Pet Columns, Office of Public Engagement, College of Veterinary Medicine at the University of Illinois at Urbana-Champaign

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Management of Pregnant Mares


Pet Column for the week of April 11, 2005


Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Kim Marie Labak
Information Specialist

As breeding season begins Dr. James Brendemuehl, an equine veterinarian at the University of Illinois Veterinary Teaching Hospital in Urbana, is often consulted about broodmare health.

Dr. Brendemuehl explains that nutrition is key to ensuring a healthy pregnancy. Proper nutritional support of broodmares improves fertility and promotes normal growth and vigor of the developing fetus. During the first 8 months of gestation, mares' energy needs are the same as those for maintenance, but energy requirements gradually increase over the last 3 months of pregnancy, when up to 65 percent of fetal growth occurs.

The growing fetus increasingly takes up abdominal space during the last trimester, requiring the feeding of some grain and good quality hay. Mares in late gestation also need increased protein intake: 10 to 12 percent crude protein in the last 3 months compared with about 8 percent for maintenance and early pregnancy.

Dr. Brendemuehl recommends focusing mineral intake on calcium and phosphorus. Mares should receive calcium at a rate of 0.2 percent for maintenance and early pregnancy and 0.4 percent for late gestation. To maintain proper phosphorus: calcium ratios for bone health, phosphorus levels should not exceed calcium levels in late gestation. Because legume hays such as alfalfa are high in both calcium and protein, feeding these in late gestation may eliminate the need for calcium and protein supplementation in the diet.

Of course, mares should always have fresh clean water and salt available.

Regular deworming is also essential for a healthy pregnancy, and is second only to good nutrition for proper management. Most deworming medications are considered safe for use during pregnancy. A deworming program should be tailored to the individual requirements of the farm or stable and monitored by periodic fecal examinations to determine parasitic egg levels. Sound pasture management, including low stocking density, pasture harrowing, and regular rotation, should be used in conjunction with deworming protocols to establish an effective antiparasite program.

Dr. Brendemuehl advises, "Administer ivermectin to the mare on the day of foaling to minimize the parasitic load of Strongyloides westeri, which can be transmitted to the foal via the milk in the first few days after foaling."

Preventive health care for the pregnant mare also includes regular immunization, to protect both the mare and the newborn foal via the colostrum. To optimize colostral antibody concentrations, vaccines should be boostered 3 to 4 weeks before the projected foaling date.
Dr. Brendemuehl recommends the following core vaccines for pregnant mares:

- equine herpes virus type 1 (EHV-1), the primary form of equine herpes virus associated with abortion and also associated with foal mortality and encephalomyelitis in adult horses. (To prevent this disease you should also separate pregnant mares from other horses.)

- encephalomyelitis, a neurologic disease also known as sleeping sickness. Eastern, western, and Venezuelan encephalomyelitis are of greatest concern among these diseases transmitted by mosquitoes.

- West Nile virus, another neurologic disease transmitted by mosquitoes.

- tetanus, a disease with high exposure and life-threatening consequences to the mare and foal.


- rabies, which has recently been detected in horses in Illinois.

Consult your veterinarian to tailor a program that meets the needs of your mares.