Progesterone priming and age of ewe affect the life-span of corpora lutea induced in the seasonally anovulatory Merino ewe by the 'ram effect'.

Increasing doses of progesterone from 0 to 20 mg, given as a single i.m. injection, increased the proportion of corpora lutea that had a normal life-span when induced in ewes by the introduction of testosterone-treated wethers from 54% (19/35) to 100% (34/34). Injection of progesterone did not affect the induction of ovulation and 95% (130/136) of the anovulatory ewes ovulated. Nevertheless, a low proportion of ewes displayed oestrus between Days 16 and 26 after the introduction of testosterone-treated wethers (Exp. 1, 47%, 92/196; Exp. 2, 50%, 502/1000). Many of the ewes that did not display oestrus also failed to ovulate again (Exp. 1, 70%, 37/53). The proportions of anovulatory adult and maiden ewes that ovulated after the introduction of testosterone-treated wethers were not significantly different but significantly fewer maiden ewes were detected in oestrus.