An Explanation for the Reduced Fertility in Merino Ewes at the First Oestrus of the Breeding Season

Laproscopy was used in 400 ewes from 3 commercial flocks to study the difference in fertility of ewes mated in the first vs. second 2 weeks of the joining period. Examination of the ovaries of the ewes at joining and 14 days after joining revealed that 23% of the ewes did not experience a 'quiet ovulation' at the beginning of their breeding season but ovulated and displayed oestrus simultaneously both for the first time after the introduction of rams. The percentage of these ewes failing to lamb was significantly higher than for the remaining ewes mated in the first 2 weeks of joining (66% vs. 38%; P < O.Ol>, or for those ewes experiencing a 'quiet ovulation' in the first 2 weeks and a normal ovulation plus oestrus in the second 2 weeks of joining (66% vs. 36%; P < 0.01). The proportion of ewes of this type, mated in the first 2 weeks of joining was sufficient to cause an 8'/o difference in ewes failing to lamb between ewes mated for the first time in the first vs. second 2 weeks of joining. The low fertility of these ewes strongly suggests that a prior progestational phase is necessary for reasonable fertility at the beginning of the breeding season.