Seasonal variation in fertility in Merino ewes : the reproductive wastage associated with mating in winter, spring, summer, or autumn

Abstract

Merino ewes of four groups were joined for mating in winter, spring, summer, or autumn. Each group remained with the rams for 8 weeks. Some of the ewes were examined at autopsy 4 weeks after mating, others radiographically 12–18 weeks after mating, and others were allowed to lamb without interference.

Some ewes failed to mate in winter and many in spring, but virtually all mated in summer and autumn. In spring, but not in other seasons, some rams served more ewes than others. In winter, but again not in other seasons, the incidence of mating was related to the physical condition of the ewes. Absence of mating, both in winter and spring, was due to ovarian inactivity.

There was significant variation between rams in returns to service and in foetal loss 4 weeks after mating. Where fertility of rams was not implicated there was no significant seasonal variation in foetal loss at 4 weeks; this averaged 13% and there was little foetal loss thereafter.

Lamb losses were high from ewes mated in summer or autumn but low from those mated in winter or spring.