Management of Pregnant Mares
by Dr. Jim Brendemuehl, Equine Extension Veterinarian, Univ. of Illinios College of Vet MEd
Mares should be managed attentively during pregnancy to help ensure the birth of a strong, healthy foal with no injury incurred by the dam. Maintaining the mare in good health, being familiar with the signs of impending parturition, and preparing a foaling environment conducive to mare and foal health will increase the likelihood of obtaining a healthy foal.
The average length of gestation in the horse is 335 to 342 days, with normal ranges of 320 to 365 days. There are seasonal effects on the duration of gestation, with mares due to foal in late winter and early spring (January to March) carrying their foals 5 to 10 days longer than mares foaling later in the spring and summer.
Preventive health care for a pregnant mare includes regular immunization for certain infectious diseases. Immunization serves two purposes: protection of the mare and eventual protection of the newborn foal through nursing. Vaccines should be boostered 3 to 4 weeks before the projected foaling date to optimize antibody concentrations in colostrum.
Core vaccines for pregnant mares include:
- 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. (Additional management practices to prevent this disease include separating pregnant mares from other horses and preventing contact with new or transient animals.)
- 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.
Regular deworming is second only to good nutrition for proper management. Most deworming medications are considered safe for use during pregnancy and products from all major classes of dewormers are approved for use during pregnancy. A valuable management practice is to administer ivermectin to the mare on the day of foaling to minimize the parasitic load of Strongyloides westeri. The infective larvae of this parasite are transmitted to the foal via the milk in the first few days after foaling.
Proper nutritional support of broodmares improves fertility and promotes normal growth and vigor of the developing fetus. Pregnant mares should be kept in good body condition. Mares should not be obese as obesity has been associated with weak, undersized foals.
Digestible energy requirements of mares during the first 8 months of gestation are the same as those for maintenance but gradually increase over the last 3 months of pregnancy, when about 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. A good rule of thumb is 10 to 12 percent crude protein in the last 3 months, compared with about 8 percent for maintenance and early pregnancy.
The primary minerals to be concerned with in rations for pregnant mares are calcium and phosphorus. The National Research Council recommends calcium be fed at a rate of 0.2 percent for maintenance and the first 8 months of gestation and 0.4 percent for late gestation. Phosphorus levels should be evaluated closely and should not exceed calcium levels in late gestation.
Because legume hays are high in both calcium and protein, feeding alfalfa hay in late gestation may preclude the need for calcium and protein supplementation in the diet.
Mares should always have available fresh clean water and salt.
Preventive health care programs for pregnant mares depend on many factors, including expected disease exposure, vaccine efficacy, farm population density, and economic constraints. Therefore, consultation with your veterinarian is important to tailor a program to meet the needs of individual mares and to fit disease control measures on farms where mares reside.
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