What to Watch For When Breeding Mares

Ensure equine breeding success with these tips.

As obvious as it may seem, a mare has to be in heat to get in foal.

Kseniya Abramova/Hemera/Thinkstock

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For mare owners, breeding season brings two sure things: anxiety and expense. Even with advances in breeding technology, some mares—young and old—will fail to deliver a live foal at term.

Critical to breeding success is good breeding management. Fortunately, many of the veterinary problems pertaining to mares are fairly easily addressed.

Make Sure the Mare Is in Good Condition 

A mare’s body condition is critical for successful breeding. Mares in good to moderate condition are more likely to successfully conceive than are mares that are too thin or too fat. Ideally, mares are adding a bit of weight at the time of breeding.

Make Sure the Mare Is Ready to Breed 

It behooves any owner to confirm that the mare’s uterus is ready to go before trying to get it to carry a foal. Common pre-breeding examinations include:

  • Rectal palpation. This helps to assess the size of the uterus as well as the presence of ovarian follicles and their proximity to ovulation. 
  • Ultrasound. This is indispensable‚Äč in mare management. Ultrasound helps to assess things like uterine thickness, the presence of uterine cysts, which are not necessarily a detriment to successful breeding, and the presence of excessive uterine fluid. Ultrasound images of the ovaries can give insight into the presence, size and type of uterine follicles. 
  • Uterine culture. In live breeding situations, the stallion owner often wants to confirm that the mare has a negative bacterial culture. A mare with a uterine infection may have an extremely difficult, if not impossible, chance to get in foal. The species and density of bacteria grown help to determine the significance for the individual mare. 
  • Uterine cytology. While a negative bacterial culture is important, the results of a uterine culture alone are, at best, not entirely reliable. Indeed, false positives are possible on uterine culture, leading to needless treatments. 

    Preparing uterine cytology smears in conjunction with uterine cultures is essential in helping determine the significance of uterine culture results. Positive bacterial cultures should be accompanied by the presence of neutrophils on the cytology smear. If neutrophils are not present, there’s a good chance the positive uterine culture is due to contamination of the uterine swab and not contamination of the uterus.

  • Uterine biopsy. Histologic examination of the uterus may be warranted, especially in older mares or in problem breeders, to help assess the chances of a mare carrying a foal to term. 

Make Sure Insemination Is Properly Timed 

First and foremost, as obvious as it may seem, a mare has to be in heat to get in foal. Determining that a mare is in heat simply by observing behavior can be very difficult. If the heat cycle can’t be identified, the mare is not likely to get in foal. However, careful attention to follicle size, texture, uterine edema, cervical relaxation and mucus stringiness makes estrus detection very accurate.

Even if a mare is in heat, if semen is not present in the uterus in close proximity to the time of ovulation, the mare will not get in foal. Attending veterinarians need to make sure that insemination, whether by artificial or natural means, is chronologically close. While ovulation can be assessed by rectal palpation, ultrasound the day after breeding is a far more accurate way to ensure uterine clearance and an appropriate decrease in uterine edema.

If the heat cycle can’t be identified, the mare is not likely to get in foal.

Various hormonal approaches such as the use of human chorionic gonadotropin (HCG) or deslorelin implants may be employed to help ensure ovulation at the proper time.

In some mares, the follicle continues to develop beyond the size at which ovulation normally occurs, and the development appears to continue for a prolonged period. Such mares are often inseminated, however, the follicles do not ovulate and the mares do not get in foal. In most cases, these mares have developed a condition called a persistent anovulatory follicle or anovulatory hemorrhagic follicle.

No one knows exactly why mares develop abnormal follicles. They are not a reflection of breeding mismanagement on the part of the veterinarian. Possible causes include various hormonal abnormalities, or hemorrhage into the interior (lumen) of the follicle.

While it should be noted that some correlation has been made between mares receiving hormones to stimulate ovulation and the presence of anovulatory follicles, the condition can develop in mares that have not been managed with breeding hormones. Fortunately, anovulatory follicles are relatively rare. Anovulatory follicles also tend to occur more commonly when mares are in transition to or from the breeding season, in the spring or fall, respectively.

While most anovulatory follicles resolve if a luteolitic dose of prostaglandin F2-alpha is given, a small percentage resist treatment.

Ensure Proper Uterine Clearance 

Uterine fluid may be seen with ultrasound post-breeding. Some uterine fluid typically occurs as a response to the presence of a foreign object: semen. Most mares will clear this fluid within 24 to 48 hours and establish a pregnancy.

However, in some mares, especially older mares that have had multiple foals, or older mares with poor conformation of their reproductive tracts, inflammatory fluid and semen may not be cleared normally. This is known as delayed uterine clearance.

If fluid is not cleared normally, low-grade endometritis may ensue, resulting in a loss of the conceptus, usually before one week post-breeding. Strategies to help remove uterine fluid, such as administration of oxytocin, may be useful in some mares post-breeding, accordingly.

Don’t Forget the Stallion 

While it’s tempting to blame the mare when she doesn’t settle, don’t forget that the stallion is a common source of breeding problems.

While some stallions have problems with semen quality, storage and shipping methods can affect fertility dramatically, especially if the semen is frozen. In order to assess semen quality, of course, semen must be examined. This is best done by someone with expertise and experience. It may be helpful to ask the stallion owner about the animal’s fertility before entering into a breeding contract so as to minimize the risk of infertility because of poor-quality semen.

A live-foal guarantee is one way to ensure that both the stallion and mare owner have an interest in a successful outcome.

Mare breeding problems are more difficult to minimize except in hindsight; veterinarians usually can’t know whether a problem is going to occur, even if everything has been done correctly.

While many other things can affect a mare’s likelihood of conceiving, most healthy mares successful conceive and carry a live foal to term. Paying attention to a few important items helps optimize her chances. 

Originally published in the February 2016 issue of Veterinary Practice News. Did you enjoy this article? Then subscribe today! 

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