Breeding Your Mare: A behind-the-scenes look at the science of mare reproduction

The weather is getting warmer, the stallion has been chosen, the contract has been signed, and you think you may have seen your mare looking a little too interested in the gelding across the fence – time to call the clinic and set up a date to drop her off for breeding!

Whether this is your first adventure into the world of mares and foals, or you have been down this road before, you may be wondering what happens between the time you say your farewells at the clinic and the time you get that wonderful news. Well, wonder no more! In this article I will explain all of the techniques, technologies, and tricks we have at our disposal to make the process as reliable and smooth as possible. Keep in mind that every mare is very different and so the management approaches are specifically tailored to each individual case.

Before we delve into this fascinating topic, let’s review the typical reproductive cycle of the mare. Horses are what we call “long day breeders”, which means their breeding season happens when the days are getting longer during the springtime, roughly April to August. This is entirely related to light-dark cycles. During the winter, the mare is in anestrous, a state during which very little is happening in her reproductive tract. She does not cycle or come into heat during this time. You may have heard of placing a mare “under lights” – this tricks her brain into thinking that the days are getting longer, therefore bringing her into an earlier heat.

 Diagram of the hormone profile during a regular reproductive cycle in the mare

Diagram of the hormone profile during a regular reproductive cycle in the mare

The typical reproductive cycle lasts 21 days, and consists of two phases – the estrous phase (in heat) and the diestrous phase (out of heat).

The anatomy of the mare’s reproductive tract is quite similar to that of many other species. The uterus is divided into the body and two uterine horns. Each horn leads to the oviduct and then the infundibulum, ending at the ovary.

 Representation of the mare reproductive tract

Representation of the mare reproductive tract

The typical reproductive cycle lasts 21 days, and consists of two phases – the estrous phase (in heat) and the diestrous phase (out of heat).

The anatomy of the mare’s reproductive tract is quite similar to that of many other species. The uterus is divided into the body and two uterine horns. Each horn leads to the oviduct and then the infundibulum, ending at the ovary.

The broad ligament carries the blood supply to the reproductive tract, and suspends the tract in place.

 Ultrasound Image of ovary with multiple follicles (dark areas) present

Ultrasound Image of ovary with multiple follicles (dark areas) present

The ovary, when active, is always changing. The main two structures you will find on the ovary are the follicle and the corpus luteum, or the CL. These two structures are extremely important in reproductive management.

The follicle produces estrogen, the hormone responsible for behavioural estrous in the mare. The follicle also contains the ovum. The ovum is released from the follicle during ovulation, to become fertilized by the spermatocyte.

 Ultrasound image of an ovary with a corpus luteum (CL) present (Solid structure in centre of ovary)

Ultrasound image of an ovary with a corpus luteum (CL) present (Solid structure in centre of ovary)

The CL is what develops from the old follicle once it has ovulated. The CL produces progesterone, the hormone responsible for suppression of behavioural estrous and maintenance of pregnancy. It regresses after approximately 14 days, which will allow for the development of another follicle, and the mare will again display signs of behavioural estrous.

Now that we have all of that anatomical and physiological stuff out of the way, we can talk about what we actually do with that information!

Our main tools for monitoring your mare are a digital ultrasound machine and palpation. We perform periodic ultrasounds and palpations via the rectum to view and feel the whole reproductive tract, especially the ovaries. By palpating and viewing the follicles, we are able to follow their growth and eventual ovulation. Several signs, including the tone or softness of the uterus, the sensitivity of the ovary, the follicle size/shape, and the uterine edema score, can provide us with an estimate of when the mare will ovulate.

When the mare is getting close to ovulation, she is then induced by administering a small amount of synthetic hormone. This allows us to predict the timing of ovulation much more accurately. This is particularly helpful for mares that will be bred with cooled semen being shipped from another province or from the USA, or for mares where only a few doses of frozen semen are available. By being able to predict when a mare will ovulate, we can use less semen than we would otherwise. Of course, not every mare remembers to check the clock before she ovulates, and so after administering the hormone we will check the mare more frequently to ensure that her ovulation is detected and she is inseminated as closely to ovulation as possible, which will help maximize our chances at getting a pregnancy.

Remember that pesky CL that produces progesterone and prevents your mare from coming back into heat? If the mare presents to the clinic with a CL on one of her ovaries, or if she unfortunately did not become pregnant from the last insemination, we are able to cause early regression or “lysis” of the CL by administering a hormone called prostaglandin specifically designed for this purpose. Once the prostaglandin is administered, the CL will regress and the mare will come back into heat in the next 3-7 days, depending on the size of the next biggest follicle on her ovaries. This process is called “short-cycling”.

There are several other management strategies that we sometimes must employ to help our less fertile mares.

  1. Uterine Lavage – to do this, we aseptically introduce a special catheter into the uterus and flush the uterus with saline or a similar liquid. The amount of fluid varies between mares, depending on our assessment of the fluid that we receive back. Mares that require this procedure usually have problems clearing excess fluid from the uterus following breeding, or tend to accumulate more fluid to begin with. To make the uterus a healthy environment for the embryo, it is important to help the mare evacuate all excess fluid if she is unable to do so on her own.

  2. Oxytocin administration – many mares that have trouble clearing fluid may simply not have enough mechanisms to help their uterus to contract. Oxytocin is the same endogenous hormone that is released during nursing and helps to clear the placenta and bring the uterus down to normal size after foaling. By administering oxytocin we help the mare to do the job of making the uterus a healthy environment.

  3. Uterine Infusions – this involves infusing an antibiotic or another drug into the uterus. This strategy is only typically used in special cases when we have diagnosed a uterine infection.

  4. Synthetic Progesterone – we will sometimes supplement a mare with a synthetic progesterone analog that will help the mare maintain her pregnancy or reset her cycle. If your mare has had problems with early embryonic loss in the past, we may suggest starting her on this supplement.

Finally, no article on mare reproduction is complete without discussing embryo transfer. Embryo transfer (ET) involves removing a fertilized embryo from one mare (the “donor”), and placing it into the uterus of another mare (the “recipient”). It is very important that the donor and recipient mares’ cycles are synchronized, so that when the embryo is placed into the recipient’s uterus, she does not reject it. Synchronizing their cycles involves inducing the recipient to ovulate about 1 day after the donor – this timing carries the best results for pregnancy. Most of the hormones and prostaglandins mentioned earlier are used to help synchronize these mare’s cycles, in addition to a few others. The advantages of ET are that a mare can mother more than one foal in a year (depending on the breed association), and mares that are at risk or may have trouble carrying a foal to term can still mother foals. Another advantage is that competition mares are then able to take less time off from their busy schedules.

Once your mare is bred, it is time to wait for the magic to happen! Using the ultrasound machine, we are able to detect signs of pregnancy as early as 14 days after ovulation. At this point, we are unable to see an embryo, but can only see the round, fluid-filled vesicle in which the embryo resides. Once the vesicle is seen at 14 days, the pregnancy is then confirmed at 25 days post-ovulation, when the embryo and heartbeat are visible.

 Ultrasound image of two vesicles present in within the uterus (consistent with twins)

Ultrasound image of two vesicles present in within the uterus (consistent with twins)

Twins. They seem like such a great problem to have, but unfortunately healthy twins are very rare and often do not survive at all. They can also carry a significant risk to the dam, which can compromise her future fertility, and sometimes, her life. Fortunately, if detected early enough, twins can be treated and avoided, leaving just one pregnancy behind to continue. Twins can usually be detected at the first pregnancy check, which is 14 days post-ovulation. They are not always treated at that time, but may require a second visit. Mares that have had twins in the past have a much higher risk of conceiving twins. Thoroughbreds statistically have a higher incidence of twinning.

 breeding mare equine vet veterinarian doctor Calgary Cochrane Alberta

All of this may seem like a lot of work just to breed a mare, but these methods and technologies can really enhance your breeding operation, no matter how big or small. So the next time you drop off your mare at the clinic, rest assured that we are using the safest and most reliable means by which to make her a mother.. you will be hearing the clip-clop of tiny hooves in no time!