Purified pig FSH increases the rate of double ovulation in mares

Summary A commercial preparation of porcine FSH (Folltropin) was used to determine whether injections of FSH early in the oestrous cycle, before a large dominant follicle is present, can stimulate multiple ovulations in cycling mares. In the 1990 and 1991 breeding seasons, after injection of 8, 16 or 32 mg FSH twice daily beginning on Day 6 and PGF 2α on Days 5 and 6, the rate of multiple ovulation was increased significantly to 1.83, 1.54, and 1.50, respectively, compared with 1.19 for saline‐injected controls. All multiple ovulations were double ovulations. In 1992, mares were injected with 8 mg FSH twice daily beginning on Day 3, with injection of PGF 2α once daily on Days 3–6 (‘early’ group) and the response compared with that of the same mares to the protocol used in 1990‐91 (‘late’ group). PGF 2α injection caused a rapid decline in plasma progesterone in both groups. FSH did not increase ovulation rate in either group but the rate of double ovulations was increased in the ‘early’ group, for saline‐ and FSH‐treated mares, in comparison with untreated control cycles for the same mares. Ultrasonographic analysis of growth and regression of individual follicles ≥15 mm and ≥25 mm in mares treated during the 1990‐92 breeding seasons revealed no consistent effect of FSH treatment on those follicular populations. These results suggest that treatment with high doses of a commercial preparation of porcine FSH can produce a modest increase in ovulation rate. Although the increase in ovulation rate was small, the response was consistent; no mare ovulated more than 2 follicles and double ovulations were synchronous, occurring on the same or succeeding days in almost all mares.