Vital statistics for an experimental flock of Merino sheep. II. The influence of age on reproductive performance

Abstract

Estimates are given for various characteristics associated with reproduction rate, at each age of ewe from 2 to 10 years. In general, reproduction rate rose with increasing age to a peak, with a subsequent fall, the 10-year-old figure, however, being in most cases above the 2-year-old one. The peak occurred at 5–6 years for number of ewes lambing per ewe joined, at 6 years for survival rate of both single and twin lambs and for number of lambs weaned per ewe joined, at 7 years for number of lambs born per ewe joined, and at 7–8 years for number of multiple births per ewe joined. The estimates are used to predict the reproduction rate of flocks with different age structures, the conclusion being that the average number of lambs weaned per ewe joined would increase with the age of the oldest group of ewes even up to 8–9 years. Data from other sources are summarized for comparison. If survival rate can be improved, particularly for lambs from multiple births, then the older ewes will show an even greater advantage. Formulae are given which enable various criteria for reproduction rate, such as lambs weaned per ewe joined, to be broken into components whose relative importance can be assessed.