Seasonal effects of PMSG and number of inseminations on fertility of progestogen-treated sheep.

The influence of pregnant mares' serum gonadotropin (PMSG) on reproductive performance of sheep bred by artificial insemination (AI) was studied in the anestrous and estrous seasons. Ewes were treated with progestogen-impregnated intravaginal sponges in June and October. One-half of the ewes in each trial received PMSG at sponge removal and were inseminated 55 h after sponge removal. One-half of the ewes in each treatment group were reinseminated 5 h later. Conception rates in June and October were 82 and 87% with PMSG and 18 and 48% without PMSG, respectively. The corresponding lambing rates were 60 and 74% with PMSG and 10 and 26% without PMSG. Litter size was unaffected by season or PMSG use. Embryonic mortality estimated over both trials was 22% after the first 2 wk of pregnancy with PMSG, but was 44% when PMSG treatment was omitted. Two inseminations were not superior to one. These data indicate that irrespective of season or double insemination, PMSG improves reproductive performance of ewes bred by AI at progestogen-synchronized estrus.