The Problem:

What I have is a mare who is a candidate for PMS! When "Vixen" is bred, she's so precious and loving. If you go to the pasture gate and whistle she comes running - wanting her lovin' and attention. And to ride - she's so willing to please and so smart and capable. She's wonderful!! BUT..... after her babies are weaned (she does get tired of the little darlings) and, if she's not bred back, she turns into what my daughter calls "The Mare from Heck"! She bucks!. She kicks! She's completely nasty and unpredicable! Every year at this time, I swear I'm selling her.

My husband always smiles and sets up another breeding and Lo & Behold, within six weeks to two months, my sweet "Vixen" reappears. Is this a normal hormone thing or should I test her for something - or is my solution "KEEP HER BACKFOOT AND PREGNANT! Help! I have babies coming out my ears!                                          S. B., Oregon

The Answer:

If we knew the complete answer to this question, we might be able to acquire a small fortune solving these kinds of problems for both mare owners and horsewomen. While we don't pretend to have it all figured out, we do believe that the answer lies in the complex relationship between hormones and disposition.

The connection between these naturally occurring body chemicals and how one views the world has been intensively studied since we decided to quit burning "witches" at the stake and began to look for the roots of their "torment". Today, we have sophisticated equipment which can determine the body levels of these naturally occurring mind altering chemicals (known as hormones) down to minute levels but, we still are not able to predict the precise effect that a given level of a hormone will have on the behavior of any one specific individual.

For example, the hormone, estrogen, which causes a mare to be receptive to a stallion's sexual advances produces different behaviors at various levels. Small amounts usually cause the mare to show mild interest in a stallion's shenanigans, although she will reject any bold advances upon his part. Under the influence of larger amounts of this hormone, a mare can be expected to be more receptive to a wider range of stallion sexual behavior. However, the amount of estrogen that determines when one mare will show maximum receptivity may, in another mare, have no appreciable effect - in other words, she rejects all advances made by the male. The difference between these two mares is, likely, related to the number of receptor sites (locations designed to allow the hormone to bond and activate).

Now if this estrogen-heat relationship sounds a bit delicate to be easily manipulated, consider this: we know a great deal more about this phenomenon of estrogen and receptivity, than we know about the effect of hormones on non-sexual interrelationships between species (case in point - S.B. and her mare). Acknowledging this void of scientific information, let's explore this pitch-black room of ignorance and grope around to see if we can find by feel what we can not see with enlightenment.

Several months ago, we had a call from a woman in Houston, Texas with a similar problem to S.B.'s. Her mare tended to have the disposition of a crone before she began daily treatments with oral progesterone - the hormone of pregnancy. By tricking the mare's body into thinking that she was pregnant, she turned the mare into a well-behaved show horse. Two years later, after taking the mare off of progesterone, a dragon lady was taking over this mare's mind mind as this "Mare from Hell" rose from her flame-breathing head.

Ladies, can you imagine the hormone crash after being fed progesterone for two years; all the normal feedback mechanisms of reproductive cycling have been overridden until suddenly one day - the drug is not longer present? The mare's hormonal balance is thrown into chaos which is reflected in the behavior of the mare.

To help stabilize the system, we suggested that she might breed the mare. This would allow the body to naturally return to the presence of progesterone and hopefully, ease the transition back into the normal cyclical nature of the female.

This advice, however, would not solve another hormone-behavior problem brought to our attention by a neighbor. While this mare could never be called "wonderful and willing to please", her owner did enjoy riding her, that is, until six weeks after the birth of her first foal. At this point the mare's attitude drastically began to change and by the time, the owner brought her to us (roughly eight months after foaling - two months after weaning), this mare was a "fruitcake".

She couldn't handle any stress and was dangerous to herself and anyone on her back. Upon questioning, we learned that the mare had not been back in heat since the birth of her foal and had just developed mastitis due to the fact that she was still producing milk - two months after weaning. This time we suspected that the mare had a hormone imbalance created when her body failed to reset after foaling.

The most probable explanation revolved around a situation known as a persistent corpus luteum - a structure on the ovary which is suppose to disappear after foaling when it is attacked by another hormone, prostaglandin. In an effort to bring the system back to normal, the mare was injected with prostaglandin and five days later, her own persnickety self re-emerged.

After wading though all this, we're sure that many horse trainers might be ready to jump in and say, "The heck with all this hormone therapy stuff. What all these mares need is some strong disciple for their negative behavior!"

Sure, it's a well known fact that punishment can modify behavior, but you had better be some kind of horseman to ride most of these hormone-deranged mares through it. And it's still not a quick fix! Punishment often creates as many problems as it corrects. Changing this type of attitude takes much more patience and understanding than strength and discipline.

Unfortunately, S.B. didn't provide many clues about the reproductive status of her newly weaned mare as she metamorphosed into a tormented creature. Was she cycling normally? If so, oral progesterone is a possible solution. If there was no reproductive receptivity, the mare might be a candidate for prostaglandin. At any rate, S.B. needs to find a specialist in equine reproduction who can try to piece the puzzle together and put the mare's hormones back together again. But, if all else fails, maybe S.B. just needs to keep that mare "Barefoot and Pregnant and work on finding a market for all those babies.

Copyright © 1994 Dr. Jim and Lynda McCall