Hormone Treatment Gets Mares Ready to Breed Rapidly

Mares receiving the synthetic hormone reFSH went from anestrus (not cycling) to ready to breed within about a week, researchers found in a recent study.

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reFSH for horse breeding
Mares treated with reFSH can be kept outside, while keeping mares under lights often requires them to be stalled, Roser said. | Photo: iStock
Artificial lighting during the winter might get your mare cycling sooner for early breeding, but it won’t happen overnight. However, scientists testing a new hormone treatment have found that it leads to similar results and good pregnancy rates in a fraction of the time.

Mares receiving a synthetic hormone called recombinant equine FSH (reFSH) went from anestrus (not cycling) to ready to breed within about a week, researchers from the University of California, Davis (UC Davis), found in a recent study.

“Mares that were in deep anestrus, without lights, responded to reFSH by developing ovulatory follicles and ovulating after treatment with an ovulatory agent,” said Janet F. Roser, MS, PhD, professor emerita in the UC Davis Department of Animal Science. “There is no second guessing as to when the mares will respond, which allows for timed breedings and less monitoring compared to when mares are under lights.”

In their study, Roser and her fellow researchers administered reFSH intramuscularly to 20 mares in deep anestrus twice a day, every day, until their follicles reached 35 millimeters in diameter, which took five to seven days in the study mares

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Passionate about horses and science from the time she was riding her first Shetland Pony in Texas, Christa Lesté-Lasserre writes about scientific research that contributes to a better understanding of all equids. After undergrad studies in science, journalism, and literature, she received a master’s degree in creative writing. Now based in France, she aims to present the most fascinating aspect of equine science: the story it creates. Follow Lesté-Lasserre on Twitter @christalestelas.

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