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: [1]
- 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
Inducing ovulation is common in both artificial insemination and live cover reproduction. This technique increases the chance the mare will ovulate within the optimum fertilization window.
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. [1]
In general, mares cycle during the spring and summer, with each estrous cycle averaging 21 days in length. [1] 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
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. [1]
- 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. [1]
- Ovulation: Estrus ends with ovulation, the release of oocytes into the oviducts in preparation for fertilization. [1]
Diestrus
Diestrus typically lasts between 14 and 15 days. [1] 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. [1]
- 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. [1]
- 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. [1] 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.
Artificial Lighting
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. [1]
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. [2][3]
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. [1]This would mean that you can re