Variations in the fertility over the joining period of ewes on hill country

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Abstract
The cumulative data from 9 flocks of sheep over the period 1967 to 1973, representing 2427 2-year-old and 5443 3- to 5-year-old (mature) ewe-matings, were analysed for variation in reproductive performance between ewe ages and among those mated by entire rams in each of 4 successive 14-day periods. Joining with the entire rams began on the 15 to 20 March and lasted 8 weeks. The percentage lambs weaned per ewes present at lambing was 71 % for 2-year-olds and 93% for mature ewes. This difference was mainly associated with the lower multiple birth rate of 2-year-olds (15% v. 31%). Only 37% of 2-year-olds and 61 % of mature ewes were mated in the first period of joining, and the conception rate for ewes mated in this period was respectively 12% and 6% lower than for those mated in the second period. The percentage of ewes lambing to mating in a single period was at a maximum (62%) for all ewes mated in period 2 and declined for those mated in periods 3 and 4. The multiple birth rate was a maximum of 21 % for the 2-year-olds conceiving in period 1 and declined to 10% in period 4. In mature ewes the multiple birth rate was 33% for those conceiving in periods 1 and 2, and 23% in periods 3 and 4.