Fooling Mother Nature: Induction of estrus and ovulation in mares

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.

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"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 ovulation.

"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."

Photoperiod

Despite hormone therapy, light is critical. Naturally, with exposure to lengthening days, the pineal gland stimulates the pituitary to release GnRH.

"The most common misconception or the biggest mistake is for veterinarians to think that light is not very important," says Patrick Burns, PhD, president of Burns BioSolutions, Lexington, Ky.

All of the hormonal treatments and drugs work best or most effectively, either after the mare has had some exposure to photoperiod (artificially), or at the beginning of the transitional season in the spring (April-May), when the mare has started the process on her own due to the increasing daylight.

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"When you try to superimpose these hormonal therapies on mares that are in debt, deep in anestrous, the results of the therapies are consistently terrible," Burns says.

In order to fool Mother Nature slightly, a treatment of as short as three weeks to six weeks of light exposure (16 hours of light per day, typically until 10 p.m.) has beneficial effects. Natural light and lengthening days alone will work, but it typically takes about 60 days. Accelerated light regimens speed the process a bit.

"No one's come up with anything better yet," Burns observes. "Typically we would like to have them on a lighting regime for a minimum of 30-40 days before any of these hormone therapies."

With some mares, the results are quite good in as little as 21 days. There is a huge difference between no light and 21 days. The 21-day regimen was shown when using exogenous equine follicle stimulating hormone (FSH). Even with exogenous FSH, which will grow the follicle to ovulation, the results are far better when the mare has had at least three weeks of light. Light initiates an entire neural-endocrine cascade into effect. We know that light has some effects on GnRH secretion, a lot of input on the pituitary and stimulation of prolactin.

"What I preach and teach is that photoperiod is number one," says Dr. Jim Brendemuehl, Dipl. ACT, University of Illinois Veterinary Teaching Hospital. "When one has the time to plan ahead, it is important to get mares under a lighting regime by the first of December. It is recommended to put mares on lights 45-60 days prior to when you want them to cycle. They should also be in a positive nutrient balance so they are gaining weight, rather than losing, and in good body condition."

Sufficient pituitary LH content

Mares must have sufficient LH in the pituitary for the hormonal drug therapies to work. Oftentimes with hormone therapy in the field, the pituitary just does not contain a sufficient amount of LH to promote ovulation in early spring, regardless of the stimulation of the system. The follicle is stimulated, but ovulation doesn't occur due to the insufficiency of LH.

Naturally, when the mare goes into seasonal winter anestrous, her LH synthesis decreases. There is not adequate LH stores until far enough along into the transitional period. LH ultimately gets there after photoperiod stimulation. As she starts cycling in the spring, FSH levels increase prior to LH. Once the mare is progressing with light stimulation, her pituitary begins to build sufficient LH content. The last contributing factor that leads up to ovulation is the pituitary gland building up enough LH in the gland to be able to trigger an ovulatory surge.

Progestins/estradiol

"In my perspective, in the transitional phase, whether mares are under artificial lights or under natural daylight conditions, when they haven't started to cycle yet, progesterone or the progesterone/ estradiol combination is very effective to give mares a kick start into their cyclic activity," says Dirk Vanderwall, DVM, PhD, a researcher with Northwest Equine Reproductive Lab at the University of Idaho. "I certainly find that very helpful and beneficial clinically in mare management in that spring transitional phase."

Progesterone seems to help the pituitary synchronize itself and to accumulate a sufficient amount of luteinizing hormone (LH), so the mare will go ahead and ovulate. Progestin has a negative feedback, which inhibits FSH and LH and essentially allows a buildup of LH at the same time it is inhibiting follicular development.

When you put a mare on progesterone/estradiol, the addition of the estradiol inhibits follicular development in the majority of mares. With the use of progestin/estradiol combination, progesterone prolongs the luteal phase; estradiol inhibits follicular growth. With the addition of estradiol, you get a more significant negative feedback, and consequently, estrus synchronization, due to both the progesterone and estrogen feedback, which inhibits FSH and LH. There is a more profound negative feedback on FSH and LH than with progestin alone, which affects the onset transition of mares in the springtime. You have greater inhibition of FSH and LH and a greater potential that LH will rise to initiate ovulation.

"One of the tricks we do with progesterone/estradiol treatment is to essentially block the release of LH and allow it to be built up to a level so that once we do remove progestins/estradiol, there is enough of an LH-surge to stimulate ovulation," Brendemuehl says. "That is part and parcel to the mechanism action of how the progesterone/estradiol therapy works."

In a study at Colorado State, Burns investigated the use of a single-injection, 10-day progesterone versus a placebo control. They looked at the average number of days from the first day the mare came into heat (with a 35-mm follicle) until ovulation, with or without treatment. The difference was quite significant. The untreated mares took 19 days from the time they had a breedable follicle until they ovulated. With treatment, it only took six days. With progesterone, there is not a prolonged heat and estrous cycle until ovulation.

"Those things really cause the average vet in the field, owners, and those shipping semen a lot of problems," Burns says.

Human chorionic gonadotropin

HCG is approved and licensed for use in veterinary practice for inducing ovulation in mares. For several years, hCG has been used effectively in inducing ovulation in mares that are in behavioral estrus with a follicle of 35 mm or greater. HCG, having LH biologic activity, is most effective given to mares that have not received it previously, presumed due to its antigenic properties. It is therefore usually limited to being given during one or two estrous cycles in a given breeding season.

GnRH analogs

Whether GnRH or deslorelin (a GnRH analog) an injection in early spring will produce a nice surge of FSH, but you'll see almost no LH release, as with other therapies because the pituitary does not contain sufficient LH early in the spring. Use of deslorelin produces a hastening of ovulation in mares with preovulatory follicles greater than 30 mm. Johnson et al 2002 demonstrated that mares induced to ovulate with deslorelin experienced suppressed gonadotropin (LH and FSH) secretion for several days and desensitization to exogenous GnRH for at least seven days post-ovulation. A deslorelin implant produced an initial ovulation two days sooner than untreated mares; their LH and FSH increased initially after implant but decreased thereafter. Though plasma LH and FSH increased immediately following deslorelin administration, they both declined for several days thereafter. Deslorelin also produced a prolonged inter-ovulatory period, between the initial ovulation and the subsequent one.

Dopamine antagonists

The dopamine antagonists probably work via their positive influence on prolactin, though additional research would be of interest to thoroughly define their mode of action. Domperidone and sulpiride increase prolactin secretion indirectly, by blocking the action of dopamine, whose action is to decrease the secretion of prolactin by the pituitary gland.

"I have used dopamine antagonists typically with a mare that is presented the first of February that hasn't been under lights, so we don't have the luxury of waiting 45-60 days for her to respond to photoperiod stimulation," Brendemuehl says. "In some of those mares in good body condition, we have had some success in putting those mares on a course of dopamine antagonist therapy of domperidone or sulpiride, and having them respond positively in terms of follicular development and to start cycling earlier."

In work done at Colorado State and Cornell, use of the dopamine antagonists, domperidone and sulpiride, did not seem to promote ovulation in mares during periods of severe cold weather.

"It probably has to do with the influence of the cold on thermoregulatory mechanisms, thyroid hormone function as it contributes to ovarian hormone function, and probably even more so to what is going on in the hypothalamus, the pituitary and the influence of corticosteroids," Brendemuehl says.

The true mechanisms are still a bit unknown. It is one of those gray areas where there has been little research.

Prostaglandins

Use of prostaglandin F2 (PF2) will shorten the luteal phase, lysing the luteal structure (CL) to bring the mare in heat earlier than she would on her own. But the prostaglandins have little use for the spring transitional mare, since she hasn't had an ovulation yet. Therefore, in the spring transitional mare, there is no biological endpoint that would be affected by prostaglandin. The other concern regarding PF2 are the possible side effects of increased heart rate, diarrhea, sweating and pelvic muscle spasms. The side effects can be reduced by giving two injections of reduced dosage 24 hours apart.