Hormonal Approach to Inducing Early Ovulation in Mares

Donald Thompson, Oberhaus, Erin  |  6/10/2017 7:34:11 PM

An industry-imposed birthday of January 1 for most registered horse breeds has created an incentive for horse owners and breeders to produce foals as early in the year as possible. However, this becomes difficult because of the mare’s long gestation period of 330 to 345 days and the seasonal nature of the mare’s reproductive cycle. Mares in Louisiana typically enter a period of reproductive inactivity, known as seasonal anestrus, in late October and do not return to a regular cycle, or cyclicity, until early April. Therefore, advancing the first ovulation of the season is desirable for breeders in order to produce maximally competitive foals – those born close to the January 1 birthdate – and has led to years of study of spring transition in mares and numerous attempts to develop pharmacological therapies for inducing early cyclicity.

Much effort is invested yearly in the effort to manipulate the mare’s reproductive cycle so she ovulates in mid-February. The common method used in the field to induce early cyclicity is artificial lighting. This has proven to be labor intensive and cost prohibitive for most farms. Of the pharmacological therapies that have been tested, the most promising is the use of prolactin (a pituitary hormone) or dopamine antagonists such as sulpiride or domperidone that stimulate prolactin. Elevated prolactin during seasonal anestrus stimulates follicular growth and hastens ovulation in most, but not all, mares. It is unclear how prolactin induces early ovulation, but it is suspected that prolactin acts directly on the ovary.

Ten years of experimentation at LSU has revealed success rates of mares ovulating early range from 50 to 90 percent in mares treated with a combination of estrogen and either domperidone or sulpiride. Because all mares did not respond to treatment, the current research project was designed to identify factors that might indicate whether a mare responds to stimulated prolactin with early ovulation.

In this experiment, mares in seasonal anestrus were treated with injections of estrogen and sulpiride in mid-January. Mares that responded with early ovulation did so approximately 10-15 days after treatment. These mares tended to have higher body condition scores and higher concentrations of the hormones leptin and insulin, which might indicate that their nutritional status was superior to mares that did not respond. Nutritional status is an important factor in reproductive success in many species, so it is not surprising that nutrition may have affected the ovarian response to stimulated prolactin.

Ovaries also were assessed to determine what changes occurred in response to stimulated prolactin. The ovarian follicles that were induced earlier produced just as much estrogen, progesterone and prolactin as the follicles occurring naturally later in the spring. The early-induced follicles also contained just as many receptors for luteinizing hormone, the hormone responsible for ovulation. Receptors for luteinizing hormone in the ovary are required for the hormone to exert its action and cause ovulation. This indicates that the early ovulatory follicles are just as mature and capable of ovulating a fertile egg as their later-season counterparts. Pregnancy rates were 75 percent in mares induced to ovulate early. This is an important aspect of the proposed treatment in terms of its practical use in the equine industry.

Ongoing research at LSU continues to focus on identifying a practical approach for the use of these dopamine antagonists in mares. An understanding of the ovarian changes that occur in the presence of prolactin will allow researchers to identify cause-and-effect relationships between prolactin and ovarian function. This knowledge could help generate new or better protocols to induce early cyclicity in more mares and may also be applicable in artificial reproductive technologies such as embryo transfer and in vitro fertilization.

Erin L. Oberhaus is a Ph.D. candidate and Donald L. Thompson Jr. is the Ralph and Lela Boulware Professor of Animal Sciences in the School of Animal Sciences.

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Erin Oberhaus, a Ph.D. student in animal sciences, prepares to draw blood from a horse at the LSU AgCenter horse unit. Photo by Tobie Blanchard

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