Abstract
Forty-two ewes were treated with intravaginal insertion of progesterone sponge and single injection of pregnant mare serum gonadotropin (PMSG) either in May (mid-anestrus; Group 1, n=14), in July (late-anestrus; Group 2, n=13) or between October and December (breeding season; Group 3, n=15). The sequence of estrus, LH surge and ovulation, thus induced in each group was compared with that in untreated cycling ewes during the breeding season (Group 4, n=10). Although intervals from the sponge removal to the onset of estrus (22.3-25.5 h) and to the LH peak (23.3-28.5 h) were not different among groups, the duration of estrus in Group 1 (19.3 ± 4.4 h) was shorter (P<0.05) than Groups 3 and 4. The intervals between the onset of estrus and the peak of cervical mucus secretion were also shorter in Groups 1 and 2 as compared with Group 4. The number of ewes ovulated during the period between 40-50 h after sponge removal was less (P<0.05) in Group 1 than Groups 2 and 3 . The conception rates were 62.5, 83.3, 100 and 100 % in Groups 1, 2, 3 and 4, respectively. The present results suggest that synchrony among timing of estrous behavior, LH surge and ovulation following the progesterone and PMSG treatments become less coordinated during anestrous season, which could be one of the primary factors responsible for low conception rate in these animals.
