Wastage of ova in young Merino ewes.

Eight sub-flocks each of 48 to 51 Merino ewes were mated for 18 days with 1 ram to each subflock. Four of the sub-flocks were of non-parous 1.5-year-old ewes and the other 4 were of parous 2.5-year-old ewes. Fertilisation rates for the subflocks of 1.5-year-old ewes varied from 83 to 94%, and the sub-flocks of 2.5-year-old ewes ranged from 81 to 94%. By day 29 post coitum 42% and 56% of 1.5-year-old ewes in sub-flocks 1 and 2 respectively were no longer pregnant. Embryonic mortality was low in other sub-flocks of 1.5- and 2.5-year-old ewes. Virtually all embryonic death occurred after day 12 post coitum. The sporadic occurrence of high ova wastage in maiden 1.5-year-old ewes in this experiment and in others is discussed in relation to the short duration of oestrus in young ewes and the great variation in service activity of rams.