Superovulation in the Mare: A Work in Progress

During the primary follicular wave of the mare's estrous cycle, it is typical for one follicle to become dominant and several subordinate follicles to regress. During estrus the dominant follicle becomes a pre-ovulatory follicle and under the control of a well-defined hormonal milieu goes on to ovulate. The percentage of double ovulations in mares is low. The success of advanced reproductive technologies in the mare would be enhanced by effective superovulation to provide multiple oocytes and multiple embryos for such techniques as embryo transfer (ET), gamete intra-fallopian tube transfer (GIFT) and intra-cytoplasmic sperm injection (ICSI). Superovulation would increase pregnancy rates in normal and subfertile mares as well as when using semen from subfertile stallions. Over the past 35 years, many investigators have used various hormones regimens such as equine chorionic gonadotropin (eCG), gonadotropin releasing hormone (GnRH), immunization against inhibin, equine pituitary extracts, (EPE) and a partially purified preparation of equine follicle stimulating hormone (eFSH) to induce superovulation in cycling mares, but the success rate has been limited or inconsistent. It is not the intent of this review to go into a detailed description of each of the hormone regimens previously studied, but to use the knowledge gained by the many studies on follicular dynamics in the mare during follicular emergence, deviation, dominance and the periovulatory surge to propose why certain hormone regimens used for superovulation appear to be more successful than others. In addition, this review will focus on the success of the more recent studies on the use of the recombinant equine gonadotropins: recombinant equine luteinizing hormone (reLH) and recombinant equine follicle stimulating hormone (reFSH) to superovulate mares.