Variation in the responses of shorn sheep to cold exposure

Abstract

Thirteen Blackface and Merino × Cheviot female sheep were shorn and each exposed for a maximum of 10 hours to intense cold (—18° C, 4 m.p.h. wind). Individual exposures were terminated earlier if rectal temperatures fell by 4·5° C. In the first year of the experiment (1963) 11 out of 13 of the sheep were in late pregnancy. There were also 12 control sheep, 9 of them pregnant, which were not subjected to cold treatment. In 1964, 12 of the previously treated sheep were subjected again to similar cold exposures. This time they were unmated and there were no controls.The cold treatment apparently caused increased variation in gestation length, but the effects on neo-natal lamb mortality were inconclusive.In 1963 and 1964 there was wide variation between individual sheep in cold tolerance—defined as the ability to maintain rectal temperature during cold exposure. Repeatability for this character was approximately 0·73, suggesting the existence of a genetic component. The average decline in rectal temperature was similar in both years: 3·9·2° C. in 1964. Pregnancy and breed had no detectable effects on cold tolerance.Skin temperatures on the midside, ear and hind foot (pastern) were measured in 1964. They all declined during cold exposure. Midside skin temperatures typically fell to about 10° C., ear temperatures to about 3° C. and foot temperatures to zero. There were temperature fluctuations due to periodic vasodilation on the ears but the feet generally showed sustained vasoconstriction.Superior cold tolerance tended to be associated with low mean body skin temperatures and a previous history of live-weight gain. It is suggested that subcutaneous fat deposition may be one factor responsible for improving skin insulation and thereby reducing heat loss in shorn sheep, but direct evidence for this was lacking.