MELENGESTROL ACETATE AND NORGESTOMET FOR THE INDUCTION OF SYNCHRONIZED ESTRUS IN SEASONALLY ANOVULAR EWES

Two commercially available progestogen products for cattle, melengestr ol acetate (MGA) and norgestomet (SMB), were evaluated for their abili ty to induce synchronized estrus in anovulatory ewes. Seasonally anest rous ewes (n = 232) were randomly assigned to one of seven treatments: 1) control (C); 2) MGA only (OMGA;.3 mg of MGA/d); 3) MGA + zeranol ( RMGA; 2.5 mg of zeranol); 4) MGA + PG-600 (PMGA; 400 IU of PMSG + 200 IU of hCG in a 5-mL dose); 5) SMB only (OSMB); 6) SMB + zeranol (RSMB) ; and 7) SMB + PG-600 (PSMB). Treatments were initiated 10 d before br eeding. Immediately preceding progestogen treatment, RMGA and RSMB ewe s were given a single i.m. injection of zeranol. Concomitant to proges togen removal, PMGA and PSMB ewes were given a single i.m. injection o f PG-600. On May 4, 1992, all treatment groups were combined into one breeding group for a 30-d breeding period. Mating at synchronized estr us was greater (P < .001) for progestogen-treated ewes. Within progest ogen treatments, more (P < .001) SMB ewes than MGA ewes were marked wi thin the first 5 d of breeding. Overall, there were no treatment diffe rences in estrus response for the 30-d breeding period. Progestogen-tr eated ewes had a shorter (P < .001) mean interval from ram introductio n to lambing. Fertility and prolificacy were not different for C, MGA, or SMB ewes. Ewes primed with zeranol before MGA or SMB treatment had fertility and intervals from ram introduction to lambing similar to t hose of ewes receiving an injection of PG-600 after progestogen treatm ent. These data indicate that progestogen products commercially availa ble for cattle may be useful in enhancing out-of-season breeding perfo rmance in sheep.