The Harry M. Zweig Memorial Fund for Equine Research


Controlled Postponement of Ovulation by Progestagen Treatment

- Dr. Robert Gilbert

GilbertDr. Robert Gilbert

Control of ovulation is important in breeding management of all breeds of horses. Precise prediction of time of ovulation facilitates management of popular sires using natural service and is necessary for optimal management of insemination using fresh chilled semen. Within limits, hastening of ovulation is possible using human chorionic gonadotropin (hCG). There remains no reliable method to delay ovulation slightly. Such a technique would be helpful in booking a natural service to a stallion with a full book and in coordinating ovulation and semen shipments for artificial insemination. We propose to test the hypothesis that administration of progestagens could delay ovulation without adverse consequences on corpus luteum formation, cyclicity, or fertility.

Specifically, we will follow 12 cyclic mares during the breeding season by transrectal ultrasonography and teasing. Once the mares are in estrus and have a dominant follicle greater than 35 mm in diameter, they will be allocated to a control cycle (twice), oral administration of altrenogest (ReguMate®; 0.044 mg/kg/day) or intravaginal application of a progesteronereleasing device (CIDR, Pfizer) for two days. Treatments will be applied in random sequence, and each treatment will be applied twice.

We will determine daily estrous behavior, follicular growth rate and time of ovulation, incidence of double ovulation, and follow serum progesterone concentration to determine normal luteal function. Mares will be bred with fresh diluted semen from a single stallion (500 million progressively motile sperm) every 48 hours, beginning at detection of a 35 mm follicle for control cycles, and starting the day after completion of treatment in treatment cycles. Cervical tone and uterine luminal fluid accumulation will be recorded as these may be influenced by progestagen treatment. Pregnancy will be detected by transrectal ultrasonography at 14 days after ovulation.

This experiment will allow us to determine:

  1. If either or both progestagen treatment is capable of delaying ovulation in mares;
  2. If spontaneous ovulation follows cessation of treatment;
  3. If normal luteal function (as determined by progesterone profiles) follows ovulation; and
  4. If fertility is severely depressed by the treatment. (Subtle depression of fertility would require a much larger trial, and might be acceptable to the industry.)