Resistance to cold stress in the newborn lamb 2. Role of body weight, birth rank, and some birth coat characters as determinants of resistance to cold stress

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Abstract
The effects of body weight and birth rank on resistance to cold stress were examined in Romney-type and Nd-type lambs. Twins exhibited lower coat depths than singles and they also appeared to have a lower summit metabolic rate per kg body weight. These effects were small and the greater rate of body cooling experienced by twin lambs was principally a function of their low body weights. Sire of the lamb exerted a small influence on its body weight and coat depth but these differences were not translated into a corresponding effect on the rate of decline in rectal temperature. Nd-type lambs had significantly greater coat depths and midside wool weights (per unit area of skin) than Romney-type lambs. The metabolic rate required by the lamb to maintain deep-body temperature was significantly related to coat depth (.beta. = -0.14 W/kg per mm) but not to midside wool weight. Rate of decline in rectal temperature was also low in deep-coated lambs. Most of the difference between Romney-type and Nd-type lambs in resistance to cold stress was accounted for by the corresponding differences in coat depth. Wetting of the lamb did not influence the relationship between metabolic rate and coat depth but did reduce coat depth in Nd-type lambs. Rate of decline in rectal temperature was more highly repeatable between tests (r = 0.84) than metabolic rate (r = 0.59). Coat depth at the midside and hip positions was also moderately repeatable between tests (r = 0.73 and r = 0.77, respectively).

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