The discussion in the table topic session on hormone therapy in the mare during the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif., followed a systematic progression through three broad areas: 1) hastening the onset of (ovarian) follicular activity and ovulation in winter anestrus (noncycling) mares; 2) synchronizing ovulation in cycling mares; and 3) blocking estrous behavior (i.e., heat) in mares.

During the discussion of methods that are used to hasten the onset of ovulatory activity, there was complete consensus on the value of an artificial lighting program for expediting the onset of follicular activity and ovulation in the spring. Then facilitators and attendees discussed hormonal methods that can be used to augment the use of artificial lights, such as:

  • Treatment with progesterone
  • Treatment with gonadotropin-releasing hormone (GnRH) or its agonists; and
  • Treatment with dopamine antagonists such as sulperide.

The discussion then turned to methods of synchronizing estrus and ovulation in mares. Again, there was a general consensus that the most reliable method was daily administration of a short-acting combination of estrogen and progesterone for approximately 10 days. Some of the participants described the use of long-acting preparations of estrogen and progesterone that have the advantage of requiring only a single treatment compared to daily treatment; however, the degree of synchrony of ovulation among treated mares might not be as "tight" compared to what is achieved with daily administration of the short-acting formulation.

The session concluded with a discussion of methods for blocking heat behavior in mares. Like the two previous topics, there was a consensus that administration of progesterone, such as the orally active synthetic progestin Regu-Mate, is the most reliable means of blocking heat behavior. Additional methods discussed were three protocols for prolonging function of the corpus luteum (the source of progesterone in mares): 1) placement of a sterile glass marble into the uterine lumen; 2) hormonal induction of a "diestrus" (the quiescent period between one estrus period and the next) ovulation; and 3) administration of oxytocin. Advantages and disadvantages of each method were discussed.

The table topic session was attended by approximately 50 participants, which created an atmosphere for lively discussion.

This Table Topic was facilitated by Karen Wolfsdorf, DVM, Dipl. ACT, of Hagyard Equine Medical Institute in Lexington, Ky., and Dirk Vanderwall, DVM, PhD, Dipl. ACT, then of the University of Idaho, now at University of Pennsylvania.

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