Prostaglandin F 2α-induced estrus in ewes exhibiting estrous cycles of different duration

Abstract

Effects of prostaglandin F 2α (PGF 2α), administered during the mid-luteal phase of the estrous cycle, were examined in ewes exhibiting estrous cycles classified as short (≤16.5 days, short-cycle ewes, n=10) or long (≥18 days, long-cycle ewes, n=9) based on the durations of two estrous cycles (cycles −2 and −1) before treatment. The ewes received (i.m.) 20 mg of PGF 2α on day 10 of the third estrous cycle (cycle 0) followed, 36 h later, by 25 μg of gonadotropin releasing hormone (GnRH) to time the events of ovulation. Duration of subsequent estrous cycles +1 and +2 were recorded, and then the ewes were treated with the same combination of PGF 2α and GnRH beginning on day 10 of estrous cycle +3. Ovaries were recovered 6 h after GnRH administration to assess development of pre-ovulatory follicles. The proportion of ewes that exhibited estrus after PGF 2α and GnRH treatment on cycle 0 was not different ( P>0.05) between short- and long-cycle ewes. Onset of estrus occurred sooner ( P<0.05) after PGF 2α injection in short-cycle ewes than in long-cycle ewes (1.9±0.1 days and 2.3±0.1 days, duration of cycle 0 was 11.9 and 12.3 days, respectively). Duration of estrous cycle +1 was 1.2 days longer ( P<0.01) than cycle −1 in short-cycle ewes. However, duration of estrous cycle +1 did not change ( P>0.05) after PGF 2α and GnRH administration in ewes having long cycles. Pre-ovulatory follicles did not differ ( P>0.05) in numbers, diameter, layers of granulosa cells nor concentrations of progesterone and estradiol-17β in follicular fluid between short- and long-cycle ewes after PGF 2α and GnRH treatment. In conclusion, ewes having short or long estrous cycles responded differently to PGF 2α and GnRH treatment with respect to the interval to onset of estrus and duration of the subsequent estrous cycle.