Reproductive patterns and follicular waves in postpartum lactating versus non-postpartum cycling mares

This comparative study between postpartum lactating (PP Lactating) and non-postpartum cycling (N-PP Cycling) mares aimed to characterize reproductive patterns, types and frequencies of follicular waves, corpus luteum and endometrial echotexture dynamics, and the influence of season and body condition. Mares from each group were paired considering the day of parturition of a PP Lactating mare. The partum-ovulation interval (POI) and the postpartum interovulatory interval (PPIOI) were evaluated for PP Lactating mares, and 2 IOIs were evaluated for N-PP Cycling mares. The following observations were made: (i) PP Lactating mares have several different reproductive patterns, such as continuous reproductive activity (i.e., short or long POIs followed by a PPIOI), ovarian inactivity after the first postpartum ovulation, or continuous ovarian inactivity (postpartum anestrous phase); (ii) a greater total number of minor waves was seen in PP Lactating mares; (iii) major primary follicular waves (i.e., ovulatory) emerge around the day of parturition in mares with short POIs; (iv) the season of parturition (spring season), decrease in body condition score, and body-weight loss can have an associated detrimental effect in PP Lactating mares by increasing the total number of minor follicular waves and, consequently, the POI length; (v) endometrial echotexture scores are higher during the POI and can be influenced by the season of parturition; and (vi) corpus luteum development and demise are similar between PP Lactating and N-PP Cycling mares. This study provides, for the first time, detailed information about reproductive physiological aspects during the postpartum period and may facilitate the interpretation of gynecological practices during the foal heat and subsequent IOI in mares.