Hormones and Mare Conception Rates

Treatment with equine follicle-stimulating hormone (eFSH) did not increase healthy mares' conception rates in a new study, researchers recently reported.

The hormone eFSH is used to stimulate donor mares' ovaries to increase the number of ovulations and resulting embryos in one cycle, but some breeders also want to use it to enhance fertility of healthy mares that are going to carry to term, explained Tal Raz, DVM, PhD, Dipl. ACT, formerly of the University of Saskatchewan's Western College of Veterinary Medicine, now in Israel at the Weizmann Institute of Science and the Koret School of Veterinary Medicine.

Researchers compared two groups of healthy mares intended to carry foals to term. One group received eFSH and one did not. The eFSH mares received the hormone for a few days and were artificially inseminated with fresh semen collected from a fertile stallion after they came into heat. The other mares were bred when they naturally came into heat.

The researchers used ultrasound to examine the mares for pregnancy. Mares treated with eFSH developed more follicles and had twice as many ovulations than the other mares, but eFSH did not increase the chances for pregnancy--50% of the eFSH mares and 62% of the untreated mares conceived.

"Although the eFSH treatment stimulated the development of more large follicles in the ovaries, 25% of these follicles failed to ovulate," Raz said. In addition, natural hormones such as estrogen and progesterone were altered in eFSH mares, affecting the tone of the uterus and cervix, and potentially harming the embryos' viability.

"Based on the study results, eFSH treatment is not recommended for cycling mares if they are intended to carry their own pregnancy," Raz said.

If eFSH is considered for embryo transfer programs, breeders should be aware this treatment can result in failure to ovulate and potentially produce less viable embryos, Raz noted.

The study, "Early effects of equine FSH (eFSH) treatment on hormonal and reproductive parameters in mares intended to carry their own pregnancy," was published in the October 2009 issue of Animal Reproduction Science. The abstract is available on PubMed.

About the Author

Marie Rosenthal, MS

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