Relationship between circulating concentrations of ovarian steroids and the superovulatory responses in anestrous ewes following a multiple-dose pFSH regimen

It has been suggested that elevated concentrations of ovarian steroids during superovulatory treatment may be a cause of poor superovulatory yields in ewes. The aim of this study was to examine the correlations between daily serum concentrations of estradiol-17β (E2-17β) and progesterone concentrations, ovarian responses and in vivo embryo production in anestrous ewes superovulated in a multiple-dose pFSH regimen. Twelve clinically healthy Rideau Arcott ewes were used in the study (May–June). Medroxyprogesterone acetate (MAP)-releasing intravaginal sponges (60 mg) were inserted for 14 days, and 12 days after insertion of the sponges, 6 im injections of pFSH (Folltropin®-V) were administered (1 × 2.5 ml and 5 × 1.25 ml) at 12-h intervals. Injections were administered at 20:00 and 08:00 h, beginning on Day 12 and finishing on Day 15 following sponge insertion. At the first pFSH treatment, all ewes received a single im injection of eCG (500 IU). Sponges were removed at the time of the 5th pFSH injection, and on the evening of Day 15 all ewes received an im injection of GnRH (50 μg). Ewes were then placed in an enclosed paddock with 2 adult Rideau Arcott rams for a 36-h period and embryos were recovered surgically 7 days after the GnRH injection. Daily blood samples (14:00 h) were drawn for the 10-day period from the onset of ovarian superstimulation. Serum concentrations of E2-17β on the day of the 1st pFSH dose were negatively correlated to the numbers of embryo (Grades 1–3) (r = −0.62; P < 0.05) and embryo viability rate (r = −0.84, P < 0.01). The total number of luteal structures was positively correlated to serum E2-17β concentrations on the 2nd and 3rd day after the onset of the superovulatory treatment (both r = 0.59, P < 0.05). There were no significant correlations between serum progesterone concentrations, from 1 to 7 days after the GnRH injection, and superovulatory responses. These findings suggest that serum E2-17β concentrations may be more critical during follicular development than early embryonic development for determining embryo yield and viability, and may be responsible for some of the variation experienced in superovulatory regimens in ewes.