The effect of season and technique on synchronized and induced estrus and the induction of lambing in the ewe in a commercial setting

At 40 day intervals, groups of 87 to 142 commercial ewes of mixed breeding were subjected to a 5 day breeding period, following one of three estrus inducing or synchronizing treatments (intravaginal sponges containing 60 mg of medroxyprogesterone acetate) for 12 days with (progestin P.M.S.G. group) or without pregnant mares serum gonadotropin (P.M.S.G.) (Progestin group) at sponge removal, or a single injection of prostaglandin F(2alpha) (breeding season and early anestrus only) (PGF(2alpha) group)). Mean pregnancy rates (ewes lambing of those treated) and lambing percentages (lambs born per 100 ewes lambing) were 31 +/- 4%, 169 +/- 6%, 20 +/- 5%, 105 +/- 2% and 18 +/- 8%, 118 +/- 3%, respectively, for the three treatment groups above. Pregnancy rates for the progestin, P.M.S.G. group were 52% in late July, prior to the breeding season, 52% in September and declined to 14% in March (early anestrus). The time from the first of two daily injections of estradiol benzoate to lambing was 34.5 +/- 4.4 hours in ewes 142 to 146 days pregnant.