Horse Weight, Other Factors Affect Mare Fertility

Uterine infection is one of the most common reasons for infertility, but there are other reasons why mares might fail to conceive. A veterinarian can usually determine why a mare isn’t settling in foal and can assist owners in treating any condition that is found on a breeding soundness exam.

Mares with equine metabolic syndrome (EMS) may have problems becoming pregnant for one of several reasons. EMS is known to affect insulin-like growth factors, which are important in the normal maturation of ovarian follicles. If follicles fail to mature and release viable eggs, the mare can’t conceive.

EMS is frequently accompanied by obesity, a condition linked to changes in estrous cycles. Even if obese mares lose weight and attain more desirable body condition scores, they may not return to normal reproductive function.

On the other end of the spectrum are extremely thin or undernourished mares or those that are recovering from a serious illness. These mares may not be ovulating regularly, and their bodies may not support the nutritional demands of pregnancy while they are extremely underweight.

Adhesions or cysts in the mare’s oviducts may block movement of the ova, though these conditions are not common causes of infertility.

In one trial, some mares that consumed feed containing zearalenone, a mold-produced toxin found in some grains and forages, had a lower conception rate than mares eating feed that did not contain the toxin. Other mares were not affected. Zearalenone has been shown to have a reproductive impact in pigs and cattle.

Finally, a particular mare’s failure to conceive might not be her fault at all. Owners of mares should check out a stallion’s credentials before assuming that the problem is with the mare. Does the stallion have a good breeding record, either with live cover or with artificial insemination? Even if he has always been able to get mares in foal, he might have a temporarily low sperm count due to a recent illness that caused a high fever.

Finding the problem of an infertile mare is the first step, and correcting the problem may take some time. For this reason, owners should schedule a breeding soundness examination with a veterinarian several months before the projected breeding date.