Patterns of oestrous cycle activity in the first breeding season were studied in 30 Suffolk × Mule ewe lambs, Twenty-eight reached puberty and the mean date of first oestrus was 27 November (s.d. 13·6 days). Mean live weight at first oestrus was 37·7 (s.d. 3·63) kg. The mean number of oestrous cycles was 3·3 (s.d. 1·6), 0·92 (65) being normal length single cycles (14 to 19 days), 0·03 (2) long single cycles (20 to 26 days) and 0·05 (4) multiple cycles (> 26 days).
Plasma progesterone profiles before and after the breeding season were studied in 10 randomly selected animals. All showed profiles suggesting one or two silent ovulations preceding the first oestrus but none showed any indication of silent ovulations following the last oestrus.
Plasma progesterone profiles were also studied on the four animals showing multiple cycles and on the two animals that did not reach puberty. These suggested that, in all four animals showing multiple cycles, one or two silent ovulations had occurred between the two overt oestrous periods. Of the two animals that did not reach puberty, one showed three abnormal silent cycles and the other showed no change in progesterone concentration from a low basal level.