Body temperatures and respiratory rates of free-ranging Merino sheep in and out of shade during summer

Abstract

Shade use by 20 unrestrained Merino wethers carrying 23-31 mm wool was monitored as they grazed dry pasture during summer (Ta max = 28-41 ¦C). During 17 24-h periods intra-abdominal (Tb) and trunk-skin (Tsk) temperatures were recorded with ambulatory data-loggers at 1 or 5 minute intervals; in 11 periods respiratory rates were also counted each hour during daylight. The four sheep that used shade least (6% of 8 h day) had mean, maximum and minimum Tb no different to those of the four sheep that used shade most (39%). Maximum and minimum Tb each day occurred 1-2 h earlier in animals inclined to stay in sun rather than shade. Tsk varied in sheep shuttling in and out of shade, but mean mid-back skin temperature was only slightly, and not significantly, higher for animals in sun than shade. Respiratory rates also were similar for animals in sun and shade. The similarity of thermal status of animals in and out of shade was unexpected. This may have arisen because animals had different rates of evaporative cooling that were not reflected in respiratory rates, or possibly different rates of heat production. Alternatively, all animals could have been effectively insulated from solar heating by their wool, though they presumably experienced some heat stimulus since all used shade at some time. Shade may be of benefit