The effect of subluteal levels of exogenous progesterone on follicular dynamics and endocrine patterns during the early luteal phase of the ewe

Nineteen Corriedale ewes were treated with an im dose of a PGF2alpha during the luteal phase to synchronize estrus. After ovulation had been detected by using ultrasonography (Day 0); the ewes were randomly assigned to 2 different groups. In 11 ewes a CIDR, which had previously been used for 10 d, was inserted on the fourth day after ovulation. The ewes then received a dose of PGF2alpha on Day 5 to induce luteolysis. The CIDR remained in place until the end of the experiment (Day 9). Control ewes (n=8) received no treatment. Blood samples were taken daily for estradiol, progesterone and FSH determinations. In the untreated ewes, 2 follicular waves were detected in all of the animals throughout the monitoring period, with a mean wave interval of 4.5 d. The total number of follicles which were greater than or equal to 2 mm decreased from Day 0 to Day 4 (8.8 +/- 1.0 to 5.3 +/- 0.6; P less than or equal to 0.05) and then increased at Day 7 (7.5 +/- 0.9; P less than or equal to 0.05). The growth profiles of both the largest and the second largest follicles of Wave 1 showed significant divergence, while no divergence was observed in Wave 2. Serum estradiol concentrations decreased significantly from the day before to the day of ovulation and then increased again during the growing phase of the largest follicle of Wave 1. Concentrations of FSH were high on the day of emergence of both waves, but while a significant decline was observed after emergence in Wave 1, the levels remained high in Wave 2. In 8 of the 11 treated ewes, the largest follicle of Wave 1 was still present on the ninth day after ovulation (persistent follicle). In the other 3 ewes, the largest follicle of Wave 1 was already regressing on the day that the treatment was administered, and the largest follicle that was present on Day 9 originated from Wave 2 (nonpersistent follicle). In persistent follicle ewes, the largest follicle of Wave 1 prolonged its lifespan significantly, attaining the maximum diameter (Day 8.1 +/- 0.8) later than in untreated (Day 3.0 +/- 0.4) and nonpersisted follicle ewes (Day 2.0 +/- 0.6). The total number of follicles decreased in persistent follicle ewes between Day 0 and Day 4 (7.9 +/- 1.5 to 4.5 +/- 0.5, respectively; P less than or equal to 0.05) and remained low until the end of the experiment. Progesterone concentrations (nmol/L) between Days 6 and 9 were significantly different between untreated and persistent follicle ewes (12.8 +/- 1.0 vs 9.4 +/- 1.0, P less than or equal to 0.02). The present study confirms that the largest follicle of Wave 1 is dominant in the ewe and that subluteal progesterone concentrations can prolong its lifespan and extend this dominance.