Synchronization of oestrus in sheep by orally active progesterone derivative

Abstract

In a preliminary experiment with 40 Merino ewes of 45 kg liveweight, oral doses of 0, 6.6, 20, and 60 mg 6-methyl-17-acetoxyprogesterone were administered daily for 14 days. The highest dose level completely prevented oestrus during treatment, and all 10 ewes came into oestrus during the 5-day period beginning 24 hr after the last dose; seven of the 10 ewes conceived and lambed. Daily doses of 6.6 and 20 mg reduced the incidence of oestrus during treatment, but produced little synchronization subsequently.

In the main experiment 77 similar ewes were treated over 17 days with either 6-methyl-17-acetoxyprogesterone (50 mg orally per day, 39 ewes), or progesterone (10 mg in oil intramuscularly every second day, 38 ewes). There were 19 untreated ewes. Some ewes from each group were marked by rams during the treatment period. In the 6 days following the final dose of progestagen 75% of ewes were marked by rams, with a peak on the third day. There was no difference between treatments.

The proportion of ewes that lambed following mating in the 6 days after cessation of treatment was 15/29 and 11/29 for 6-methyl-17-acetoxyprogesterone and progesterone treatments respectively.

In the main experiment half the ewes were injected with pregnant mare serum gonadotrophin 24 hr after the last dose of progestagen, and 24 hr later, with human ohorionic gonadotrophin. This supplementary treatment did not further reduce inter-animal variation in time of onset of oestrus, but tended to reduce the proportion of ewes exhibiting oestrus.