Ultrasound and endocrine evaluation of the ovarian response to PGF2α given at different stages of the luteal phase in ewes

The purpose of this study was to evaluate the ovarian response of ewes to two treatments with PGF(2alpha), using transrectal ovarian ultrasonography and hormone measurements. Fifteen milligrams of PGF(2alpha) was given to six cyclic Western White Face (WWF) ewes early in the estrous cycle (Days 4 to 7) and to six late in the cycle (Days 10 to 12 after ovulation), and a second treatment was given 9 days after the first. Ultrasound scanning and blood sampling started 7 days prior to the first PGF(2alpha) treatment and ended 10 days (scanning) or 19 days (blood sampling) after the second PGF(2alpha) treatment, for both groups of ewes. Mean ovulation rate (2.6 +/- 0.7) did not differ significantly between the ewes first treated early or late in the cycle, or after the first or second treatments with PGF(2alpha). The time from treatment to ovulation was longer in ewes first treated early (4.0 +/- 0.3 days) compared to late (2.8 +/- 0.4 days) in the cycle (P < 0.05). Both the number of ovulations (range: 0-7) and time from treatment to ovulation (range: 1-9 days) were highly variable. This variability appeared to be due to the extension of the life span of ovulating follicles that emerged prior to PGF(2α) administration and also ovulation of some follicles that emerged after treatment. When results for first and second treatments were pooled, the total number of follicles ≥5 mm. in diameter on the day of treatment that failed to ovulate in response to PGF(2α) was higher in ewes first treated early (0.8 +/- 0.2/ewe) compared to late (0.3 +/- 0.2/ewe) in the cycle (P < 0.05). The proportion of detected luteal structures relative to the number of ovulations was lower in ewes first treated early compared to late in the cycle (60 and 86%, respectively; P < 0.05). Disruption of ovulatory follicle dynamics and normal luteogenesis, and variability in the timing of ovulation after PGF(2α), treatments could all contribute to poor or variable fertility when prostaglandins are used for estrus synchronization. (C) 2002 Elsevier Science Inc. All rights reserved.