An investigation of conventional oestrous synchronization techniques in the ewe
Barrett, David M. W.
Studies reported in this thesis were conducted to examine endocrine factors, follicle dynamics, ovulation rate, and subsequent luteogenesis in response to the conventional oestrous/ovulation synchronization protocol of two injections of prostaglandin F₂α (PGF₂α) in cyclic sheep and the effects of pregnant mare serum gonadotrophin (PMSG) given after treatment with a progestogen sponge in sheep in and out of the breeding season. Cyclic ewes given two injections of PGF₂α nine days apart, with the first injection given early or late in the oestrous cycle. The time from injection to ovulation was longer in ewes first injected early compared to late in the cycle. Both the number of ovulations (range: 0 to 7) and time from injection to ovulation (range: 1 to 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 PGF₂α and also ovulation of some follicles that emerged after treatment. When results for first and second injections were pooled, the total number of follicles ≥5 mm in diameter on the day of treatment that failed to ovulate in response to PGF₂α was higher in ewes first injected early compared to late in the cycle. The proportion of detected luteal structures relative to the number of ovulations was lower in ewes first injected early compared to late in the cycle (60 and 86%, respectively). Disruption of ovulatory follicle dynamics and normal luteogenesis, and variability in the timing of ovulation after PGF₂α injections could all contribute to poor or variable fertility when prostaglandins are used for oestrous synchronization. Five hundred IU of PMSG is commonly given to anoestrous ewes, at the end of a twelve- to fourteen-day treatment with progestogen impregnated intravaginal sponges, to induce synchronous oestrus and ovulation. If PMSG is omitted very few ewes ovulate. During the breeding season, this progestogen/PMSG treatment has also been used for oestrous synchronization; however, there is probably little advantage to its use in terms of fertility. The exact way in which PMSG affects ovarian antral follicle dynamics to enhance fertility in anoestrous ewes but not in ewes in the breeding season is unclear. PMSG treatment did not result in any changes in follicle numbers, size, or most growth characteristics of all follicles ≥4 mm in size. The only exception was that the length of the growth phase of follicles ≥4 mm in diameter detected on the day of sponge removal was longer in control than PMSG treated anoestrous ewes. Of the PMSG treated and cycling control ewes, eight ovulated follicles from the last two, five only from the last, and five ewes only from the penultimate follicle waves emerging before sponge removal. After sponge removal, serum progesterone concentrations were lower in control ewes compared to PMSG treated ewes out of season. When daily serum oestradiol concentrations were normalized to the day of ovulation, and analysed for the period after sponge removal in anoestrous ewes, there was a significant interaction between group and day of experiment (P