Late Gestation Mare Management

The final months of a mare's gestation period are anxious ones. As an owner during those final weeks, one is filled with anticipation now that the date of birth is approaching. At the same time, there are those disquieting concerns about potential complications and the worry as to whether you have done everything necessary during the pregnancy to ensure a live foal and an uneventful birth.

During those final weeks and months of gestation, the mare's body and the fetus undergo changes that bring about new demands in the way of nutrition and, depending on what is being asked of the mare, a possible change in the exercise regimen.

David Beckman, DVM, whose Anchorage, Ky., practice concentrates on reproduction, puts it like this:

"Most fetal growth occurs in the final trimester. At six months of pregnancy, the fetus is only about the size of a cat. Almost all of the growth occurs after that--most of it between seven and 11 months of gestation.

"At the same time, nutrient requirements change. The mare now needs more protein for the rapidly growing fetus. During the early months of pregnancy, there is little change in nutrient requirements, but in the latter stages, the mare's body needs more. It is difficult from the sixth or seventh month on to feed the mare enough to maintain her body weight, support the fetus, and provide enough energy for performance."

What this means, Beckman explains, is that if the mare is involved in a racing or performance career, it is recommended that exercise demands be reduced.

In addition to the change in nutritional demands, the mare simply is becoming heavier as the fetus grows and her own body weight increases, making it difficult for her to perform with the same alacrity as she did early in gestation or before becoming pregnant.

A foal might weigh 125 pounds at birth, and when water weight and additional fat are added, it could mean that the mare has gained 250 pounds or more during the pregnancy. This additional weight also means that her center of gravity has shifted--it has moved rearward--and produces additional strain on the abdominal muscles. Heavy exercise during this period would increase that strain. This, of course, does not mean the mare no longer needs exercise. Quite the contrary. However, the exercise should be obtained in a paddock rather than on the track or in a show ring.

The fact that it is difficult to feed a mare enough to maintain her weight, sustain the growing fetus, and handle exercise demands during the latter part of gestation means that the mare, if she and the fetus are to remain healthy, should be in good body condition early on in the gestation period.

Here, again, there must be a happy medium. While a mare which is too thin might compromise the health of a foal (and her own health as well), an overly fat mare might do the same. The fetus of a mare which is overly fat might have its growth retarded, and the mare might have a limited supply of milk once birth has occurred.

One tool that can be used by owners and managers in determining correct body weight is the body score. The scale ranges from 1 on the bottom for very thin mares to 9 or 10 on top for those that are too fat.

Frederick Harper, PhD, University of Tennessee, recommends that on a scale of 1 to 9, mares should have a minimum body score of 5.5. For most mares, he believes, the score should range between 5.5 and 7.3.

Mares which are in this body score range are in a condition where the ribs can't be seen, but can be felt with some fat between them. Some fat is also deposited along the sides of the withers, behind the shoulders, along the neck, and around the tailhead. A slight crease might appear down the back.

While fetal development is greatest during the last three months of gestation, the mare might gain more weight during the second trimester. Some pregnant mares do not gain a good deal of weight during the final trimester.

During those final months, the mare might need to be fed some grain even though she is on good-quality pasture. Her nutritional needs will have increased for energy, protein, calcium, phosphorus, and vitamin A.

Vaccination and Other Concerns

Broodmares should be on a regular vaccination, deworming, and hoof care schedule throughout their reproductive careers. The vaccinations should be for such potential afflictions as Eastern and Western encephalomyelitis, influenza, and rhinopneumonitis (a disease that often is devastating when it strikes because it can cause abortions). In many cases, abortions from "rhino" will occur during the last four months of gestation.

About 30 days prior to foaling, pregnant mares should be immunized for tetanus, Eastern and Western encephalomyelitis, and influenza. Not only does this provide protection for the mare, but it does the same for the newborn foal. The foal is born without any immunization and receives its first protection through immunoglobulins in the colostrum. Its own immune system does not really kick in until about 30 days of age.

Mares should be dewormed regularly, but not during the last 30 days of pregnancy. Just how often one deworms depends on both geography and environment. Mares kept in close confinement in a warm, humid climate will need deworming more frequently than those in an arid climate with each mare having a good deal of space.

Geography also plays a role in another potential problem. In some parts of the country, tall fescue grows in profusion. In many cases this grass is infested with an endophytic fungus that can cause serious problems. For some mares, this means a thickened placenta, prolonged gestation, retained placenta, a lack of milk, and a weak or stillborn foal.

Pregnant mares should be removed from fescue pastures several months before foaling. If possible, it is best to keep pregnant mares off infected tall fescue throughout the gestation period.

Normally, the length of gestation is 342 days, give or take 20 days. Mares have been known to go a full 12 months and beyond, causing serious consternation for owners and attendants.

Generally, a mare will follow a pattern as to gestation length. For this reason, it is wise to keep complete records throughout the pregnancy, including previous foaling dates.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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