare to be bred. To do this, the mare is brought to the stallion or the stallion is brought to the mare, and they are allowed to meet over a fence called a teasing board. This fence is usually covered with mats for protection from injury to stallion and mare. The mare is assessed to see if she shows signs of estrus, if she is disinterested in, or if she is aggressive towards the stallion. The other method is via palpation and ultrasound. Palpation is used to determine the tone of the uterus and cervix, and the size of the ovaries. Palpation is typically followed by ultrasound to look for visual changes in the uterus and to measure follicular size. This is commonly done when artificial insemination is being used to ensure timing is appropriate for breeding.

Rectal palpation of a mare is done to assess uterus and ovaries. Photo: Dr. Dawne Nairn

There are many different options for breeding a mare, under two main categories – live cover or artificial insemination. Live cover may be by pasture breeding with the mare and stallion together in the pasture long enough for the mare to have a normal cycle and be bred. Live cover may also be accomplished by hand-breeding, where both the stallion and mare are held by handlers and the mare is mounted by the stallion in a more controlled environment. Many mares bred this way do not require palpation or ultrasound to determine their cycle stage as a mare will not normally stand for a stallion if she is not in estrus.

Live cover by pasture breeding – the stallion and mare are together in the pasture long enough for the mare to have a normal cycle and be bred. A mare will not typically stand for a stallion if she is not in estrus. Photo: Shutterstock/Julia Siomuha

For mares that have trouble becoming pregnant or cannot carry a pregnancy, or for stallions with limited semen, more advanced breeding techniques are typically employed. For mares that cannot carry a pregnancy or owners who wish to have multiple foals from one mare in a season, an embryo transfer procedure can be performed. In embryo transfer, the donor mare is bred near the time of ovulation, and then the fertilized embryo is flushed out approximately seven days later. The embryo is then transferred into a recipient mare, who carries the pregnancy to term and in essence becomes the surrogate mother to the foal. Another option that is gaining popularity is intracytoplasmic sperm injection, or ICSI (pronounced “ick-see”) for short. In this procedure, the eggs are aspirated straight out of the mare’s ovaries and then fertilized with a single sperm in a petri dish. The fertilized egg is allowed to mature for a few days and is then implanted into a recipient mare to be carried to term. This procedure is much more complicated and expensive, and so is reserved for mares that cannot become pregnant but still have viable oocytes, or mares to be bred to stallions whose semen is limited. The latter situation is usually when the stallion has been gelded or has passed away, and there is only a small amount of frozen semen left.

When bred via artificial insemination, fresh-extended, cooled, or frozen semen may be used. These mares are followed more carefully by a veterinarian with palpation and ultrasound in order to breed as close to ovulation as possible. Each of these methods has its own pros and cons.

Breeding using fresh semen is usually done by live cover of the mare by the stallion – either through pasture breeding or hand breeding. The stallion can also be collected, the semen filtered and extended, and then the mare is bred via artificial insemination immediately thereafter. The major advantage of using fresh semen is that it is of the highest quality for breeding and useful in stallions that are experienced for live cover, or those whose semen does not cool or freeze well. Live cover is also the only method of breeding allowed by The Jockey Club for registration of racing Thoroughbreds. The use of teasing and live cover also eliminates the need for expensive equipment, such as an ultrasound and collection facility, making it a simpler option for breeders who own their own stallion and mares. Fresh semen tends to have the best longevity, remaining viable within the mare for about two days after breeding (or even up to five days for some stallions with excellent semen). This means that the timing of breeding does not have to be as close to ovulation as with other methods of breeding. The disadvantage is that the mare and stallion have to be in the same physical location, which is not always feasible, and some mares will react to the unfiltered semen from live cover (in filtering, much of the extra components of the ejaculate is filtered out leaving just the semen). Live cover is the most dangerous method of breeding for the mare and stallion if either are inexperienced.

Cooled semen has first been collected from the stallion, then filtered, extended, and placed in specialized cooling boxes for shipment to the mare. The advantages of cooled semen are that the mare and stallion don’t have to be in the same location (or even the same country), and that most of the viability of the semen is preserved as compared to frozen semen. Cooled semen will remain viable in the mare’s uterus for about 24 hours (sometimes up to 48 hours) so that the timing of breeding and ovulation must be closer together than when using fresh semen. The main disadvantage of using cooled semen is the requirement for excellent timing and mare management. As cooled semen is often “ordered” a few days in advance, then received by the mare management team approximately 24 hours after it is collected and processed, managing the timing so that the mare is close to ovulating when the semen arrives can be difficult. This is compounded by a stallion’s show or collection schedule, or challenges in getting semen from outside the country. However, in terms of balancing viability of semen with the ability to breed horses that are not in the same location, cooled semen is an excellent option.

Frozen semen has first been collected from a stallion, then processed to be frozen in liquid nitrogen. This preserves the semen for as long as the semen remains in liquid nitrogen (indefinitely). The advantage of frozen semen is that it can be shipped to where the mare is being managed well in advance of when she will be ready to breed, which eliminates the need to synchronize the mare with the stallion’s schedule. Another benefit is that mares can be bred to stallions that have passed away or have been gelded, or those on different continents. Unfortunately, the semen of some stallions does not freeze well, and it can be difficult to get mares pregnant with poor semen. Frozen semen remains viable in the mare for only about 12 hours after insemination, which means the mare should be bred as close to ovulation as possible, and this calls for much more intense mare management. The average pregnancy rates with frozen semen tend to be about half of those using cooled semen, making it less advisable to use frozen semen when breeding an older mare or a mare known to have trouble becoming pregnant. Because of this, the amount of time and money spent trying to achieve a pregnancy in a mare with frozen semen can be quite a bit higher than when using fresh or cooled semen in a comparable mare.

Once a mare has been bred, it is time to plan her pregnancy checks. Early on, these are typically done via transrectal palpation and ultrasound. As the fetus ages and grows, pregnancy checks can be done via palpation only, and then via transabdominal ultrasound.

It is recommended that the initial pregnancy ultrasound be performed at 14-16 days after the mare’s last breeding or ovulation date, when the embryo is of sufficient size to be visualized on ultrasound. The embryo is mobile within the uterus at this point, and if there are twins (which, when carried to term are usually dangerous for both the foals and the mare), this is the best time to eliminate one of the twins, leaving one embryo. If the mare is not pregnant, she will just be coming back into heat and can be tracked and bred in her upcoming cycle. At this check the pregnancy is called a vesicle, and as it does not yet have a heartbeat, we say that there are “signs consistent with pregnancy.”

Ultrasound image of twin vesicles in a mare. Ideally a twin reduction procedure would be performed in order to reduce it to a singleton pregnancy, as allowing twins to be carried to term in horses is usually dangerous for both the mare and the foals. Photo: Dr. Lindsay Rogers

A second pregnancy ultrasound is recommended at 25-30 days of gestation to ensure there is an embryo with a heartbeat present.  If there is, at this point we can say that the mare is pregnant.    In most instances, mares are not checked again for pregnancy after this point unless there is a concern or a reason for checking again.

Fetal sexing, if desired by the owner, can be performed at days 55-60 of gestation; however, this can be difficult if the fetus is not in the right position.

Another pregnancy check, can be performed at five months of pregnancy before starting the mare on a three-dose series of herpesvirus vaccinations to prevent infectious abortion. This check can be beneficial to confirm whether the mare is still pregnant prior to administering vaccines and increasing her feed as she gets closer to term.

The end goal – a happy, healthy, spunky foal. Photo: Clix Photography

While you may have found the perfect stallion to match your mare in terms of athleticism, bloodlines, and temperament, it is also important to consider the logistics of breeding your mare. By being well informed you will better understand and appreciate the costs and processes that will lead to the birth of your beautiful foal the following year!

This article was originally published in the March/April 2017 issue of Canadian Horse Journal.

Main article photo: Clix Photography