In the mare, the primary corpus luteum (CL) secretes progesterone after ovulation and is responsible for the maintenance of early pregnancy. Its effects wane between days 10 and 30 of gestation. Progesterone concentrations slowly decline until the resurgence of the primary CL, which is stimulated by equine chorionic gonadotrophin (eCG) after day 35 of pregnancy. The luteinising activity of eCG also results in the formation of accessory corpora lutea (CLs) which provide an additional source of progesterone in the early placentation stage of pregnancy. In comparison, oestrogen concentrations during early pregnancy are reported to be low until Day 35 of gestation when concentrations of oestrogen increase along with progesterone. Previous studies of oestradiol-17β and progesterone in mares in early pregnancy do not specify the reproductive or lactational status of the experimental mares. Lactation and parity may be important factors that affect reproductive hormones concentrations and these are investigated here.
Oestradiol-17β and progesterone concentrations were measured in maiden mares (n=7) and lactating, multigravid mares (n=4) from day 23 to day 43 of gestation. While progesterone concentrations were not different between the 2 groups, oestradiol-17β was significantly higher in the lactating, multigravid mares. It could not be determined from this study if the elevated oestradiol were associated with parity or lactation. A second study was undertaken to further investigate these findings. Four groups of mares: maiden mares (n=6), multigravid, non-lactating mares (n=5); primigravid, lactating mares (n=5) and multigravid, lactating mares (n=6) were used for this study. Jugular blood samples were collected each morning from the day of ovulation (GD 0) to Day 50 of gestation (GD 50). Plasma was stored at -20℃ until later assayed of oestradiol-17β and progesterone by radioimmunoassay with a commercial kit and an in-house kit respectively. Lactating mares had significantly higher concentrations of oestradiol-17β than non-lactating mares. There are no significant differences in progesterone concentrations from each group. Parity was not associated with this difference in concentration of oestradiol-17β. It was also found that the concentration of oestradiol-17β decreases in lactating mares relative to the day of parturition.