Two hundred and forty mature Merino ewes were used in a factorial experiment to investigate the effects of single injections of either 0, 40, 80 or 160 mg progesterone on the length of the subsequent oestrous cycle. Equal numbers of ewes (15 per group) received subcutaneous and intramuscular injections, and equal numbers were treated either on the day of oestrus or the following day.

Oestrous cycle length was significantly reduced in all treated groups, with a minimum cycle length of 7–10 days following treatment with 80 or 160 mg progesterone. Neither route of administration nor day of treatment significantly affected the response. The conception rate achieved by treated ewes at the first post-treatment oestrus did not differ significantly from that in comparable controls.

The results suggest that a single massive dose of progesterone on the day of oestrus is an effective means of shortening the oestrous cycle of the ewe without loss of fertility.