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

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.