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

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Virginia Tech


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; determined by blood serum progesterone concentration) of mixed breeding were randomly assigned within broad age groups to one of seven treatments: 1) control (C); 2) MGA only (OMGA); 3) MGA + zeranol (RMGA); 4) MGA + PG-600 (PMGA; 400 IU pregnant mare's serum gonadotropin + 200 IU human chorionic gonadotropin in a 5 mL dose); 5) 5MB only (OSMB); 6) 5MB + zeranol (RSMB); and 7) 5MB + PG-600 (PSMB). Beginning 10 d before breeding, OMGA, RMGA, and PMGA ewes were fed .3 mg MGA/d provided through a mixture of shelled com and a commercially prepared pelleted supplement containing MGA. Concomitantly, OSMB, RSMB, and PSMB ewes were given a 3 mg norgestomet implant inserted subcutaneously on the back of the ear. Immediately preceding initiation of the MGA and 5MB treatments, RMGA and RSMB ewes were given a single i.m. injection of 2.5 mg zeranol. At the end of the 10-d treatment period, MGA feeding was discontinued and the norgestomet implants were removed. Concomitantly, PMGA and PSMB ewes were given a single i.m. injection of PG-600 (5 mL). All treatment groups were combined into one breeding group on May 4, 1992, with a ram to ewe ratio of 1: 17 for a 30-d breeding period. Mating to synchronized estrus was greater (P < .0001) for progestogentreated ewes. Within progestogen treatments, more (P < .000 1) 5MB ewes were marked within the first 5 d of breeding than MGA ewes. Overall, there were no treatment differences in estrus response for the 30-d breeding period. Blood serum samples collected during the first 14 d of breeding were analyzed for progesterone as an indicator of corpora lutea formation. Even though a large proportion of C ewes displayed luteal activity, only 12 % exhibited behavioral estrus within the first 17 d of breeding. Progestogen treated ewes exhibited a shorter mean interval (P < .0001) from ram introduction to lambing. Fertility and prolificacy were not different for C, MGA, or 5MB ewes. Of the two progestogen treatments used alone, lambing rate was 85 and 59 % (P < .03) for OMGA and OSMB ewes, respectively. Ewes plimed with zeranol before MGA or 5MB treatment exhibited similar levels of fertility and intervals from ram introduction to lambing compared with ewes receiving an injection of PG-600 after progestogen treatment. These data indicate that progestogen products commercially available for cattle may be useful in enhancing out-of-season breeding performance in sheep.