An analysis has been made of the reproductive performance of a breeding flock of Clun Forest ewes in Northumberland. Data are based upon 941 breeding females and 75 males. The average lambing percentages were 43·9 115·4 and 137·7 for ewes aged 1, 2 and over 2 years. Total lamb mortality to 3 or 6 weeks of age was 23·7% for lambs from 1-year-old dams, 16·6% from 2-year-olds, and 12·9% from older ewes. Mortality for all ages of dams was 12·1 % for single lambs, 15·0% for twins and 34·0% for triplets. Birth weight was strongly associated with survival within all birth types and maternal ages.

Analyses of weights at birth, 21, 42 and 112 days showed that much of the observed variation in lamb weight was associated with age of dam and type of birth and rearing. Single lambs and those from mature dams were significantly the heaviest. The effect of age of dam on lamb weight decreased sharply from birth to weaning, whereas the type of birth and rearing was the major influence at 21 and 42 days, and was still important at 112 days.

The mean weights of all lambs adjusted by least squares analysis to the basis of male twins from 3-year-old dams were 9·2, 19·7, 30·6 and 62·7 Ib. at the four ages. Four- and 5-year-old ewes reared the heaviest lambs. Interactions between the effects studied were significant both within and between years and accounted for from 7 to 10% of the total variation. A single set of general correction factors at each age was shown, on average, to account for 80–90 % of the variation between subclasses within years and may have applicability to comparable flocks.