Melengestrol acetate and norgestomet for the induction of synchronized estrus in seasonally anovular ewes.

Two commercially available progestogen products for cattle, melengestrol acetate (MGA) and norgestomet (SMB), were evaluated for their ability to induce synchronized estrus in anovulatory ewes. Seasonally anestrous 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 breeding. Immediately preceding progestogen treatment, RMGA and RSMB ewes were given a single i.m. injection of zeranol. Concomitant to progestogen removal, PMGA and PSMB ewes were given a single i.m. injection of PG-600. On May 4, 1992, all treatment groups were combined into one breeding group for a 30-d breeding period. Mating at synchronized estrus was greater (P < .001) for progestogen-treated ewes. Within progestogen treatments, more (P < .001) SMB ewes than MGA ewes were marked within the first 5 d of breeding. Overall, there were no treatment differences in estrus response for the 30-d breeding period. Progestogen-treated ewes had a shorter (P < .001) mean interval from ram introduction 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 those of ewes receiving an injection of PG-600 after progestogen treatment.(ABSTRACT TRUNCATED AT 250 WORDS)