Published October, 2013

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 little when they are in heat, others exhibit signs of heat such as an elevated tail and frequent urination, among others. While these attitude changes might not pose physical problems, they could potentially distract and impair a mare’s overall performance in the show ring or on the racetrack. Horse handlers, therefore, often prefer to suppress estrus in mares to prevent potential behavioral problems, Squires said.

For the study, mares received oxytocin daily on Days 8 to 10, 8 to 12, or 8 to 14 after ovulation. The researchers evaluated the mechanisms behind treatment with oxytocin that would result in continual production of progesterone from the CL on the ovary. Death of the CL occurs in the absence of pregnancy at the end of diestrus phase, while oxytocin treatment prolongs the period the mare is not in heat. If a mare does not become pregnant, prostaglandin (a hormone coming from the uterus) release normally causes the CL to die and the mare enters a new cycle. However, several previous studies have confirmed that administering oxytocin after ovulation prolongs the length of time the CL in mares continues to produce progesterone.

“As research has previously proven, we found oxytocin administration when given eight days after ovulation was effective in preventing the corpus luteum on the ovary from dying, and heat-related signs were reduced accordingly in these mares,” Squires said. “However, the oxytocin had to be given for several days to get this response. We were not able to shorten the number of days oxytocin was given and still have the mare stay out of heat for an extended period of time.”

The researchers also collected tissue from the endometrium (uterus lining) to evaluate the cellular changes in the uterus with oxytocin treatments from Days 8 to 14 after ovulation.

They found that administering oxytocin regulates an enzyme called cyclooxygenase 2, preventing prostaglandin production that is responsible for death of the CL. Thus, oxytocin injections allowed the CL to continue producing progesterone and the mare to stay out of heat.

“Instead of having to give the mare injectable or oral progesterone as a supplement, we tricked the mare into producing her own progesterone longer in the cycle, but at this point we still need further studies to determine if the number of injections can be decreased perhaps by giving a longer acting oxytocin,” Squires said.

Shaila Sigsgaard is an editorial assistant for the Bluegrass Equine Digest.