Use of progestins in open and pregnant mares

Abstract

Administration of progestin to mares has become commonplace in the equine industry. Progestins can either be administered orally in the form of altrenogest, or injected intramuscularly suspended in oil or more recently in the form of microspheres. Progestins have been used for numerous clinical reasons. Early in the breeding season, progestins are administered in an attempt to control the erratic estrous periods typical of the transition period. Synchronization of estrus in normal cycling mares with progestin has been used to improve reproductive efficiency for use of cooled, transported semen and synchronization of embryo transfer recipients. Progestins can be administered either daily for 14 days or, with the addition of prostaglandins, a shorter treatment of 7–10 days can be utilized. Estradiol has been added to progesterone in an attempt to obtain greater control of follicular development during steroid treatment. Alternative uses of progestins include long-term suppression of estrus for show or race mares, and the delay of the postpartum estrus. Studies have demonstrated that fertility is not adversely affected with either of these two approaches. Administration of progestins to mares with a history of early embryonic loss has become commonplace. Both injectable progesterone and altrenogest have been shown to maintain pregnancy in ovariectomized mares.