Genetic selection for cold resistance in Scottish Blackface lambs

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Abstract
Hypothermia with starvation is a major cause of neonatal mortality among lambs born outdoors. The results of selection for resistance to hypothermia (cold resistance) in newborn Scottish Blackface lambs are described. Cold resistance was defined as the time required to induce a decline in body temperature of about 4·5°C by means of tests involving part-immersion in a cooled water bath. From about 200 tested lambs (half males) four ram lambs with the lowest cold resistance and four with the highest resistance were selected and each mated randomly at 7 months of age to about 30 ewes to establish upward and downward selection lines. The progeny were subsequently tested for cold resistance and the selection process was repeated for four male generations. Tested females were allocated at 18 months of age to high or low lines according to their performance as lambs.Preliminary half-sib analysis and sire-offspring regression gave estimates of 0·3 for the heritability of cold resistance. Response to selection was rapid but asymmetrical with a realized heritability for cold resistance of 0·27 (s.e. 0·13) for upward selection, 0·01 (s.e. 0·16) for downward selection and 0·17 (s.e. 0·09) for the line divergence.Body weight at test was phenotypically, but not genetically, correlated with cold resistance. Rate of recovery from induced hypothermia was not significantly correlated with cold resistance. The results are discussed in relation to lamb mortality in the field.