Progesterone For Equines

It is critical for horse breeders and managers to understand the estrous cycle of mares. Since mares breed seasonally, the breeding of mares cannot be successfully achieved throughout the whole year. Mares are only able to breed when daylight length is more than 16 hours (Spring through early Fall).¹ This period is known as the breeding season.

Mares have been known to be particularly unpredictable as regards the timing of ovulation, leading to uncertainty on the part of breeders and managers, and difficulty in planning breeding operations.² The estrous cycle of the mare is mainly controlled by gonadotropins, which control follicular development and ovulation. Mares exhibit unique ovulatory events which are not observed in other species. A luteinizing hormone (LH) surge occurs for several days, with levels of LH reaching their peak after ovulation.¹

During the breeding season, healthy mares produce normal, ovulating follicles; at this time, they are cycling normally and exhibiting sexual receptivity to the stallion. In the Western Hemisphere, the equine breeding season begins in middle of Spring  and continues through the Fall. The mare’s estrous cycle is approximately 21 days with 5-7 days of estrus and 14 to 15 days of a diestrus period.¹

During the reproductive season, the mare's overall attitude and behavior may change along with her hormonal activity. During her estrous cycles, the mare’s ovaries swell and become active rather than dormant. The mare’s cycle repeats itself at 21 to 23-day intervals until she becomes pregnant or until she reverts to anestrus as light decreases in late Fall.

During estrus (the “heat” portion of the cycle), follicular maturation and ovulation occur,³ with ovulation usually occurring between 24 to 48 hours prior to the end of estrus. During diestrus, the mare is not receptive to the stallion. The follicle that came about due to ovulation develops into the corpus luteum (CL).⁴ If the mare has not become pregnant, the corpus luteum will be absorbed and follicular development will proceed at the end of diestrus.

Chemical Manipulation of Ovulation

Manipulating estrus is essentially a method of “tricking” a mare’s reproductive system into altering her hormonal balance, thereby allowing her to remain in a nonestrous phase or to bring on estrus faster and shorten its duration.

Delicate balances of progesterone and estrogens influence the phases of the mare’s estrous cycle, estrogen being the predominant hormone during estrus. In diestrus—the period between estrous cycles— progesterone exerts more influence. Progesterone is the hormone which promotes pregnancy by encouraging the uterus to accept and hold on to an embryo.

Manipulation of the light cycle (with artificial light) is a long-held method of managing ovulation. For more effective control, chemical methods are used. Scientists and equine researchers have developed many drugs to manipulate estrus, employing equine hormones, hormones from other species, and synthetic variations of hormones. These bring about a shift in the hormonal balance, thereby changing the estrous cycle.^4,5

Progesterone Warnings

Progestin medications do carry some side effects which can be life-threatening in carnivores, particularly domestic dogs and cats. Increased appetite and weight gain are common, and progestins have been associated with lethargy, hair loss, and hair discoloration.³ In horses, the administration of progesterone in oil can bring about injection site reactions, which can be managed with NSAIDs.

Equine Breeding Management Using Progesterone

Progesterone can be used to suppress heat during transition into the ovulatory season in mares that have histories of long and erratic estrous cycles during this time. Progesterone has also been used with less consistent results to maintain pregnancy in mares with abortion history. One protocol that enhances the normal estrus cycles involves placing mares on exogenous progesterone for 10 to 15 days. Once removed from progesterone, most mares will return to estrus within 3 to 4 days and ovulate 9 to 10 days following progesterone withdrawal. In order for this to be effective, mares must have sufficient activity on their ovaries (several follicles of at least 20-25mm or larger).^1,8

Since the flurry of hormonal activity during estrous cycles significantly impacts the mare’s attitude and behavior. Some of this is (by nature) designed to attract the stallion. These behaviors, however (such as frequent urination, squealing, tail swishing, and aggression) are less than ideal for mares that perform or are used in riding. As a result, progesterone is also frequently used to manage these “moody mares.”¹

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.

¹Yoon, Min. (2012). The Estrous Cycle and Induction of Ovulation in Mares. Journal of Animal Science and Technology. 54. 10.5187/JAST.2012.54.3.165.

²equine-reproduction.com.

³Hemberg 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.

⁴Merck Veterinary Manual.

⁵Beijerink, et. al. Basal and GnRH-induced secretion of FSH and LH in anestrous versus ovariectomized bitches. Theriogenology 67 (2007) 1039–1045.

⁶equimed.com.

⁸Volkmann D. Rational Use of Progestagen Therapy During Pregnancy. Western Veterinary Conference 2010 2010. 2010.