Birth weight and survival of Merino, Corriedale, and Polwarth lambs

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Abstract
The effects of year of birth, age of dam, sex, and litter size upon birth weight of lambs were examined by least squares analysis in eight flocks in Western Victoria (4 Merino, 3 Corriedale, and 1 Polwarth). Male lambs were approximately 0.8 lb heavier (P<0.01) than female lambs, single lambs about 1.6 lb heavier (P<0.01) than twin lambs. Lambs, born of 5-year-old ewes, were generally heavier (P<0.01) at birth than lambs from ewes of other ages. A general set of adjustments of birth weight for these sources of variation are presented for each breed. The effects of the same sources of variation on survival of lambs to weaning were calculated by least squares analysis. Regressions of survival on birth weight (BW), BW2 and BW3 were included in the analysis. Male lambs did not survive as well as female lambs and a higher proportion of twin lambs died than single lambs. Lambs from primiparous ewes generally did not survive as well as those from multiparous ewes. Maximum survival occurred at birth weights of about 10 lb (Merino), 11 lb (Corriedale), 8 lb (Polwarth).

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