Equine Pregnancy: the Last Trimester

Mike Yoder, NCSU Extension Horse Specialist

Late Fall into early winter is usually a quiet time for the broodmare. Most mares are entering the last trimester of their pregnancy, a time when many changes occur. Through the fall and into the early winter there are two primary concerns for the broodmare manager, nutrition and disease prevention. Maintaining the mare in good body condition and preparing her to ward off any diseases that may interfere with the pregnancy.

Nutrition

At no point during the last trimester of pregnancy should a mare be losing weight. Mares that lose weight are said to be in a negative plane of nutrition and have been shown to lose pregnancies at a higher rate than mares that are maintaining or gaining weight (positive plane of nutrition). While we do not want mares to be obese, a little extra body condition serves two purposes: first, when the mare foals and begins to nurse the foal, her energy demands increase as much as 50 percent. Increasing her feed intake quickly to meet this sudden requirement for energy, may make the mare susceptible to colic or founder. However, a mare that is slightly fat at foaling has stored energy to draw upon for 7 to 10 days while you are gradually increasing her feed intake to meet energy demands.

The second benefit of having mares on a positive plane of nutrition at foaling is the ease with which they rebreed. Mares on a positive plane of nutrition rebreed sooner than mares who are on a negative plane of nutrition and are more likely to maintain the pregnancy. The point to be remembered is that mares should be gaining or maintaining weight at foaling and then energy should be increased to maintain at least a moderate body condition until the foal is weaned. Quality forage, regardless of the kind, should be the basis of your nutrition program with grain and mineral supplements used only as needed to balance nutrients and maintain body condition.

Vaccinations

Rhinopneumonitis, EHV-1, is a virus that may cause a mare to lose her pregnancy, or in some cases, perinatal foal mortality. EHV-1 is a herpesvirus acquired by inhalation. It is generally recommended that mares should be vaccinated against Rhinopneumonitis using a killed vaccine. The following table provides information on the most common EHV-1 vaccines.

Name	Pneumabort-K	Prestige II	Prodigy	Rhinomune
Type	Killed	Killed	Killed	Attenuated live
When to Give	5, 7, 9 months of gestation		5, 7, 9 months of gestation	After 2nd month Then every 3 months.
Notes	Vaccinate pregnant and non-pregnant mares at same time.	EHV-1, EHV-4, and influenza subtypes A1 and A2.	Effective against virus-associated abortion.	No adverse reactions have been reported in pregnant mares vaccinated with this product.

Herpes virus vaccines usually do not provide long term protection, thus most are given every 2 to 3 months. Of the vaccines listed above, Pneumabort-K is listed as effective against an EHV-1 outbreak, but there is some controversy over whether or not any of the vaccines are effective for use after the disease is established in a herd. To minimize the chance of EHV-1 infection, resident mares should not be exposed to outside mares. Stress may also make mares more susceptible to the virus, so maintaining a stress free routine for the mare is beneficial.

Mares should also be vaccinated, using killed or inactivated vaccine, for Influenza, Tetanus, and Encephalomyelitis. Boosters for each of these should be given to pregnant mares 30 – 45 days prior to foaling to enhance passive immunity in the foal. Always discuss your vaccination program with your veterinarian and heed their advice concerning modifications to these recommendations.

Maintaining your mare’s immunity and a desirable body condition through the last trimester of pregnancy will help position your mare to produce a healthy foal and rebreed in a timely manner. Be sure to observe your mare at least twice daily to quickly identify any nutrition or health issues that may impact your mare and/or the foal she is carrying.