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.
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