The effects of progesterone and oestradiol-17beta treatment on plasma hormone levels and on the reproductive behaviour of ewes in late anoestrus and early in the breeding season.

Ewes in late anoestrus or early in the breeding season were treated with a subcutaneous progesterone implant in combination with injections of oestradiol-17β. The effects on plasma hormone levels and reproductive behaviour were studied. In non-cyclic ewes removal of the implant was followed by a surge of plasma gonadotrophins at 20-48 h. The time of day at which the implant was removed had a significant effect on the interval to this surge. The interval was shorter (7–20 h) in ewes injected with oestradiol at the time of implant removal. Cyclical activity was induced in 21 of 23 non-cyclic ewes by the treatments and, of these, 15 showed oestrous behaviour during the four days following implant removal. Only one ewe conceived at this first induced oestrus, whereas conception rate was 82 per cent for the 17 ewes showing behavioural oestrus at 15–21 days. In cyclic ewes, removal of the progesterone implants had little effect on their oestrous cycles and there was no plasma gonadotrophin response 7–20 h after oestradiol administration. Nevertheless, injection of oestradiol at the time of implant removal resulted in some degree of oestrus synchronisation in cyclic ewes.