There are a variety of hormonal therapies available to the equine practitioner to help bring mares into estrus and hasten
ovulation. They include human chorionic gonadotropin (hCG), various progestin/estradiol combinations, gonadotropin releasing
hormone (GnRH) analogs, and dopamine antagonists. These hormonal therapies push Mother Nature along to help make the breeding
process more efficient, reduce excessive handling of mares and stallions, and help ensure successful breeding and conception,
whether a maiden, a previously barren or a cyclic mare.
Normally mares will be in heat for five to seven days, and will ovulate around the fourth or fifth day of heat, about
24-48 hours before they go out of heat.
A brief sequential overview of the regular estrous cycle
"If you don't use a hormone to induce ovulation, you may end up breeding that mare two or even three times during a given
estrous cycle," states Dr. Patrick McCue, Dipl. ACT, associate professor in equine sciences at Colorado State University.
Induction of ovulation is used to limit the number of times the mare must be bred so a "timed" ovulation can be achieved,
resulting in the need to breed the mare only once or maybe twice.
An ovulation-inducing agent is typically used when breeding mares in a frozen or cooled-semen breeding program so it can be
ordered in time to inseminate the mare prior to ovulation, especially if semen is in limited supply. In a Thoroughbred breeding
situation (where the mare needs to be covered), it can be used to limit the number of times a given mare has to be covered
by a stallion or to ensure a timely foal the following year, close to Jan. 1. For a lot of busy stallions, a mare is not going
to be able to get booked a second time in a heat cycle.
"When the mare goes to the breeding shed, the veterinarian or manager must be able to, with some degree of assuredness, know
that that mare will ovulate following breeding," McCue says.
Even if the amount of semen is not limited or if the stallion is not that busy, one also must understand mare issues that
prompt use of ovulation inducing agents.
"Some mares may have a rather dramatic inflammatory response post-breeding or post-insemination," McCue says. "In those particular
mares, you're only going to want to breed that mare one time during a given estrous cycle. After that, you may be more focused
on cleaning out her uterus. You may want to limit the number of times she is bred, and therefore give her hCG or a GnRH product
to get her to ovulate so that she doesn't need to be covered again."
For anestrous mares, the hormonal therapies initiate estrus in mares that might not cycle on their own.
To successfully induce ovulation, the mare should be cycling, in heat, with a developing dominant pre-ovulatory follicle of
at least 35 mm or greater. On ultrasound, she typically should have some edema in her uterus, which would correlate to whether
or not she is in behavioral heat. She needs to be toward the end of the transition period for these agents to actually induce
"It is important that veterinarians know that there are also breed differences in response," McCue says. "For a Warmblood
mare, one would typically wait until the follicle is 40 mm or greater before contemplating giving something like hCG to induce
ovulation. They don't typically ovulate with smaller follicles."
Despite hormone therapy, light is critical. Naturally, with exposure to lengthening days, the pineal gland stimulates the
pituitary to release GnRH.