Variation in Fleece Weight, Weaning Weight and Birth Weight of Sheep under Farm Conditions1

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Abstract
A study was made of the variation in fleece weight, weaning weight and birth weight of four breeds of sheep maintained under farm flock conditions. The breeds studied were Corriedale, Hampshire, Shropshire, and Dorset. The data consisted of 1965 fleece weight records from 660 ewes, 1295 weaning weight records of lambs from 463 ewes, and 2158 birth weight records of lambs from 560 ewes. The data were analyzed to obtain estimates of the effects of certain environmental factors, and the repeatability and heritability of these traits. The findings were generally in agreement with similar studies of sheep maintained under range conditions. Age differences among ewes and differences associated with years were the major sources of tangible environmental variation in fleece weight. Differences associated with years, age of dam, age of lamb, sex, and type of birth and rearing were important sources of variation in weaning weights in the Corriedale, Hampshire and Shropshire data. Year of birth, age of dam, sex, and type of birth affected birth weights significantly in all breeds. In the Dorset weaning weight and birth weight data the season of birth was an additional source of variation. Age of dam was found to have no significant effect on weaning weight of Dorset lambs. Breed differences were large for all traits studied. Repeatability estimates obtained from the adjusted data were 0.608, 0.078, and 0.119 for fleece weight, total weight of lambs weaned, and total weight of lambs born, respectively. The heritabilities of fleece weight, individual weaning weight, and individual birth weight were estimated as well as the heritability of total weight of lambs weaned. The values obtained were 0.415, 0.073, 0.334, and 0.194, respectively.