Pregnant Mares

by Atwood C. Asbury, DVM, DACT • courtesy of AAEP

The mare from which you have always wanted to raise a foal has just been pronounced pregnant by your veterinarian. Now you just have to wait about 11 months and see what she produces, right? Not so fast! There is much more involved than just waiting if you want the foal to have the best chance of being born healthy. Your first concern is to maintain that newly detected pregnancy.

Barry Ball, DVM, PhD, Dipl. ACT (reproduction specialist), University of California, Davis, showed early embryonic loss occurred at a rate of nine percent in young, fertile mares by Day 14 after fertilization, with a loss rate more than 60 percent for aged, sub-fertile mares during this same time period. Therefore, rechecks of the pregnancy are a must so if an early loss occurs, re-breeding is an option.

Can You Stop Early Embryonic Loss?

One possible defect, inadequate progesterone secretion, is a rare occurrence, but many breeders supplement progesterone or synthetic compounds with similar actions such as altrenogest (brand name Regu-Mate) during early pregnancy. Progesterone causes the cervix to remain tightly closed, the uterus to remain firm and helps block production of prostaglandins (hormones that arise from inflammation in the uterus).

Why Not Give Progesterone To All Mares In Early Pregnancy? 

It is expensive, potentially harmful to handlers, and unnecessary in many cases. Progesterone deficiency can be documented by monitoring progesterone levels frequently during the first few weeks of pregnancy. Repeated early pregnancy loss is a reason for concern, and your veterinarian’s input is needed. Managing stress can offset this problem in many cases. Minimize stress by eliminating sudden feed changes, extremes in exposure to adverse weather, threatening dogs chasing the horses, loud noises, etc.

Good management practices for all horses include control of intestinal parasites, prevention of infectious diseases, feeding high-quality rations, and care of feet and teeth. Pregnant mares require more. The major concern centers on the antibodies passed to the foal through colostrum (first milk). The equine placenta does not permit passage of maternal antibodies to the fetus. During the final weeks of pregnancy, the mare needs to concentrate proteins that carry antibodies to the foal. Therefore, a carefully planned immunization program during gestation is critical. Consult your veterinarian for a vaccination schedule since there are regional differences to consider (such as botulism).

Problems with colostral transfer of immunity include a low antibody concentration, premature lactation and loss of antibodies and the foal’s inability to suckle. Measuring IgG (the immune antibody protein) in the foal’s serum at 24 hours of age will determine the amount of antibody transferred. There is a correlation of low levels of IgG and susceptibility to infections. Treating foals with immune plasma can modify the problem.

Mare Health 

Pregnant mares should gain weight moderately during pregnancy and have a natural glow to their coats. Careful inspection of the udder and external genitalia should occur at frequent intervals throughout gestation. Any discharge from the vulva warrants a call to your veterinarian. Your records should clearly indicate which mares are sutured so they can be opened in time for foaling. Inspect the udder for premature lactation. Left on their own, foaling mares will select an open space within a sheltered area with clean ground and easy escape routes. Those factors are evolutionary. So, it is to our advantage to provide similar facilities—protected and quiet—for the “civilized” mare.

Should We Supervise All Foalings? 

The statistics say no, due to the low incidence of problems. But when there are problems, they tend to be big ones, and immediate attention can make a difference. The small breeder might benefit from technologies that alert the foaling attendant.

One final management tip: Beware of the late pregnancy mare who seeks to eat all of the hay put out for her and three or four other mares. Invariably, you’ll end up with one overly stuffed mare and several hungry ones. This is bad news during foaling since the abdominal pressure during foaling can cause ruptures in the large bowel and cecum. This rupture is likely to occur when the large bowel is distended by feed and, when it occurs, it is fatal.

At best, horse breeding is an art that allows minor adjustments to nature. The satisfaction of seeing that foal born, standing, growing, and competing is more than adequate compensation for the breeder.