Reproductive season affects inhibitory effects from large follicles on the response to superovulatory FSH treatments in ewes

The main objective of this study was to compare the effect of the presence of large follicles at the start of FSH treatment on the superovulatory response in ewes in the breeding and nonbreeding seasons. A second objective was to verify the effect on the superovulatory response of the presence of a corpus luteum at the start of the FSH treatment during the breeding season. Fifteen ewes in breeding season (October) and 14 in nonbreeding season (May-June) were treated with 40 mg FGA sponges (Chronogest®) for 14 days, together with a single dose of 125 μg cloprostenol on Day 12, considering Day 0 as day of progestagen insertion. Superovulatory treatments consisted of eight decreasing doses (1.5 ml x 3, 1.25ml x 2 and 1ml x 3) of Ovagen™ twice daily from 60 h before to 24 h after sponge removal. Ovarian structures were assessed by transrectal ultrasonography using a 7.5 MHz linear array probe. Luteal activity at progestagen insertion (Day 0) and presence of corpus luteum and of large follicles at first FSH dose (Day 12) were determined. There were no significant differences between the breeding season and nonbreeding season for ovulation rate (11.6 ± 1.4 versus 11.6 ± 1.3), number of recovered embryos (8.0 ± 1.1 versus 9.6 ± 1.3) or number of viable embryos (7.2 ± 1.1 versus 5.8 ± 1.2). During the breeding season, there were fewer recovered embryos in ewes with a large follicle (≥6 mm) at first FSH dose (6.9 ± 1.1 versus 12.3 ± 1.8, P < 0.05) and fewer viable embryos (5.0 ± 1.2 versus 10.5 ± 0.5, P < 0.05) than in ewes without such a follicle. During the nonbreeding season, however, there were no significant differences between ewes with or without a large follicle for either recovered (9.0 ± 2.5 versus 11.3 ± 1.2) or viable embryos (6.3 ± 2.3 versus 8.1 ± 1.2). Analysis of seasonal differences in ewes with a large follicle showed a lower number of recovered embryos in the breeding season (P < 0.05) due to a lower recovery rate (65.7% versus 92.3%, P < 0.05), since mean number of corpora lutea in response to the FSH treatment was similar (10.9 ± 1.3 versus 10.0 ± 2.5). These results indicate that, in sheep, the inhibitory effects of large follicles during the nonbreeding season are not as obvious as during the breeding season. © 2003 Elsevier Science Inc. All rights reserved.