Achieving Multiple Ovulation In The Mare - Possibilities And Limitations

Superovulation in domestic animals is an important tool for routine use in embryo transfer (ET) programs to reduce the costs and to enhance the efficiency. The failure to produce more than one embryo by donor per cycle is the most important reason of the low efficiency and the high costs of equine embryo transfer programs. Typically, the success rate of embryo recovery from a single-ovulating mare is about 50% per cycle. However, mares that spontaneously double ovulate or are induced to ovulate multiple follicles have higher embryo recovery rates. With an increased embryo recovery rate in horses, the overall costs of ET would be reduced. Furthermore, if multiple embryos could be obtained on a given cycle, then embryos could be frozen for transfer at a later time. Historically, the induction of multiple ovulations has been tried through hormone manipulation of follicular waves in the mare. Manipulation of hormones associated with development of dominant follicles and regression of subordinate follicles are the basis of superovulation. The number of follicles that ovulate in response to exogenous hormone treatments is limited in mares as compared with sheep and cattle. This limited success may be due to the unique anatomical configuration of the mare's ovary. The tissue of the ovary in the mare is arranged as the inner cortex and the outer medulla, so its structure is "inside out" compared to the ovary of other domestic species. In addition, only a small portion of the ovary has germinal epithelium that will accommodate ovulation, termed the ovulation fossa. Several protocols have been studied in the equine species to achieve multiple ovulations, but the results are still inconsistent. This article focuses on the main species-specific physiological ovarian dynamics with an emphasis on the possible effects on previous superovulation treatment attempts and limitations done so far.