Dominant follicle and gonadotropin dynamics before ovulation in postpartum lactating mares

The aim of this study was to compare the dominant follicle (DF) and gonadotropin dynamics for 9 days before ovulation in postpartum lactating (PP Lactating) versus non-postpartum cycling (N-PP Cycling) mares. Every PP Lactating mare on the day of parturition was paired with a N-PP Cycling mare, and the data analyses considered the partum-ovulation interval (POI) and the postpartum interovulatory interval (PPIOI) in PP Lactating mares and two interovulatory intervals in N-PP Cycling mares. The results of the present study revealed several novel and unique aspects of DF development and FSH and LH dynamics before ovulation in PP Lactating mares when compared with N-PP Cycling mares. The most remarkable differences between both groups of mares were the following: (1) a shorter interval to ovulation in PP Lactating mares during the foal heat (POI ≤22 days) compared with all other intervals; (2) a larger DF in PP Lactating mares, and an earlier day of DF at maximum diameter during the foal heat; and (3) lower gonadotropin levels in PP Lactating mares. Regarding the particularities, PP Lactating mares had lower LH levels during the POI than the PPIOI, demonstrating a strong partum effect; spring-foaled mares had longer POIs, larger DF diameters, and lower LH levels; and lower body condition scores and higher body-weight loss led to longer POIs and smaller DF diameters in PP Lactating mares. This study contributes to a broad understanding of ovarian function in the postpartum mare.