Researchers from the University of Kentucky (UK) Gluck Equine Research Center evaluated the effectiveness of a hormonal treatment (oxytocin) in preventing mares from showing estrus (heat) for an extended period of time.

Currently, owner must give mares an oral hormonal (progesterone) product (marketed as ReguMate) every day to prevent them from showing estrus. This is both labor-intensive and expensive. In the UK study researchers investigated a simpler, less-costly approach based on injecting a hormone called oxytocin. Earlier studies had shown oxytocin to be effective in suppressing estrus in mares but it required frequent injections. The goal of the current project was to determine if fewer oxytocin injections could still suppress estrus effectively.

“This work is a huge leap forward in unfolding the mechanisms behind the effects of chronic oxytocin treatment in mares,” said Ed Squires, PhD, Dipl. ACT (hon.), professor and executive director of the UK Gluck Equine Research Foundation. “Treatment with oxytocin may be one option to prevent the mare from going into heat and could be the least costly method of suppressing heat in show horses and possibly racehorses.”

A mare’s normal cycle is 21 to 22 days long, as measured by counting days between ovulation. The length of time the mare is in heat varies, but is generally four to seven days. Once a mare ovulates a follicle, the corpus luteum (CL) forms on the ovary and produces progesterone, which prevents the mare from coming back into heat for about 14 days. This phase of the mare’s cycle is called diestrus.

While some mares’ behavior alters very li