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Mare & Foal


The breeding industry has come along way from the time a stallion was simply placed in a field of mares and watched to see what would happen and when it would happen. The things the observers were looking for were when the mares would actually be covered by the stallion and then, equally important, they would note when the mare stopped accepting the stallion. This information was accurately recorded and then used the following year to indicate when the mares were due to foal. This natural procedure is called "pasture breeding."

Of course it worked, but not very efficiently. As in all arts and sciences, the procedure was improved upon. What the farm manager knew was that after the mare had been covered, if she were not back in heat after 16 days or so, she was probably pregnant. Now we know exactly why that is.

Today we not only know when a mare is in foal, but with modern technology, primarily the use of the ultrasound machine, we know just what’s going on in her reproductive tract. This tells us the optimum time to breed her so that she will become pregnant.

So, you’ve decided to retire your race mare. She’s been a consistent campaigner, earning over $100,000 and taking a piece of a little stake at Del Mar. She’s still sound, but it’s springtime now and she just doesn’t seem to care about winning anymore. It’s time for her to ship to the breeding farm. You expect she’ll be bred within the next few days and you’ll have a healthy foal on the ground next spring. Simple as that. Boy, do you have a lot to learn.

It’s just not that simple. If it were, we’d have huge herds of wild horses, but the fact is the percentage of mares that produce foals in the wild is very low.

"The horse is probably the only domestic species that isn’t selected on fertility," said Dave McGlothlin, farm manager of Harris Farms. "If you have a cow and she doesn’t produce a calf she ends up as hamburger. With horses we select for everything but the ability to reproduce. We select for speed, for color, athletic ability, and that’s why the fertility percentages are so much lower for equines than they are for other species."

The fact is, in order to get your mare pregnant in a timely fashion, there is quite a bit of help from modern technology. And there are quite a few things that an owner should understand.

"One of the first things that owners need to understand," said McGlothlin, "is that mares don’t cycle year-round. Owners will send a mare into a breeding farm and just assume that she’ll be ready to breed in a couple of weeks, get pregnant on the first cover and be ready to go home from there. Unfortunately, depending on the time of year, a mare may not be cycling. These mares may respond to lights or photo period manipulation to help them start cycling. Some of the mares that shipped into us in January are just now (March 20), beginning to get serious about cycling. Some owners are inpatient and think we can simply program a mare to meet a schedule, but in reality it’s the mare and Mother Nature that dictate when the mare is ready to be bred."

Mares are reproductively active during the periods of long daylight. "Being under lights," as McGlothlin refers to is one way that technology has helped Mother Nature along.

"Starting mares on a regimen of natural and artificial photo periods that equal 16 hours of light per day will start a series of hormonal events that make the mare believe that it’s actually the middle of May (the natural breeding season), and therefore that she’s supposed to be reproductively functional," explained McGlothlin. "This allows breeders to start breeding in early February rather than the middle of May."

"When a mare arrives on the farm we start teasing her with our teaser stallion and have our vet, Dr. Jeanne Bowers, evaluate the mare’s ovarian activity and see if the mare already appears to be cycling," said McGlothlin.

In determining her ovarian activity the veterinarian would perform a rectal palpation, and on some farms, an ultrasound of the uterus and the ovaries. What the veterinarian would be looking for are follicles and changes in the uterus indicating that the mare is in heat. The follicles are structures within the ovaries that contain the eggs or ovum. Sometimes abnormalities are seen, such as fluid in the uterus and cysts. Cysts need to be noted so they are not confused with early pregnancies.

Diagram of Mare

Probably one of the greatest advancements in helping man better manage breeding decisions is the ultrasound machine. With ultrasound, the veterinarian can see the amount of follicular activity, visualize ovulations and early pregnancies and see if the uterus has edema, which would indicate the mare is in heat.

The term "in heat" is important to understand. It describes the mare’s psychological attitude when she is receptive to the stallion for mating and her physiological state of being ready to be bred. The mare is in heat as a result of estrogen, which is produced by the follicles as they grow in size. Normally a mare will have several follicles and usually one will dominate and go to ovulation or the releasing of the ovum or egg. This follicle is called the graafian follicle and would routinely be from 35 millimeters to 60 millimeters in diameter. When the follicle ovulates or ruptures, the egg is released and travels down the oviduct (fallopian tube), where it will hopefully meet the stallion’s sperm. A normal stallion’s sperm will be viable for 48 hours and should be waiting in the mare’s fallopian tubes for the egg. The ultrasound can be very helpful when a mare is in heat because it can be used to measure follicle development and single or multiple ovulations, which may result in twins. It can then be determined if the pregnancies should be terminated.

If it is determined that the mare has had a prior ovulation, is not pregnant or was not bred and is no longer in heat, she could be given a shot of prostaglandin, which should start the cycle again. This is sometimes called "short cycling."

 "A lot of owners think that you can give the shot at any time, which is not true," explained McGlothlin. "The mare has to be cycling and have a functional corpus luteum, which is the structure that develops after a follicle has ovulated. It is responsible for the production of progesterone."

 Now, thanks to modern technology, the time of ovulation has been determined in your mare and she has been covered accordingly. When do we know if she is in foal?

 One of the things that the farm manager will continue to do is tease the mare to see if she comes back in heat.

 "The following morning, after the mare has been bred, she would be palpated again to determine if she had ovulated," continued McGlothlin.   

"If she had, then that’s considered day zero. Routinely we would scan (ultrasound), the mare at day 16. That will determine if there is a pregnancy or multiple pregnancies."

Do multiple pregnancies occur often?

"Yes, actually, they do," said McGlothlin. "Multiple pregnancies can be managed depending on the location of the vesicles or pregnancies at the time of examination. If there is more than one vesicle the veterinarian will try to disrupt all but one."

You might be wondering if there is any way to determine which vesicle to keep and which to terminate.

"We usually look at the saddlecloth to see if it’s a stakes winner," said McGlothlin.

If it were only that easy. There really isn’t any way of knowing which would be the better horse, but there are some things that do determine how the veterinarian would handle terminating one of them.


"When you determine that there are multiple pregnancies, they can be in several different locations," explained McGlothlin. "The mare’s uterus is a "Y" shaped organ, which consists of the body and two horns. Ideally, if you’ve got twin pregnancies there would be one in one horn and one in the opposite horn. These are relatively easy to manage because they are far enough apart that the veterinarian can go in, identify where they are, go after one and pinch it, disrupting that pregnancy. All too often the vesicles will be close together. When they are very close, but not touching each other probably creates the biggest problem because they are not as likely to reduce to a single pregnancy on their own, but they are too close for the veterinarian to go in and get one of them without disrupting both. You’ll follow these along and you’ll probably have to make a decision at some point to terminate both."

"Another scenario," continued McGlothlin, "is when they are actually touching or contiguous. The majority of those will reduce. Sometimes you will get a double reduction where you lose both pregnancies. But a lot of times you’ll have one that dominates the other so the mare will go on and have a normal single pregnancy."

If it is determined that the mare has experienced multiple ovulations, she would be scanned at 14 days because the vesicles are still moving about in the uterus at that point.

"If the vesicles were right next to each other you might turn the mare out for an hour and when she’s brought back they might have migrated apart," explained McGlothlin. "In which case you go after one of them and terminate it. But after they get out about 16 days they’re no longer moving around inside the mare’s uterus."

Uterine cysts can also complicate pregnancy determination. Small cysts can be easily confused with a vesicle.

The ultrasound, in combination with palpation, has made the job of determining when a mare is ready to breed, and when she has ovulated, much more precise. Many farms mix and match these modern techniques. For example, many veterinarians who palpate mares do not use the ultrasound machine, mainly because it is not practical to use out in the pasture. You need a chute for the mare, a table or stand for the machine and fairly dark quarters to see the screen. But as you can see, the veterinarian who is set up to use an ultrasound machine does have a huge advantage when close decisions have to be made about when to breed your mare. The ultrasound has also become an indispensable tool for pregnancy determination.

Before you even start breeding the mare you have to make sure that you have a healthy environment for the vesicle. A mare’s conformation lends itself to bacterial contamination.

"The reproductive system in a mare is not well designed from a sanitary standpoint," said McGlothlin. "When the mare defecates she can sometimes aspirate fecal material into the vaginal vault and set up an ascending infection that will travel up the reproductive tract. It’s not very common in maiden mares, but in older mares that have had multiple foals it is not unusual. As the mare becomes older the seal on the vulva (outer lips of the vagina), is not as tight as it was when the mare was young. Consequently, it opens up the possibility of bacterial contamination."

As a preventative measure, Dr. E. A. Caslick, back in 1937, discovered that sewing the lips of the vulva closed helped keep mares clean and pregnant. This is actually called a Caslick operation.

During the ultrasound exam, if fluid or excess edema is noted in the uterus, an intrauterine culture is used to determine if there is infection present. It would then be determined which antibiotics those organisms are sensitive to and the mare would be treated with uterine infusions utilizing those antibiotics in order to eliminate the infection.

"One of the things that has come into vogue," continued McGlothlin, "and that is very helpful is the intrauterine lavage, or the intrauterine flush. It is used to ‘flush out’ undesirable fluid in the mare’s uterus. Some mares, particularly older mares, have a problem with what’s called urine pooling. With multiple pregnancies over time the suspension system, so to speak, of the uterus becomes stretched and allows it to hang down into the mare’s abdomen. When the mare urinates, normally the urine is expelled over the pelvic rim, but as the suspension system breaks down and the uterus sags, some of that urine may go internal, which results in a pool of urine in the vaginal vault right around the cervix. When the mare is in heat and the cervix opens up, that urine can spill into the uterus. This fluid may cause a hostile environment for sperm or pregnancy and should be removed. The veterinarian will infuse sterile saline into the mare’s uterus which will mix with uterine contents and then everything is recovered. This will hopefully produce a healthy environment for pregnancy."

  "We may also use oxytocin," continued McGlothlin. "This is a naturally occurring compound in the mare and it causes contraction. If the mare has fluid in her uterus you can give her oxytocin and it will cause the uterus to contract and hopefully, essentially, squeeze the fluid out."

  As you can see, there is a whole army of maladies that can keep your mare from becoming pregnant. When breeders only had Mother Nature to rely on, they would often times lose the battle. But thanks to modern technology blended with Mother Nature, breeders can understand, overcome and beat this army of maladies.

The initial costs of these modern techniques are by far overshadowed by the early detection of problems that can delay pregnancy.

It’s now day 28 on the road to your mare’s first foal. Congratulations! Because of the farm manager’s and the veterinarian’s expertise in employing these methods the ultrasound shows a picture of an embryo. That embryo has a heart, and you can actually see the heart beating! Hopefully, after all everyone has been through, what you’re seeing is the strong heartbeat of a winner.