Variation in the incidence of twin ovulation in Merino ewes on a constant plane of nutrition

Abstract

The incidence of twin ovulation was studied in 49 Merino ewes held at constant body weight, excluding wool weight, throughout the year. Two groups were used: one (40 ewes) in which the ewes were kept apart from rams, and the ovaries examined at intervals of 10 or 12 weeks, and the other in which the occurrence of oestrus only was observed. In the former group, there was a statistically significant variation throughout the year in the ratio of twin to single ovulation. The variation fitted a curve which was sinusoidal in form, with a peak in the late autumn and a trough in the spring. There was a similarly timed but less apparent variation in the occurrence of oestrus in the ewes run with rams. It is concluded that the variation in the incidence of twin ovulation was controlled by changes in the physical factors of the environment.