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

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.