Sources of variation in the weaning weight of Romney Marsh lambs

The effects on weaning weight of type of birth and rearing, age of dam, year of birth, sex, and age at weaning were studied on 2,655 records from one flock and 649 records from another flock of Romney Marsh sheep. In the first flock, differences in weaning weight between wether lambs and ewe lambs, lambs from five‐year‐old and two‐year‐old dams, and single and twin lambs were, respectively, 3.0 lb, 4.8 lb, and 10.2 lb. The regression on age at weaning was 0.28 lb per day. Effects of the same order were found in the second flock. The differences due to type of birth and rearing were found to be the most important source of variation. The heritability of weaning weight calculated by doubling the regression of offspring on dam was 0.35.