All You Need to Know About Breeding Your Horse

There is nothing quite so magical, and yet ordinary, as the miracle of reproduction. Seeing a mare through pregnancy is one of the most rewarding experiences in an equestrian’s life, but can also be one of the most anxiety-inducing! Though pregnancy is as natural an experience as any other, countless questions are bound to arise. 

What is everything you need to know about horse breeding and pregnancy? Compared to other livestock, horses usually receive more veterinarian management and intervention, which can make the process expensive. Horses can also be pregnant anywhere from 320-380 days. After a certain time, pregnant mares have diet restrictions that can make their care more tedious.

Keep reading to learn the basics (and more!) of horse breeding and pregnancy.

Horse Breeding: When is the Right Time?

While some female animals experience induced ovulation, horses are cyclical. A mare will experience her first heat around 12-15 months of age and, from there, will sustain a 21-day estrus cycle until around the age of 20. Out of these 21 days, a mare will be in heat, fertile, and receptive for approximately 5-7 days. This is a generally short cycle with a generous fertility window, making the timing of the mating somewhat convenient. If you are interested in breeding your mare, you will want to keep a close eye on her cycles. Here are some signs that she may be in heat:

• Behavioral changes – a mare may be more irritable (or more affectionate) when she is in heat.
• Interest in stallions – a mare in heat may urinate in front of a stallion or gelding, may strike a straddling pose, and may move her tail to the side in invitation.
• Swollen vulva – you may notice changes to your mare’s reproductive organs.

Though a filly may experience her first heat as early as 12 months of age, she should not be bred until much later. A horse is not finished growing until 4-6 years old, depending on the breed. While many breeders choose to start their mares earlier, I wouldn’t recommend you breed a horse under the age of 4 without the blessing of a trusted veterinarian.

To learn more about how to tell whether your mare is in heat, visit my article How to Tell if a Horse is in Heat: Complete Guide.

Horse Breeding: Artificial Insemination vs. Live Cover

There are two ways that mares are bred: naturally (romantically called “live cover”) and artificial insemination. Horses are not as prolific as, say, rabbits – with only 50-60% of matings resulting in conception. Typically, artificial insemination sees an even lower success rate. With both methods, there are often a few “tries” to help make up for these lackluster percentages.

If your preferred stallion is not local, this choice has been made for you – you will artificially inseminate your mare. You can do this with either frozen semen or fresh, cooled semen. Depending on your distance from the stallion, fresh semen may not be an option. If your preferred stallion is local, you may have the option to request live cover. This may be done by leaving your mare with the stallion for a couple of weeks so that they can have as many opportunities as possible to seal the deal.

Horse Pregnancy: Did it Work??

Whichever method of insemination you’ve decided on, you’re going to be anxious to know as soon as possible whether a conception has occurred. The only way you can tell with certainty that your mare is expecting is via ultrasound. An experienced veterinarian can detect a fetus as early as two weeks from conception, though they may recommend waiting for a week or two beyond that to ensure the best “view.” An ultrasound will cost around $50, but this doesn’t include the vet call to your farm.

Aside from an ultrasound, there are a few other signs (and tests) that may indicate your mare has conceived:

• Blood test – a sample can be taken to test for elevated progesterone, an indication of pregnancy. This test is not always reliable, however, and some mares have naturally higher progesterone levels than what is typically seen.
• Absence of heat – if you’ve been watching your horse’s cycles and can generally detect when she is in heat, the absence of that may indicate your horse has conceived. Accuracy here depends on the horse – some mares experience “silent heat” and display almost no outward signs.
• Disinterest in stallions – where a mare in heat seeks the attention of stallions, a mare who has conceived will generally ignore or reject a stallion.
• Increased girth – a mare with a visibly larger girth may be pregnant, though this is not very reliable. A mare will not “show,” if at all, until the later portion of her pregnancy. There may, too, be other reasons for a change in belly size – we once had a rescue horse who had been advertised as being exposed to a stallion. We were told by a professional that she appeared to be pregnant based on the size of her girth, but after an ultrasound, it was determined she was simply experiencing a distended abdomen due to malnourishment. 
• Udder changes – a mare’s udders will begin filling with milk at the very end of her pregnancy. If this is the first sign you’ve noticed that your mare is pregnant, you don’t have much time to prepare – birth is generally within days to weeks at this point.

Horse Pregnancy: What to Expect When Your Mare’s Expecting

The most common question about horse pregnancy usually revolves around gestation length. While most sources will note horse gestation as being 11 months, in reality, anything between 320 and 380 days is within the norm. There is a common saying that goes, “The fetus determines the day of delivery, and the mare determines the hour.” 

It is important to have regular veterinary care during your mare’s pregnancy. While most mares will conceive a single foal, twins are not unheard of and, in most cases, will need to be “reduced” to avoid complications for both mare and foal. The occurrence of twins varies by breed, with Thoroughbreds experiencing the highest rate at 25-35%. Quarter Horses, for comparison, experience twin pregnancies at a rate of about 5-10%. In more than 90% of discovered cases, the smallest twin is removed in utero.

Mares will need regular vaccinations and de-worming during pregnancy to ensure the greatest chance of a healthy delivery and a healthy foal. Specific vaccinations and scheduling will depend on your region, which is yet another reason to involve a trusted veterinarian in your mare’s pregnancy. It is best to have these vet checks on your ranch if possible, as unnecessary transportation may increase stress on your mare.

Finally, though you may be tempted to shower your mare with extra nutrients and sweets during her pregnancy, this may do more harm than good. A healthy mare will not need any increased calories until the very end of her pregnancy and during lactation. Though she may be requesting more treats from you, overweight mares generally have a more challenging time during birth. Your mare will, however, experience increased hydration needs for both her growing fetus and for milk production.

Dietary Restrictions for Pregnant Horses

There is one dietary restriction every horse owner needs to know when it comes to taking care of a pregnant mare. Upon entering the last trimester, or about 90 days before the given birth date, the mare should be taken off of fescue grass. There is a fungus produced by fescue grasses that can lead to complications in equine pregnancies.

Before you decide to breed your horse, you need to look at your current set-up to ensure you can remove your mare from any Fescue grass that may be on your property. Fescue is a common grass in the United States, so this can be easier said than done. You may have to create a dry lot space or re-seed a new area beforehand.

What Can Horses Breed With?

This may sound surprising at first, but horses are not actually limited to their own species when breeding. In fact, horses breed outside of their species all of the time. When a horse conceives with a donkey, the resulting offspring is called a mule. Mules are valued for their large statute and even temperaments and are used for both riding and agricultural purposes. Mules are sterile, so the only way to create more mules is by breeding horses to donkeys.

In the same way, horses can also breed with zebras, the resulting offspring being a “zorse.” Of course, a zorse is not as commonly seen as a mule, because zebras are not typically kept as livestock like donkeys are. Still, they are quite striking if you ever come across one!

Of course, horses can also breed with other horses. Any breed of horse can breed with any other breed of horse, with the offspring of unknown heritage simply called a “grade horse.” Horses of different sizes can also breed together – in fact, a pony can even reproduce with a larger breed, with the general consensus being that a foal will grow only as large as the mare’s body will allow.

Differing Sizes Between the Mare and Stallion

There is a science to producing a certain type of horse. For example, many people love draft crosses. They like the calm demeanor and sturdy build of a draft horse, and the athleticism and height of a light riding horse, so they mix them together. However, depending on the size and breed of the mare and stallion will depend on how the baby looks coming out.

With draft crosses, you’ll see some with more draft horse characteristics than light horse characteristics, and vice versa. If the mare was the draft horse and the stallion was the light horse, the baby would come out looking more like a draft horse. If the mother was a light horse and the father a draft horse, the baby would come out looking more like a light horse.

The biggest thing to remember on this front is that, ideally, the mare should be the bigger horse. If a larger stallion breeds with a much smaller mare, it can become a health risk for the mare.

Is Horse Breeding and Pregnancy Complicated?

While there will always be inherent risks involved in the reproduction of larger animals, horse breeding really isn’t all that complicated. After all, horses reproduced naturally for thousands of years and still do so in the wild. Most mares do just fine on their normal rations during pregnancy and, more often than not, will be able to birth their foals without any assistance at all.

Of course, most human pregnancies are also uncomplicated, but being under the regular care of an obstetrician is still highly recommended. I would advise the same for any horse owner interested in breeding. While most horse pregnancies will go off without a hitch, being under the regular care of an experienced vet is invaluable and can prevent potential complications. 

Once your mare gives birth, the fun has just begun! Foals are chaotic troublemakers who can be a blast to have around the farm! To learn about how foals grow and mature, check out my article Horse Growth Guide: When Do Horses Stop Growing?