Studies on seasonal variation in the level of fertility in Merino ewes. I. Observations on mating, pregnancy, and lambing at intervals of four months.

Abstract

A flock of Merino ewes was divided at random into three groups which were mated to the one lot of Merino rams for periods of two months at intervals of four months. The mating periods were April to June, August to October, and December to February. Observations were made on the occurrence and distribution of mating throughout each mating period, on attainment of advanced pregnancy, and on lambs born. There was a high incidence of mating from the start of the first and second mating periods, but a high incidence of mating did not occur until the third week of the third period. Most of the ewes mated on one occasion only during the first and third periods and most of them eventually lambed. Many ewes returned to service on one or two occasions during the second period and many failed to bear lambs. A high proportion of the pregnancies from the first mating period and a moderate proportion of those from the second were multiple, but all pregnancies from the third mating period were single. As measured by the ratio of lambs born alive to ewes mated, the first mating was accompanied by the highest level of fertility and the second by the lowest.