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The Broodmare
By Barb Crabbe, DVM
Then and Now In l7th century England, a pregnancy test required nothing more than a glass of water poured into the mare’s ears. If she shook only her head, she was pregnant. If she shook her whole body, the mating had failed and it was necessary to breed her again. Today’s evaluation of pregnancy is a little more involved. Ultrasound examination performed as early as ten days following conception allows for visualisation of a pregnancy. A heartbeat can even be seen in the developing embryo before the first month has passed.
Before Breeding Begins Before she’s bred, the broodmare should be in optimal condition. She should be at a good weight, currently vaccinated for any infectious diseases, and have had routine dental work performed. Vaccination in the early months of pregnancy is not recommended, and any procedures that might be stressful or require medication should be avoided to minimise the potential for adverse effects on the developing fetus.
At least once a month prior to the anticipated breeding date, the mare should undergo a pre-breeding examination, to ensure that she is healthy and that her reproductive system is normal. This examination is likely to include rectal palpation and ultrasound, and uterine culture and cytology (examination of samples taken from the uterus for signs of infection or inflammation). Uterine biopsy may be recommended for older mares or mares with a history of reproductive problems. If abnormalities are detected with any of these diagnostic tests, the veterinarian will recommend treatment prior to beginning the breeding process
The Breeding Process Successful breeding requires that the mare be bred within an appropriate time frame around the release of an egg from the ovary during the estrous cycle. The most common methods likely to result in a pregnancy include live cover, artificial insemination with fresh semen and artificial insemination with frozen semen.
Live Cover With live cover, the mare’s heat cycle is usually monitored through ‘teasing’ or observation of her behaviour when she’s exposed to the stallion. When the mare exhibits signs of estrus (such as winking vulva or squatting to urinate, she will be bred to the stallion every other day until she no longer accepts him, indicating that she has ovulated (released the egg).
Artificial Insemination with Fresh Semen Artificial insemination requires that the veterinarian closely monitor the mare’s heat cycle, typically using daily palpations and ultrasounds, in order to determine the optimal time for insemination. The vet observes the growth of one or more follicles (fluid-filled structures on the ovary that mature and eventually release an egg), and changes that occur within the mare’s uterus as the heat cycle progresses. Ideally, a single follicle will begin to dominate, and will eventually release a single egg.
If semen is being shipped from a stallion in a remote location, the vet will request semen when the follicle reaches a certain size. The goal is to have the semen in hand for insemination just before the mare ovulates, or releases the egg from the ovary.
As soon as the semen arrives, the mare will be inseminated. Semen is deposited directly into the mare’s uterus using a long insemination pipette. Many stallion managers will send a second dose of semen for a subsequent insemination the following day. After the mare has been inseminated, a small sample of the semen will be examined under the microscope to assess its viability.
Ideally, the mare will ovulate within twenty-four hours of insemination, although many mares still conceive with transported semen even if ovulation does not occur until the forty-eight hour point. Much depends on fertility of the stallion, and a very important factor in working with fresh, transported semen is maintaining good contact with the stallion manger
Artificial Insemination with Frozen Semen Frozen semen offers the advantage that the semen is ready and waiting before the breeding process even begins. The disadvantage is that timing of insemination must be much closer than with fresh transported semen (usually within six hours of ovulation), and pregnancy rates are lower. Because the timing is so critical, the mare may be examined with the ultrasound as often as three or four times a day during the final days of her heat cycle. The goal is to inseminate within six hours of ovulation. In some cases, the vet will inseminate both before and after ovulation for optimal results.
Post-Breeding Treatment When the mare is inseminated, her uterus may become inflamed in response to the semen. This can cause fluid and inflammatory debris to accumulate within her uterus, which can be detrimental to the embryo and prevent a healthy pregnancy from being established. This problem is much more common with frozen semen because the natural protectants in the seminal fluid have usually been removed as part of the freezing process. It takes approximately six days for the fertilised egg to travel down from the ovary into the uterus, giving the vet that amount of time to clean up the uterine environment.
Post breeding treatment may include the following:
• Oxytocin: An injection of this hormone stimulates uterine contractions that help push unwanted fluid and debris out of the uterus. It can be given as early as four to six hours after insemination, and may be repeated several times during the first day or so following insemination to help clear fluid.
• Uterine Infusion: A small volume of saline is placed into the uterus to help wash out unwanted fluid and debris. If the mare has a history of uterine infection, this infusion may include an antibiotic.
• Uterine Lavage: If the mare accumulates a large volume of fluid, this procedure, which involves washing the inside of her uterus with several litres of saline, will be recommended. A large Y shaped tube is inserted into the uterus, allowing fluid to be moved in and out.
Manipulation of the Estrous Cycle In domesticated horses, an earlier breeding season is often imposed in order to produce foals born early in the season. Lighting adjustments during winter months (northern hemisphere) often help to stimulate an earlier cycle. (Note: This still occurs in the southern hemisphere to lengthen the daylight hours in the early part of the breeding season when days are shorter). Lighting adjustments during winter months may help to stimulate an earlier cycle. In addition, a wide variety of different hormonal treatments may be used throughout the breeding process at any time of year in order to facilitate timing for successful breeding. The following are the most common strategies used for manipulating the mare’s estrous cycle.
Lights In nature, daylight hours stimulate the mare to begin cycling by stimulating release of the gonadotropin-releasing hormone (GnRH) from the hypothalamus, which eventually leads to the onset of ovarian activity. To encourage this process to begin sooner, a mare can be placed under artificial lights during winter months. As a rule of thumb, sixteen hours of light per day are required to stimulate cycling.
Prostaglandin This hormone injection, if given to the mare a minimum of five days after end of her previous heat cycle, should stimulate a new heat cycle to begin within three or four days. It works by breaking down the corpus luteum, the structure that developed following ovulation of a follicle from the previous heat cycle, an event that must occur before a new follicle can develop. When the mare is given a prostaglandin injection, she may sweat, cramp, or even act a little colicky for approx forty-five minutes. This is a normal reaction that should not be cause for concern.
Progesterone This hormone can be used to regulate heat cycles. For example, the mare might be administered the oral synthetic progesterone such as Regu-Mate® (Intervet) daily for a period of ten to twelve days. When the Regu-Mate® is discontinued, most mares will begin to cycle within three to five days.
Progesterone may also be administered as an injection either by itself or in conjunction with estrogen. Daily injections for ten to twelve days or a single injection of a long-acting version of these hormones, similar to Regu-Mate®, will be given to prime the uterus for breeding. With either oral or injectable progesterone or progesterone/estrogen combinations, a prostaglandin injection may still be administered on day ten of treatment to reliably stimulate the heat cycle.
Progesterone or progesterone/estrogen combinations may also be administered after breeding, or after pregnancy has been confirmed to help maintain the pregnancy.
Human Chorionic Gonadotropin (HCG) This injectable hormone may be used to help stimulate ovulation. It assists with the timing of breeding by helping to co-ordinate the time of insemination with the time of ovulation. In most cases, HCG will stimulate ovulation within forty-eight hours of administration.
Deslorelin An alternative to HCG, this hormone may also be administered to stimulate ovulation. It is extremely reliable at inducing ovulation within thirty-six to forty-eight hours, and it is especially useful for mares that have a tendency to develop very large follicles prior to ovulation.
Further information on Pregnancy, care of the pregnant mare, foaling and insights into issues when ‘things go wrong’ can be found in this very informative book. Available through Capricorn Link at www.capricorn link.com.au
Reproduced with the kind permission of Sterling Publishing from The Comprehensive Guide to Equine Veterinary Medicine, available from all good equine suppliers.
For wholesale enquiries, please contact Capricorn Link at sales@capricornlink. com.au
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