Q: My 5-year-old mare has a bit of a “witchy” attitude. I eventually would like to get a foal from her, and I’ve been told that if I breed her, the hormones of pregnancy will help her settle down and improve her attitude. Is this true? If so, do the effects last only while she is pregnant? I may breed her sooner than originally planned if it makes her easier to handle. Name withheld on request

A: There are no guarantees in life or in horses. One person’s experience with the same scenario
may be quite different from another’s. That said, it is likely that your mare’s social behavior and attitude will be altered by pregnancy.

When a mare ovulates (at or near the end of her heat period), whether she is bred or not, she normally develops a structure called a corpus luteum (CL). This structure is responsible for producing progesterone, a hormone that naturally suppresses so-called “witchy” behavior. You should notice a change in her behavior when she goes out of heat. If you do not, she may not respond any differently when pregnant. If you did not want to breed her now, her behavior may also be altered artificially by giving her a daily dose of Regu-Mate (altrenogest). This will increase the progesterone effect in her system and may provide a clue as to whether or not her behavior might be altered by pregnancy.

Because they are also under the predominant hormonal influence of progesterone, pregnant mares are typically more docile, compliant and less reactive than other horses. Some pregnant mares, on the other hand, may become more aggressive, dominant and irritable. Those that do may be reacting to more estrogen and/or testosterone in their hormonal matrix due to the influence of a developing male fetus. Fetal gonads (specifically testicles) produce a large amount of estrogen, which is converted into testosterone.

Many young or maiden mares do settle down and become less “witchy” after they become pregnant. This behavior may persist after foaling, becoming a lifelong change. However, to a large extent a mare’s behavior is determined by her social dominance or place in the hierarchy in the herd. This is more difficult to alter by hormonal influence (through pregnancy or altrenogest supplementation) and may in your mare be the underpinning of her “witchy” attitude.

William B. Ley, DVM, MS
Diplomate, American College of