129 Ovarian dynamics and gonadotropins during selection of the dominant follicle in postpartum lactating versus non-postpartum cycling mares

Abstract

The mare, compared to other livestock, has the shortest interval from partum to the first ovulation. In monovulatory species, the follicle deviation process in a wave is characterised by the continued growth of the dominant follicle (DF) and regression of the subordinate follicle. Although follicle diameter deviation, a key event of follicle selection, has been investigated during the oestrous and menstrual cycles, the occurrence of this phenomenon before the first postpartum ovulation seems to be unclear in all species. This study aimed to compare the follicular dynamics and gonadotropin profiles around the follicle diameter deviation day in postpartum lactating (PP Lactating; n = 24) versus non-postpartum cycling (NPP Cycling; n = 15) mares. On the day of parturition, every PP Lactating mare was paired with an NPP Cycling mare, and ovarian follicles (>4 mm) were tracked daily by transrectal ultrasonography, and blood samples were collected. Data were analysed in the PP Lactating group according to the length of the partum-ovulation interval (POI; ≤22 and >22 days) and the postpartum interovulatory interval (PPIOI), and in the NPP Cycling group during two interovulatory intervals (1st and 2nd IOI). In addition, regardless of group, all four intervals were compared. We performed the FSH and LH assays using radioimmunoassay. Ovarian and hormonal parameters were analysed using ANOVA for sequential data. The day and diameter of the DF at the deviation (overall mean: 14.9 ± 2.5 days; 21.7 ± 0.4 mm, respectively) were not different (P > 0.05) between PP Lactating and NPP Cycling mares. However, when considering the length of POI, follicle deviation occurred 4.4 ± 0.8 days earlier (P < 0.001) in mares with POI ≤ 22 days than in mares with POI > 22 days. No difference was found between PP Lactating and NPP Cycling mares within and between groups for the intervals from deviation to maximum diameter of the DF (10.4 ± 0.4 days), for the intervals from deviation to ovulation (12.1 ± 0.5 days), or for the growth rates of the DF from deviation to maximum diameter (2.6 ± 0.1 mm per day). The growth rate of the DF from deviation to ovulation (2.4 ± 0.1 mm per day) did not differ between PP Lactating and NNP Cycling mares; however, this growth rate was lower (P < 0.03) in the POI, PPIOI, and 1st IOI compared with the 2nd IOI. The mean diameter of the DF around deviation (days −3 to 3; 22.5 ± 0.3 mm), and systemic FSH (days −4 to 4; 10.3 ± 0.2 ng mL⁻¹) were not different between PP Lactating and NPP Cycling mares. Level of LH was lower (P < 0.0001) around deviation (days −4 to 4) in the PP Lactating (0.7 ± 0.0 ng mL⁻¹) versus the NPP Cycling mares (1.8 ± 0.1 ng mL⁻¹). Results demonstrated that a partum effect occurs only on the day of deviation in mares during the foal heat (POI ≤ 22 days) and that low levels of LH during both intervals in PP Lactating mares were not detrimental to prevent ovulation.