The effect of stilboestrol administered during an Autumn mating on reproduction in Merino sheep

Abstract

Ewes were injected subcutaneously with stilboestrol for 3 weeks prior to and during an autumn mating.

Doses of 32 and 128 µg daily during mating almost completely prevented pregnancy. The conception percentage during treatment in a group receiving 8 µg was 58, whereas 75% is expected in untreated ewes. This and other evidence indicated that doses as low as 8 µg have adverse effects on fertility.

No effect of 8 µg was observed on oestrus, but 32 and 128 µg initially stimulated but later inhibited oestrus. The higher doses had little effect on the average length of oestrous cycles, but increased the variance by causing abnormally long or short cycles. Normal oestrous cycles did not recur in many ewes within 9 weeks of the last injection. However, fertility, the ratio of pregnancies to services, appeared somewhat recovered within about 3 weeks after the last injection.

These results indicate that the fertility of Merino ewes is very vulnerable to adverse effects of oestrogen administered during an autumn mating. The significance of this is discussed in relation to possible permanent and temporary oestrogenic effects of subterranean clover pastures, and the importance of losses which may be caused by ingestion of oestrogens during mating.