Veterinarians at Rood and Riddle Equine Hospital answer your questions about sales and healthcare of Thoroughbred auction yearlings, weanlings, 2-year-olds and breeding stock.
QUESTION: How does foal heat differ from a regular heat cycle, and do I need to do anything differently to breed a mare back on foal heat?
DR. MARIA SCHNOBRICH: Foal heat refers to the first estrus or “heat” that a mare experiences after foaling. In most mares, the beginning of this estrus can start as soon as four or five days after foaling with the mare demonstrating receptive behavior and ovulation usually occurring around day 10 after foaling.
What is remarkable about the mare is how rapidly, after the dramatic and traumatic event of foaling, her reproductive tract has repaired and is capable of supporting a new pregnancy. Because of the type of placenta the mare has (epitheliochorial), the six layers of tissue between the fetus and dam blood allow a less traumatic event at birth when the fetal tissues separate than in other species. This facilitates rapid repair of the uterus and studies have shown that by 14 days after foaling the endometrium has a normal histologic appearance.
Foal heat is a fertile estrus and historically was preferred by some, as it was considered preferable to breed back and prevent the mare from developing “issues”. Breed, age, and reproductive history are major factors in the success of the foal heat breeding. Several studies have looked at the fertility of the foal heat breeding and found some evidence that it may be ideal to skip the first estrus and breed later. One study evaluating 2,003 Thoroughbred mares showed that per-cycle pregnancy rates continued to improve, and embryonic loss decreased up to 70 days after foaling.
The decision to breed on foal heat should be made based on the veterinarian's evaluation and assessment of the mare's reproductive tract in the first week after foaling. Transrectal palpation and ultrasound as well as a speculum exam of the vagina and cervix are essential to determine that the reproductive tract is normal.
Regarding post-breeding management of foal heat, administration of a uterine lavage and oxytocin (with or without antibiotics) have been demonstrated in some studies to increase pregnancy rates in post-foaling mares. It is best to breed on a foal heat if ovulation will occur 10 days or more after foaling, as earlier ovulations are associated with lower pregnancy rates and higher pregnancy loss in Thoroughbreds. Unless time or other factors push towards a foal heat breeding, it is advisable to skip the foal heat breeding. Either short-cycling the mare back (decreasing the length of the foal heat with the use of prostaglandin) or waiting until the natural 30-day heat are more advisable options.
Dr. Maria Schnobrich grew up in Boston where visits to her grandparents' farm and riding lessons at a young age sparked her interest in horses and large animals. Dr. Schnobrich graduated Magna Cum Laude at Brown University followed by attending veterinary school at the University of Pennsylvania. She is a Diplomate of the American College of Theriogenology
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