Puberty in the Merino ewe with special reference to the influence of season of birth upon its occurrence

Abstract

Observations were made on the occurrence of mating and lambing in Merino ewes of three groups born in the spring, summer, and winter respectively. The lambs ware weaned at 13½–14½ weeks of age, and joined with rams within the following 2 weeks, some with fertile rams, others with vasectomized rams. Thereafter they ran with rams continuously or until lambing occurred. With few exceptions the first oestrus occurred between late November and May (summer and autumn), and the first conception between November and June. Of the ewes born in the spring, summer, and winter, 78, 100, and 95% respectively exhibited oestrus, and 63, 95, and 80% respectively conceived during the first spring, summer, and autumn following birth. All of the remainder experienced oestrus and all except one conceived during the same period in the following year. Both the age and the body weight of the ewes when oestrus and when conception first occurred varied widely. Both were least in the animals born in the spring and greatest in the animals born in the summer. The sexual season was shorter in the young ewes than it was in mature ewes. Further, the first sexual season of the youngest ewes at the time (those born in the spring) was only one-third of the length of the second sexual season or of the first sexual season of the ewes of the other two groups. There was little delay between the first opportunity to conceive and conception. However, among the ewes which mated youngest the proportion which lambed was lower than that in the other young ewes, which in turn was lower than in mature ewes. In addition, the gestation period in at least half of the ewes which lambed 3 youngest was substantially longer than it was in all except a very few of the other ewes