One hundred and sixty mature Merino ewes were randomly allocated to a factorial experiment designed to investigate the effects of short-term progesterone administration on oestrous cycle length. Groups of ewes received a series of four daily injections of 0, 10, 25 or 40 mg progesterone in peanut oil beginning either on the day of oestrus (day 0) or the next day. Equal numbers of ewes (ten per group) received subcutaneous and intramuscular injections, and those in the control groups received injections of the vehicle only.
Oestrous cycle length was significantly reduced in all progesterone-treated groups, but a consistent trend for this reduction to be greatest in ewes treated first on day 0 gave rise to a significant ‘day x dose’ interaction (P < 0·001). Route of progesterone administration had no significant effect on subsequent cycle length which, in control ewes, averaged 17·2 ± 0·1 days. For ewes first treated on days 0 and 1, mean oestrous cycle lengths for groups receiving 0, 10, 25 and 40 mg of progesterone per day were 17·3, 14·6, 12·1 and 10·7 days and 17·1, 15·2, 13·6 and 13·0 days respectively.