Estrus manipulation involves altering the natural course of the mare’s reproductive cycle to support breeding program goals. This is a common technique employed by many breeders and reproductive veterinarians.
Breeders may choose to manipulate the estrous cycle in order to increase the length of the breeding season, maximize chances of pregnancy after insemination, or to synchronize multiple mares for advanced reproductive techniques. Most estrus manipulation strategies involve hormone administration.
Breeders commonly manipulate a mare’s cycle to have foals born closer to January 1st and to provide subfertile mares with more time to become pregnant during a breeding season.
Having foals born closer to the annual January 1 birthdate is believed to confer advantages in age-related competitions, such as the Kentucky Derby for 3 year-old Thoroughbreds.
Methods to lengthen the breeding season include a combination of artificial lighting programs and administration of medications such as dopamine antagonists, progestogens, or gonadotropin-releasing hormone.
Implementing effective estrus manipulation techniques can optimize breeding outcomes, ensuring healthier or more competitive foals and improved reproductive efficiency in mares.
Manipulating a Mare’s Reproductive Cycle
Manipulation of estrous cycles or “heat cycles” refers to the intentional alteration of a mare’s natural reproductive cycle. Estrus manipulation in horses typically involves using artificial lighting or hormone administration to alter the breeding season.
Exerting increased control over a broodmare’s estrous cycle offers benefits such as: 
- Having mares start cycling earlier in the year so foals are born as close to January 1st as possible
- Increasing the length of the breeding season to improve chances of conception
- Triggering ovulation to occur at a specific time
- Synchronizing estrus among mares for advanced reproductive techniques
Manipulating the reproductive cycle requires knowledge of the different phases of estrous as well as the hormones that are most active during each phase.
By altering the mare’s hormonal balance through lighting and administering hormones, the veterinarian can shift when key stages of the estrous cycle occur, allowing for the desired reproductive outcome.
Reasons to Manipulate Estrus
Veterinarians use medications such as human chorionic gonadotropin or deslorelin, which trigger ovulation within 48 hours of administration.
Estrus synchronization is a critical component of assisted reproductive technologies, including embryo transfer. In order for pregnancy to be successful, the donor and recipient mare must ovulate within 1-3 days of each other.
To synchronize the mares, veterinarians typically use either progestogens or prostaglandins to lengthen or shorten the diestrus phase as needed. Then, ovulation-inducing agents can trigger ovulation at the appropriate time, once both mares are in estrus.
The Estrous Cycle
Mares are seasonally polyestrous, meaning that they have multiple “heat” cycles per year, but only during certain times of year. 
In general, mares cycle during the spring and summer, with each estrous cycle averaging 21 days in length.  The two main phases of the estrous cycle are estrus, the sexually receptive phase, and diestrus, the phase where the reproductive tract prepares to sustain a pregnancy.
Estrus last for between 4 and 7 days, during which time the mare is receptive to breeding and exhibits signs of being “in heat.” In the estrus phase, the ovaries prepare one to two follicles, which each contain an oocyte (egg).
- Follicle Maturation: Follicle maturation starts with the production of gonadotropin-releasing hormone in the hypothalamus.
- Hormone Signaling: This hormone stimulates the production of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which are the major hormones that drive follicle maturation in the ovaries. 
- Estrogen Production: As the follicles mature, they produce estrogen, the major sex hormone that induces sexual receptivity and signs of being “in heat” in the mare. 
- Ovulation: Estrus ends with ovulation, the release of oocytes into the oviducts in preparation for fertilization. 
Diestrus typically lasts between 14 and 15 days.  During this phase, the mare is not receptive to breeding and her body prepares for a potential pregnancy in the event that a fertilized embryo is detected.
- Corpus Luteum: In the diestrus phase, the remnant of the recently ovulated follicle (corpus luteum) is present on the ovary and produces progesterone. 
- Progesterone: Progesterone prepares the reproductive tract to receive the fertilized embryo, if present.
- Prostaglandin: In the absence of an embryo, the endometrium (uterine lining) produces prostaglandin, a hormone which triggers the corpus luteum to reduce in size. 
- Regression: As the corpus luteum regresses, progesterone levels decrease, and the mare enters another estrus phase.
Lengthening the Breeding Season
Lengthening the breeding season is one of the most common reasons for manipulating the estrous cycle in mares. In most cases, breeders aim to extend the breeding season by having mares start cycling earlier in the year.
This increases the likelihood of a mare giving birth to a foal closer to January 1st.  Foals born at the beginning of the year will be larger and more mature compared to foals born later in the year. This may give them an edge competitively or make them more desirable at yearling sales.
Some breeders may also use these strategies for subfertile mares, giving them more estrous cycles to become pregnant during a particular year.
The two main methods to increase the length of the breeding season are artificial lighting strategies and administration of hormones. Typically, a combination of the two methods produces the best results.
Mares cycle seasonally, and their bodies use the length of daylight available to tell them when to begin cycling. Spring and summer months have longer days compared to fall and winter. As the days start to lengthen in the spring, the increased light exposure causes mares to naturally begin their reproductive cycles.
Breeders can use this natural phenomenon to encourage mares to start cycling earlier in the year, by exposing them to 14 to 16 hours of artificial light per day. 
Typically, lighting changes take 8 – 10 weeks to have an effect on a mare’s estrous cycle. Most artificial lighting protocols begin in early December or earlier to influence the following breeding season. 
Artificial lighting must be of the correct intensity to successfully induce estrous cycling. Studies show that a minimum intensity of 10 foot-candles (one lumen per square foot) at the mares’ eye level is best to induce cycling. This would mean that you can read the print in a paperback book at arm’s length.
In stall or paddock settings, it’s essential to ensure there are no shadows or dark corners where the mare might linger, potentially reducing her exposure to the intense lighting. 
Some breeders use specialized face masks with a built-in light that shines directly into the mare’s peripheral vision to stimulate cycling while the mare is out on pasture.
Dopamine antagonists medications, such as sulpiride and domperidone, can also trigger earlier onset of estrous cycles, effectively lengthening the breeding season.
Dopamine antagonists are substances or drugs that prevent the activity of the neurotransmitter dopamine.  This increases the secretion of the hormone prolactin, which commonly increases during normal estrous cycling. However the exact mechanism of increased prolactin activity triggering estrous cycling is unknown. 
Progestogens are a group of hormones that include natural progesterone and synthetic analogs. Progesterone is the main hormone that sustains pregnancy in horses.
Under normal circumstances, progesterone inhibits the release of luteinizing hormone (LH), which is essential for ovulation and the formation of the corpus luteum. When this inhibition is lifted, a surge of LH can trigger the onset of estrus.
By giving a mare daily progesterone during anestrus (when she’s not cycling), LH accumulates in the pituitary gland. Once the progesterone treatment ceases, the sudden surge of LH initiates the estrous cycle. 
Altrenogest is the most common synthetic progesterone used for this purpose, often marketed under the brand name Regu-Mate. By mimicking the actions of natural progesterone, altrenogest helps in regulating the estrous cycle of mares, making it easier for breeders to manage breeding schedules.
Women need to be ultra-cautious when handling an altrenogest or giving Regu-Mate to their horses. Mishandling, such as getting it on your skin, can cause adverse and even severe reactions to a woman’s menstrual cycle.
Some practitioners use progestogens in combination with estrogens, such as estradiol. The combination of the two hormones causes even more significant inhibition of LH. 
Around 50% of breedings are successful in GnRH-induced cycling, however these pregnancies may have an increased risk of early embryonic loss. 
Studies show that administering analogues of FSH and LH directly, rather than GnRH, can also induce ovulation in anestrus mares and result in estrous cycling during non-breeding seasons. 
Veterinarians will often induce ovulation after performing artificial insemination. This ensures that the oocyte is available for fertilization within 24 – 48 hours after the insemination procedure.
Inducing ovulation reduces the risk that natural variations in the mare’s estrous cycle will prevent timely ovulation after insemination.  This is particularly important when using frozen semen or poor-quality semen, as the optimal fertilization window is much smaller. 
Some veterinarians also induce ovulation after live cover breeding, for similar reasons.
There are two main methods for inducing ovulation: the administration of human chorionic gonadotropin (hCG) and the use of gonadotropin-releasing hormone (GnRH) agonists.
Both methods stimulate the release of an egg from the ovary, but they work through different hormonal pathways and may be chosen based on specific circumstances or preferences.
Human Chorionic Gonadotropin
Human chorionic gonadotropin (hCG) has a similar structure to luteinizing hormone, and promotes ovulation when administered to mares.  Following administration, most mares ovulate within 48 hours. 
A recent study suggests that hCG may be more effective at inducing ovulation over deslorelin (a GnRH agonist) for mares in spring transition or those who are receiving sulpiride to lengthen the breeding season.  For other mares, hCG and deslorelin have similar effects. 
There is some debate over whether hCG maintains its efficacy when given multiple times to the same mare. Studies show mares can develop antibodies against hCG, which could interfere with the efficacy of the drug. 
Some studies show no changes in ability to induce ovulation with repeated administration of hCG to the same mares, but others show variable efficacy after repeat administration.  As a result, some veterinarians avoid giving multiple doses of hCG in one breeding season to reduce the risk of antibody development. 
GnRH agonists are hormones that have a similar structure and function to GnRH, resulting in increased production of FSH and LH. When administered to mares, GnRH agonists can stimulate follicular development and induce ovulation.
The main GnRH agonist used in mares is deslorelin, however buserelin and histrelin are also available.  Deslorelin is available as an implant for subcutaneous injection into the lips of the vulva.  Typically, mares will ovulate within 48 hours of implant insertion. 
The major downside of deslorelin in a breeding program is that it can prolong the diestrus period by 4 – 6 days, lengthening the time before the mare can be rebred if she does not become pregnant. 
Removing the implant can reduce the risk of prolonged diestrus, but must be performed within 1 -2 days after ovulation. 
Estrus synchronization is key for successful embryo transfer or assisted reproductive technology use. In these reproduction methods, the donor mare and recipient mare must ovulate within 1 – 3 days of each other to maximize the chance of a successful pregnancy in the recipient. 
Once the mares are in estrus, ovulation-inducing agents can trigger ovulation within a short timeframe. The main methods of altering diestrus length are prostaglandin and progesterone. 
Prostaglandin is a hormone that stimulates regression of the corpus luteum, triggering the mare to enter estrus.  Administration of prostaglandin usually results in the onset of estrus within 2 – 4 days, and ovulation within 7 – 12 days.
Common prostaglandin products for horses include cloprostenol and luprostiol. However, there is some evidence to suggest that luprostiol may be more effective in synchronization programs. 
Prostaglandin is only effective on a fully matured corpus luteum, so many practitioners use two doses of prostaglandin, 14 days apart, to ensure efficacy.  With this protocol, approximately 90% of mares will show signs of estrus following the second injection. 
Once estrus occurs, the veterinarian must monitor the mares closely for follicle development, so that administration of deslorelin or hCG to induce ovulation can be timed appropriately. 
Prostaglandins can cause side effects such as severe sweating, diarrhea, and abdominal discomfort for up to 20 minutes after administration. This makes some practitioners and breeders hesitant to use the medication. 
However, there have been no reports of long-term ill-effects or death from prostaglandin use, making the drug safe despite the temporary adverse effects. 
Veterinarians can use progestogens to lengthen the duration of diestrus in a mare, synchronizing her return to estrus with another mare.  Typically, progestogens are given for at least 10 days, followed by a dose of prostaglandin to induce regression of the corpus luteum. 
Again, women need to be ultra-cautious when giving Regu-Mate to their horses. Mishandling, can cause adverse reactions to a woman’s menstrual cycle.
- Estrus manipulation can increase the length of the breeding season, induce ovulation to improve conception rates, or synchronize mares for advanced reproductive techniques
- Hormone administration is the most common technique for estrus manipulation, but artificial lighting programs can also be used
- Working closely with your reproductive veterinarian provides the best chance of a successful pregnancy
Is Your Horse's Diet Missing Anything?
Identify gaps in your horse's nutrition program to optimize their well-being.
- Brinsko. S. P. and Blanchard. T. L., Eds.Manual of Equine Reproduction, 3rd edition. St. Louis, Mo: Mosby/Elsevier. 2011.
- McKinnon, A. O. et al. Equine Reproduction. Wiley-Blackwell, 2010.
- Squires, E. L. Hormonal Manipulation of the Mare: A Review. Journal of Equine Veterinary Science. 2008.
- Mari, G. et al. Administration of Sulpiride or Domperidone for Advancing the First Ovulation in Deep Anestrous Mares. Theriogenology. 2009.
- James, A. N. et al. Efficacy of Short-Term Administration of Altrenogest to Postpone Ovulation in Mares. Journal of Equine Veterinary Science. 1998.
- Roser, J. F. & Meyers-Brown, G. Enhancing Fertility in Mares: Recombinant Equine Gonadotropins. Journal of Equine Veterinary Science. 2019.
- Fanelli, D. et al. hCG Is More Effective than the GnRH Agonist Buserelin for Inducing the First Ovulation of the Breeding Season in Mares. Equine Veterinary Journal. 2022.
- McCue, P. M. et al. Comparison of Compounded Deslorelin and hCG for Induction of Ovulation in Mares. Journal of Equine Veterinary Science. 2007.
- Lindholm, A. R. G. et al. Comparison of Deslorelin and Histrelin for Induction of Ovulation in Mares. Journal of Equine Veterinary Science. 2011.
- Serrano, Y. R. & Crabtree, J. R. Efficacy of Deslorelin Acetate Injection in Mares. Equine Veterinary Journal. 2019.
- Kuhl, J. et al. Efficiency of Two Prostaglandin F2? Analogs for Synchronization of Estrus and Ovulation in Mares. Journal of Equine Veterinary Science. 2016.