Early effects of equine FSH (eFSH) treatment on hormonal and reproductive parameters in mares intended to carry their own pregnancy

Superovulatory treatment may potentially increase the embryo recovery rate and the per-cycle pregnancy rate in normal or subfertile mares that are managed properly. However, some studies suggest a possible negative effect of superovulatory treatment on ovarian follicular maturation and embryo viability. Objectives of the present study were to investigate the early effects of eFSH treatment in reproductively normal mares in terms of: folliculogenesis, pregnancy rate, early embryonic development, reproductive tract parameters (tone and edema), and serum estradiol-17β and progesterone concentrations. Reproductively sound mares (n = 26) were evaluated daily by transrectal palpation and ultrasonography. Five days after spontaneous ovulation, mares were randomly assigned to one of two treatment groups. In the eFSH group, mares (n = 16 estrous cycles) were administered eFSH twice daily; beginning when a follicle ≥20 mm was detected, and continuing until at least one follicle reached a diameter of ≥35 mm. PGF2α was administered 2 days following initiation of eFSH therapy, and hCG was administered approximately 36 h after cessation of eFSH therapy. In the control group, mares (n = 26 estrous cycles) were administered PGF2α 7 days after spontaneous ovulation, and hCG when a follicle ≥35 mm was detected. All mares were bred with fresh semen, monitored for ovulation (Day 0), and evaluated for pregnancy on Days 11–16. Serum estradiol-17β and progesterone concentrations were analyzed using radioimmunoassay on the Day of hCG administration, and Days 8, 11 and 16. Mares treated with eFSH had more follicles ≥30 mm at the time of hCG administration (2.6 ± 0.4 compared with 1.1 ± 0.1; P < 0.01), and more ovulations (2.3 ± 0.5 compared with 1.1 ± 0.3; P < 0.01). However, pregnancy rates were not significantly different between groups (50%; 8/16 compared with 62%; 16/26). Mean overall daily growth rate of embryonic vesicles from Day 11 to 16 was not statistically different between the two groups (3.3 ± 0.3 compared with 3.7 ± 0.1 mm/day) (P = 0.2); however, was more variable (P < 0.01) in the eFSH group (95%CI: 2.6–3.8 mm/day) than in the control group (95%CI: 3.5–3.9 mm/day). Administration of eFSH modified the reproductive tract variables and serum concentrations of progesterone and estradiol-17β on the days that oocyte maturation, fertilization, and early embryonic development are expected to occur. These alterations may be related to the greater incidence of non-ovulatory follicles (25% compared with 0%), fewer embryos per ovulation rate (0.3 ± 0.1 compared with 0.6 ± 0.1), and the lesser than expected pregnancy rates in the eFSH-treated mares.