What's New in Equine Estrus Manipulation

Current options for owners who want to control their mares’ heat cycles for breeding or performance reasons

Your mare comes equipped with the signals and structures necessary to reproduce. But nature doesn’t always have the most convenient timing. Like that time all the shipped semen went bad waiting for Ruby to ovulate. Or that embarrassing moment Ellie stopped in the middle of a dressage test and winked her vulva at a flashy Hanoverian stallion. Oh estrus.

All About Estrus

In the five to seven days before ovulating, mares go through estrus, says Christine Aurich, DVM, PhD, head of the Graf Lehndorff Institute for Equine Science, in Neustadt, Germany. During this time, the uterus is preparing to accept a pregnancy, and because semen can live several days inside the mare’s reproductive tract, she becomes “ready”—both physiologically and behaviorally—to receive a stallion.

Breeding centers must predict estrus accurately for well- timed mating or insemination, says Aurich. “With the help of induction of estrus and ovulation, breeding processes are much easier and, overall, fertility (conception rate) is much improved,” she says.

Likewise, success of assisted reproduction techniques such as intracytoplasmic sperm injection and embryo transfer require careful control of the estrous cycle to eliminate the complication of finding a recipient mare at the right stage of estrus, says Marco Antonio Alvarenga, PhD, of the Department of Animal Reproduction and Veterinary Radiology at Sao Paulo State University, in Botucatu, Brazil.

In performance mares owners might prefer to delay estrus to avoid its undesirable behavior aspects, says Robyn Ellerbrock, DVM, PhD, Dipl. ACT, assistant professor of theriogenology in the University of Georgia’s College of Veterinary Medicine, in Athens. “Manipulation is necessary in mares that become difficult to work with when in heat,” she says.

Changing the Hormone Balance

Manipulating estrus is essentially a game of altering hormonal balance to “trick” a mare’s reproductive system into staying longer in a nonestrous phase or to bring on estrus faster and shorten its duration, our sources say.

Delicate balances of primarily progesterone and estrogens induce the various phases of a mare’s estrous cycle, explains Aurich. Estrogen is the predominant hormone during estrus. In diestrus—the period between estrous cycles— progesterone takes the lead. It’s the “pro-gestation” hormone, meaning it promotes pregnancy by encouraging the uterus to accept and hold on to an embryo. Very basically, estrus makes a mare sexually excited, whereas progesterone keeps a mare calm and “maternal.”

Scientists have developed drugs to manipulate estrus, using equine hormones, hormones from other species, and synthetic variations of these hormones. They force a shift in the hormonal balance, changing the estrous cycle.

The reproductive system can also be “encouraged” to alter that balance without drugs, our sources say. By changing aspects of the mare’s environment—including her uterine environment—at different points in the estrous cycle, her body “believes” it needs to be releasing different proportions of hormones than it normally would.

Altrenogest: Still the Gold Standard for Performance Horses

Oral altrenogest (Regu-Mate) is today’s gold standard in estrus manipulation for performance horses in the U.S. and many other countries, says Ellerbrock. “This is the safest (for the mare) and most effective option, with the most predictable outcome,” she says. It’s also the only FDA-approved product for suppressing estrus, she adds.

Altrenogest is a synthetic hormone that shifts the hormone balance to become progesterone-dominant by interacting with the mare’s progesterone receptors, says Callum G. Donnelly, BVSc (Hons I), Dipl. ACT, ACVIM (LA), of the Finno Laboratory in the Department of Population Health and Reproduction at the University of California, Davis, School of Veterinary Medicine. Given orally as a liquid every day during the breeding season, altrenogest generally holds off estrus until about five days after treatment stops.

An injectable form reduces contamination risk for drug tests (more on this in a bit) but increases the risks of skin reactions, Ellerbrock says.

A new, though not yet FDA-approved, alternative is a long-term oxytocin injection, which is a “cheaper option that doesn’t risk hormonal exposure for the barn staff,” she explains, potentially interfere with a woman’s reproductive system if absorbed through the skin.

Oxytocin injections, given daily for a week, can cause cramping and don’t work in all mares. When they do, they can prolong diestrus by up to 60 days, says Donnelly.

The Horse - May 2020This article continues in the May 2020 issue of The Horse: Your Guide to Equine Health Care.

Subscribe now and get an immediate download of the issue, including this in-depth article on the latest approaches for owners who want to control their mares’ heat cycles. Already a magazine subscriber? Digital subscribers can access their May issue here. Domestic print subscribers who have not received their copy should email circulation@thehorse.com.