Superovulatory response in anestrous ewes is affected by the presence of a large follicle.

Superovulatory response to conventional treatment with eCG (1200 IU) and progestagen sponges (MAP, n = 9; FGA, n = 9; or controls without sponge, n = 6) was studied in Corriedale anestrous ewes. The follicular population just before the administration of eCG and the total ovarian response (large anovulatory follicles plus normal CL and prematurely regressing CL) to treatment were determined after laparotomy. Pretreatment with progestagen did not modify the number or class of follicles greater than 1 mm observed on the ovarian surface at the time of eCG administration (19 +/- 2.2 follicles vs 19 +/- 2.9 follicles, for pooled progestagen-treated groups and control groups, respectively; mean +/- SEM) but significantly decreased the number of large anovulatory follicles (4.7 +/- 1.0 vs 10.2 +/- 2.6; P < or = 0.01) observed following treatment. Progestagen-treated animals were classified according to the presence (n = 13) or absence (n = 5) of a large follicle (LF: > or = 4 mm diameter) on the ovarian surface at the time of eCG treatment; a qualitatively better superovulatory response was observed in ewes without large follicle (large anovulatory follicles: 1.6 +/- 0.7 vs 5.8 +/- 1.3, P < or = 0.05; normal CL: 7.0 +/- 1.4 vs 3.8 +/- 1.0, P < or = 0.1; normal CL/total ovarian response: 78.7 +/- 10.1 % vs 34.9 +/- 8.2 %, P < or = 0.01; for ewes without LF and ewes with 1 to 2 LF respectively). No differences were observed in the individual ovulatory response when comparing ovaries ipsilateral or contralateral to LF in a same animal, indicating that the effect of LF on the superovulatory response would be fundamentally systemic. This work shows that, similar to what occurs in cows, the presence of a large follicle at the time of gonadotropin administration decreases the superovulatory response in anestrous ewes.