Progesterone For Equines

In equine breeding programs, whether private or commercial and regardless of scope, horse owners and managers often have a need to cause a mare to enter into estrus and ovulate within a specific time period. Reasons for this can include the intent to use frozen semen to impregnate the mare; others often involve a stallion being delivered for a specific (limited) time, or in synchronizing the mare's ovulation with a shipment of semen.

Mares have the capacity to be notoriously unpredictable in the timing of their ovulation. This can lead to frustration, aggravation and distress in the owners and attending veterinarians.¹ Thus, the question then becomes one of what is the optimal method for achieving the objective of timing control.

During the ovulatory season, healthy mares cycle normally and exhibit sexual receptivity to the stallion on a regular basis; they are also producing normal follicles that ovulate. The ovulatory season begins in the middle of Spring (early April) and continues through the Fall unless the mare becomes pregnant. The mare undergoes a succession of cycles, with each being around 22 days in length.

The equine estrous cycle is divided into two parts: estrus and diestrus. Estrus is usually referred to as the “heat” portion of the cycle, and is the time of follicular maturation and ovulation.² During diestrus, the mare is not receptive to the stallion. Ovulation usually occurs between 24 to 48 hours prior to the end of estrus. The duration of estrus is around three to seven days. The mare will enter diestrus following ovulation and the end of estrus. The follicle that came about due to ovulation develops into the corpus luteum (CL).³ If the mare has not become pregnant, the corpus luteum will be absorbed and follicular development will proceed once again at the end of diestrus.

Strategic Timing: A Necessity

Several drugs are useful to hasten ovulation and to coordinate a schedule that works with breeding farms.³ While there are a number of effective compounds that can help horse owners and managers induce ovulation, few are as widely applicable as progesterone, an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis.

While progestins are routinely used to manage ovulation, studies have shown that by themselves, they will not necessarily control follicular development and it is possible to have ovulations occurring during treatment, or large pre-ovulatory follicles present at the end of treatment, either of which will result in unpredictable timing of estrus and ovulation.⁴

Warnings

Progestins have some side effects which are potentially life-threatening in carnivores, particularly domestic dogs and cats. Increased appetite and weight gain are common across species.² Progestins also have been associated with lethargy, hair loss, and hair discoloration.³ In equines, the most typically reported side effect attendant to the administration of progesterone in oil is injection site reaction. These can be addressed with low dose NSAIDs.

Progesterone for Equine Breeding Management

Progesterone has been used for many years to prevent mares from coming into heat, to synchronize estrus cycles for better breeding efficiency, to organize or regulate heat cycles during the mare's seasonal transition, and to help maintain pregnancy. Progesterone is also used to modify estrus-related behaviors that interfere with performance and pleasure riding in non-breeding mares, and sometimes in stallions.⁵

Because an increase in estrogen accompanies the development of follicles, there is a marked impact on the mare’s behavior,⁷ which is calculated (by nature, of course) to attract a stallion. Unfortunately, these behaviors (frequent urination, squealing, tail swishing, and threatening other horses) are disagreeable in horses that have to work with humans. Consequently, progesterone has been widely used to manage “moody mares” as well as for breeding management.

Synthetic progestins can have effects which militate against fertility as regards the reproductive process. These include impeding the progression of sperm and eggs to the site of fertilization and interfering with embryo implantation. “Progestins also have a negative feedback effect on areas of the hypothalamus and pituitary involved in reproduction by suppressing the hypothalamic-pituitary-gonadal axis, progestins prevent the hormonal cascade that stimulates estrus and ovulation in females and spermatogenesis and libido in males. Progestin treatment of females can block ovulation by suppressing luteinizing hormone (LH) release from the pituitary, but levels of follicle-stimulating hormone (FSH) may remain high,” which can stimulate follicle growth and estradiol production.⁴

Where to buy Progesterone

Progesterone is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

¹equine-reproduction.com.

²Hemberg E, Lundeheim N, Einarsson S. Successful timing of ovulation using deslorelin (Ovuplant) is labour-saving in mares aimed for single AI with frozen semen. Reprod Domest Anim. 2006 Dec;41(6):535-7. doi: 10.1111/j.1439-0531.2006.00709.x. PMID: 17107513.

³Merck Veterinary Manual.

⁴Beijerink, et. al. Basal and GnRH-induced secretion of FSH and LH in anestrous versus ovariectomized bitches. Theriogenology 67 (2007) 1039–1045.

⁵equimed.com.

⁷Volkmann D. Rational Use of Progestagen Therapy During Pregnancy. Western Veterinary Conference 2010 2010. 2010.