Resistance to cold stress in the newborn lamb 1. Responses of Romney, Drysdale × Romney, and Merino lambs to components of the thermal environment

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Abstract
Newborn lambs aged 1-30 h were exposed to a range of environmental conditions (5-30°C ambient temperature|0.0-1.9 mls air speed|wet or dry) and their metabolic rates, rectal temperatures, and hind-limb skin temperatures examined. Wet lambs exhibited substantially greater metabolic rates (by 3.8-7.0 W/kg) than those tested when dry. The results indicate that wet Romney lambs may attain their summit metabolic rate of 12-22 W Ikg and become hypothermic at ambient temperatures as high as 15°C. Conversely, dry lambs are unlikely to become hypothermic unless air temperatures are well below freezing point. Lambs exposed to air movement (1.0-1.9 m/s) had elevated metabolic rates (by 0.8-1.5 W/kg) compared with those in still air. Drysdale x Romney lambs were better able to conserve body heat than Romneys. Wet Romney and Drysdale x Romney lambs exposed to a series of declining ambient temperatures were able to minimise the hind-limb skin temperature-air temperature gradient but dry lambs generally failed to do so. It is suggested that Merino lambs may differ from Romney and Drysdale x Romney lambs in the degree to which they employ peripheral vasoconstriction when exposed to mild cold.