Readers ask questions of equine reproduction experts

Earlier this year, The Horse made a request to its Horse Health E-Newsletter subscribers: What questions do you have for specialists about breeding your mares? Our e-mail boxes were flooded with replies. After narrowing questions down to several themes to be featured in this article, the questions were presented to two specialists in the field of equine theriogenology (the study of animal reproduction) and to one behavior specialist.

Michelle M. LeBlanc, DVM, Dipl. ACT, is a professor at the University of Florida College of Veterinary Medicine and a member of The Horse's Editorial Advisory Board. She was named the 2000 Theriogenologist of the Year by the American College of Theriogenologists (ACT).

Patrick McCue, DVM, PhD (Comparative Pathology), Dipl. ACT, is an assistant professor in the Department of Clinical Sciences at Colorado State University. His research interests include reproductive endocrinology and ovarian abnormalities.

Sue McDonnell, PhD, Certified Applied Animal Behaviorist, is the founding head of the Equine Behavior Program at the University of Pennsylvania's School of Veterinary Medicine, where her work includes clinical, research, and teaching activities focused on horse behavior. She is an editorial advisory board member for The Horse and author of the magazine's Behavior column (see page 79).

We thank readers for responding to our request. The remaining questions not addressed in this article will be considered for future article topics both in The Horse magazine and in The Horse Health Weekly E-Newsletter (sign up at

How old is too old to breed a mare?

LeBlanc--The maximum acceptable age to breed a mare varies with her reproductive conformation and her well-being. What we do know is that when mares get older than 18 years of age, not every egg they ovulate is viable, similar to women when they get older. So we have to deal with poor egg quality, and some mares might have had many foals and have had wear and tear on their reproductive systems, which might make them more prone to urine pooling and uterine infections. When someone brings me a mare over 20 that is still having foals, I let them know they're very lucky.

I do a complete exam on older mares which have never been bred, because many times their cervix doesn't open properly during breeding. When they have semen put into them, either by natural cover or artificial insemination, they aren't able to clear inflammatory debris that occurs as a reaction to the semen. The mare's body reacts to the semen because it is perceived as foreign material, and her body has a white blood cell reaction to the debris. If the by-products of that inflammation reaction remain in contact with the uterine lining for more than 12 hours, they damage the uterine lining.

I pay particular attention to the cervix when I'm evaluating mares during estrus (heat). If the cervix doesn't open well, then we know we're going to have to treat the uterus accordingly after they are bred so that they can conceive. If they retain fluid and inflammation in their uterus, it is toxic to the embryo when it comes down to the uterus at 5 1/2 days, and the contamination will lead to the embryo's death.

Is it better to breed a mare that is "fatter" in terms of body condition scoring (see "How Does Your Horse Score?")?

LeBlanc--There's not a lot of scientific data on overly fat or overly thin mares, but the work that came out of Texas years ago indicates that overly fat is not good for a breeding mare. So, if you're going by a body condition score scale of 1 to 9, you'd really like to have them between a 4 and a 7. Some of these mares with foals at their sides have their body condition scores brought down to a 3 or 3.5, but still conceive successfully. An overly fat condition is related to a lower pregnancy rate in horses. Typically, you want to breed a mare with a body score of about a 4.

If a mare you want to breed is thin and doesn't seem to be gaining weight, check her teeth and her deworming history, and increase her feed. If she is in with a group of broodmares, make sure she's not at the bottom of the herd's pecking order. If so, get her out of the group and put her by herself or in a more amicable environment.

How long do you have to wait before breeding a mare after you discontinue use of Regumate?

McCue--This question is simple in that the time frame is fairly well understood. If a mare has been on Regumate for 10 days, two weeks, or any length of time longer than a few days, she should come into heat two to four days after coming off of Regumate. Most mares will be ready to breed within five to nine days after coming off of Regumate. In the mare, progesterone therapy doesn't narrow the window down as much as one would like it to, but it does give you the option of scheduling breedings. Also, the use of Ovuplant or human chorionic gonadotropin (hCG), both of which hasten ovulation, helps in this predictability process so that you can breed according to the availability of the stallion.

Can a mare that has had a rectal tear have another foal? What about a mare with a uterine tear?

McCue--Presumably, mares with severe rectal tears do not survive. The outcome is often fatal--if the mare has a full rectal tear, she will most likely be euthanized. Mares with mild rectal tears are given a block of time off from rectal examinations. They certainly can be bred, get pregnant, and foal 11 months later, but the difficulty lies in the examination period. If the mare has had a mild rectal tear, any exams during the next several days or the next week or two should be handled very cautiously. A lot of us depend on transrectal ultrasound examinations to determine preparedness for breeding or pregnancy. If the mare has had a rectal tear, sometimes we cannot do an ultrasound exam safely for several weeks. The mare potentially can be bred by natural cover or inseminated every other day while she's in heat, and the rectal exams avoided.

Virtually all uterine tears occur during spontaneous, unassisted foalings or dystocias (slow or difficult labors or deliveries). Tears can also be caused by blunt force of the fetus, or by an obstetrical manipulation.

Mares with severe uterine tears are at risk of facing severe peritonitis (inflammation of the smooth membrane that lines the abdominal cavity) and resulting death, or they might have to be euthanized. Additionally, peritonitis can lead into laminitis.

Small uterine tears might go unrecognized, or might result in only mild medical problems in the mare. After the tear heals, such mares can be bred, and in all likelihood, successfully carry a foal to term. Knowledge of mild uterine tears is very limited because we don't clinically recognize them. If a uterine tear is diagnosed and repaired successfully, the mare should recover nicely. It's kind of like the healing after a Cesarean section. If the damage is the result of blunt trauma incurred from the fetus or an obstetrical manipulation, the prognosis for the uterine wall is not as good as after the clean incision of an elective Cesarean section.

If a mare has had a known uterine tear (during foaling), she should not be bred back for a period of several weeks post-foaling--i.e., skip breeding on the foal heat and possibly on the subsequent heat.

Are there really mares which cycle "quietly?" If so, are these mares more likely to be aggressive or "marish?"

McDonnell--First of all, there are mares that cycle normally and are completely normal, but they're very subtle about their estrus. I don't know of any research on this, but in my experience, those mares are usually not aggressive or "marish" in other situations. They tend to be shy and not too demonstrative in all of their interactions with other horses and people.

We've taken a lot of mares like that (in the behavior program at New Bolton). If you tease them (to ascertain estrus) long enough and quiet enough, and every other day, you usually will get to know the mare's attitude and will notice a change. You can say, "For the last 10 days she didn't do any of that." Breeding mare managers of the farms will tell you that you should "know your mares," and know each mare's subtle signs (of estrus). Even though a breeding farm might miss mares with subtle heats, if you upgrade the type of teasing and change the method of teasing, you will see such a reaction. For example, have a mare loose in a paddock and a stallion in an adjacent stallion box so that the mare approaches him on her own terms. You'll see her approaching more. This type of mare might never show in a classic teasing situation.

In terms of the shy mare who cycles but does not show estrus behaviorally--we know the stallion can figure it out. These mares are the type that are sometimes scared of the stallion if you turn them out together after the mare has ovulated. Their fear and defensive behavior overrides estrous behavior. But, if you were to leave them together with plenty of space, the stallion would eventually get her bred.

With this type of situation, have the mare in a pasture with a safe fenceline, and the stallion across an alleyway. The two horses will show you a change in behavior. It might not look as obvious as you would like, but they will show you a change. You might have to watch them for two or three estrus cycles before you're confident. Gradual introduction to the stallion is better for this type of mare rather than hobbling her, which would interfere with her expression of estrus.

There are some mares which don't show estrus, and act aggressive or "marish." I think it's very important to be very comprehensive in the evaluation of that mare's ovaries and hormones. As a behavior cluster, those mares usually end up having problems such as an early granulosa cell tumor that might be suppressing their normal behavior toward horses and people. Every time we get the mare who's not showing (estrus) and is aggressive, it turns out that there was something very unusual going on with her ovaries and hormones. As a veterinarian in that case, you would want to advise the owner or manager to get a mare reproduction specialist to do a thorough exam of her reproductive tract and hormones to rule out such things as an early granulosa cell tumor or some problem (physiological or hormonal) that's disturbing her ovarian function.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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