Breeding Your Mare

Photo credit: Valerie Hinz

Photo credit: Valerie Hinz

So, you want to breed your mare.  You think you have the perfect stallion to match her and so it should be easy, right?  Not quite.  Nowadays, it can be much more complicated than just getting a mare and stallion together.  There are many different options available for breeding your mare.  In this blog post, we will discuss the mare’s reproductive cycle, the different types of semen available, and when to have pregnancy checks done on your mare. 

Mares are seasonally polyestrus.  This means that they have multiple estrus cycles (meaning multiple chances to breed) over the breeding season which typically extends from late March to early September in the Northern Hemisphere.  They then go through a period of not cycling, or anestrus, through the winter.  In the early spring and late fall mares go through a period of transition – either from anestrus to cycling in the spring, or from cycling to anestrus in the fall. 

A typical estrus cycle in the mare is about 21 days long and contains a period of estrous, when the mare is in heat and ready to breed, and a period of diestrus, when the mare is not in heat and won’t become pregnant.  The typical length of estrous is 5-7 days with diestrus being around 14 days.  These lengths can be variable between mares, but typically a mare will be consistent within her own cycles.  Mares are typically bred towards the end of their estrus period when they are close to ovulation (release of an unfertilized egg from the ovary to the oviduct).  After ovulation, the egg is fertilized in the oviduct and then the fertilized egg descends into the uterus about 6 days after fertilization.  

As mares age, they have a decline in fertility due to the fact that their oocytes (eggs) develop when they are still a fetus in their dam’s uterus.  They typically have good fertility to about 15 years of age, with a slow decline in fertility from 15-20 years, and then a rapid decline when they are older than 20 years old.  Many mares stop cycling once they reach their mid-20s.  This creates problems when trying to breed an older mare, especially if they are a maiden and usually results in increased cost and time managing the mare.  This is something to keep in mind when deciding whether or not to breed your mare.

Once you’ve decided to breed your mare, there is the consideration of which type of semen should be used (this decision may be made for you depending on your choice of stallion).  There are three basic types of semen, each processed a little differently and each with their own pros and cons. 

The first type of semen is fresh semen.  Breeding with fresh semen is typically achieved by live cover, where the stallion breeds the mare directly – either in pasture, or by hand breeding in a more controlled environment.  This semen requires no processing or special equipment for breeding.  Fresh semen can also be collected from the stallion, filtered and extended, and then the mare is bred via artificial insemination immediately thereafter.  The extender is a product added to raw semen to help increase its longevity when not in the mare.  Fresh semen will be of the highest quality and longevity, lasting 48 hours or more once in the uterus.  Unfortunately, it can be more inflammatory in mares that tend to react to semen after breeding as it is unfiltered and contains all of the secretions from the stallion’s accessory sex glands.  This is a benefit of using fresh-extended semen – it has the longevity of fresh semen, but tends to be less inflammatory than when it is unfiltered.  Unfortunately the ability to collect a stallion and process the semen is required for fresh-extended semen.  The main con of using fresh semen is that the mare and stallion have to be in the same location. 

Another type of semen is cooled-transported semen.  Semen is collected and then filtered and extended before being cooled in specialized transport boxes.  This allows for shipment of the semen across countries.  Usually the shipment is overnight so the semen arriving where the mare is to be bred is 24 hours old at time of insemination.  The semen will typically survive for about another 24 hours  once in the mare.  The benefit of cooled, transported semen is that it is still of fairly high quality when it reaches the mare and mares and stallions can be in very different locations and still be bred to each other.  Unfortunately this type of breeding requires increased management of the mare as the stallion station needs to know ahead of time when to collect and ship the semen from their stallion.  Mares need to be followed with palpation and ultrasound fairly closely by a veterinarian or breeding facility in order to breed them within about 1 day of ovulation.

Frozen semen is the third type of semen readily available.  A stallion is collected and his semen is processed to prepare it for freezing in liquid nitrogen.  Once frozen, this type of semen can remain usable for years as long as it is stored appropriately in liquid nitrogen.  The benefit of frozen semen is that it can be shipped to the mare well ahead of when it is needed and stored until the mare is ready to be bred.  It can be shipped across oceans so that mares and stallion on opposite sides of the world can be bred to each other.  Unfortunately it also tends to be the lowest quality and most inflammatory to the mare’s uterus.  The process of freezing semen is damaging to the semen which has a great impact on the quality and longevity of the semen.  Once thawed, frozen semen usually only lives for about 12 hours within the mare’s uterus meaning that the mare needs to be managed diligently through her cycle and bred as close to ovulation as possible (within 12 hours).  Some stallions have semen that does not tolerate freezing well and so while they may be quite fertile using fresh or cooled semen, their fertility declines greatly with frozen semen.  It is also the most inflammatory to the mare meaning that mares who have trouble becoming pregnant are poor candidates for breeding with frozen semen.

Once the mare is bred, it is time to plan pregnancy checks.  Typically a mare’s first pregnancy check is performed at 14-16 days after ovulation or last breeding date.  This pregnancy check is very important to check for twins.  In horses, twins are rarely carried to term successfully – they will typically abort part way through the pregnancy, or if carried to term can result in trouble foaling that is dangerous for both the foals and the mare.  If twins are found at a 14 day pregnancy check, one of the vesicles (called a vesicle at this stage as it does not yet have a heartbeat) can be reduced resulting in a singleton pregnancy.  A pregnancy check at this time is also useful if the mare is not pregnant as she will be showing signs of coming back into heat and can be bred again on her next cycle.

Ultrasound image of twin embryos in a mare at approximately 17 days gestation.

Ultrasound image of twin embryos in a mare at approximately 17 days gestation.

The next pregnancy check is typically done at 25-30 days after ovulation or last breeding date.  This is the heartbeat check at which point a heartbeat in the embryo can be assessed to ensure a viable pregnancy. 

After these two pregnancy checks, the remaining checks are optional for the owner – fetal sexing can be done around days 55-60 of gestation, and some owners will request another pregnancy check around 5 months of gestation, before starting the mare’s rhinopneumonitis vaccination program.  A final pregnancy and general health check in the month before the mare is due is also beneficial to ensure the mare and foal are healthy and ready for foaling.

While breeding your mare can seem like a stressful endevour, the end result, a happy foal in the next year, is well worth the investment.