The preferred mating period in Greece is spring and early summer. Conception rate to natural mating at the end of one oestrus period or early in the next is low, leading to a lengthy lambing period. Some 10% of flocks are synchronised during this period by a standard progestagen/PMSG treatment. There is considerable variation in ovulation rate, conception rate and litter size which could be related to within-flock variation in oestrous activity.

Fourteen adult ewes from each of three breeds -Chios, Karagouniko and Serres - were maintained as a single flock at Diavata (latitude 40.5°N), without mating, from the end of the milking period in September. From the following April, oestrous activity was checked daily by vasectomised ram for 18 months. Live weight was maintained approximately constant at 59, 54 and 54 kg, respectively, for Chios, Karagouniko and Serres. Plasma progesterone was measured 10 days after detected oestrus and after 28 days when there was no intermediate activity. A silent oestrus in the breeding period was counted as oestrus but when it occurred prior to the first or after the last detected oestrus, it was not used to determine the breeding period.