Ovarian and endocrine responses to prostaglandin F2α (PGF2α) given at the expected time of the endogenous FSH peak at mid-cycle in ewes

In the ewe, a rise in circulating concentrations of FSH preceding follicular wave emergence begins in the presence of growing follicles from a previous wave. We hypothesized that prostaglandin F2α (PGF2α) given at the time of an endogenous FSH peak in cyclic ewes would result in synchronous ovulation of follicles from two consecutive waves, increasing ovulation rate. Twelve Western White Face (WWF) ewes received a single i.m. injection of PGF2α (15 mg/ewe) at the expected time of a peak in FSH secretion, from Days 9 to 12 after ovulation. The mean ovulation rate after PGF2α treatment (2.3 ± 0.3) did not differ (P > 0.05) from the pre-treatment ovulation rate (1.7 ± 0.1). Five ewes ovulated follicles from follicular waves emerging before and after PGF2α injection (3.0 ± 0.6 ovulations/ewe) and seven ewes ovulated follicles only from a wave(s) emerging before PGF2α treatment (2.0 ± 0.3 ovulations/ewe; P > 0.05). The mean interval from PGF2α to emergence of the next follicular wave (1.0 ± 0.4 and 4.0 ± 0.0 d, respectively; P < 0.001) and the interval from PGF2α treatment to the next FSH peak (0 and 3.5 ± 0.4 d, respectively; P < 0.05) differed between the two groups. Six ewes ovulated after the onset of behavioral estrus, with a mean ovulation rate of 1.7 ± 0.2, and six ewes ovulated both before and after the onset of estrus (3.0 ± 0.5 ovulations/ewe; P < 0.05). None of the ovulations that occurred before estrus resulted in corpora lutea (CL) with a full life span. At 24 h before ovulation, follicles ovulating before or after the onset of estrus differed in size (4.1 ± 0.3 or 5.5 ± 0.4 mm, respectively; P < 0.05) and had distinctive echotextural characteristics. In conclusion, the administration of PGF2α at the expected time of an FSH peak at mid-cycle in ewes may alter the endogenous rhythm of FSH secretion and was not consistently followed by ovulation of follicles from two follicular waves. In non-prolific WWF ewes, PGF2α-induced luteolysis disrupted the normal distribution of the source of ovulatory follicles and may be associated with untimely follicular rupture and luteal inadequacy.