A Study of the Variation in Response to Climatic Stress within and between Breeds of Sheep

This thesis concerns an investigation of the ability of sheep to withstand cold. It includes studies of the initial cold resistance, the ability to acclimatize to cold, and of the variation within and between breeds of sheep. The cold resistance of shorn Scottish Blackface, Southdown and Welsh Mountain sheep, equal numbers on high or maintenance levels of nutrition, was measured by the rate of decline of reotal tempera¬ ture under acute cold exposure (-20°C; 4 m.p.h. wind). Their ability to acclimatize to cold was determined by the effect of three types of cold exposure on subsequent cold resistance. These were; 2 weeks at a moderately subcritical temperature (+8°C), up to 8 hrs. acute exposure, or a combination of the two. Associated physiological responses such as changes in skin temperature, heart rate, respiration rate and skinfold thickness were also measured. Cold resistance was greatest in the Blackface and least in the "elsh sheep. The mean rates of decline of rectal temperature were 0.6}6, 1.105 and 1.791 °C/100 min. exposure for Blackface, Southdown and Welsh sheep respectively. High plane sheep had greater cold resistance than low plane, but there was no evidence for interactions between breed and nutrition. There was considerable variation between individuals within breeds; coefficients of variation ranged from 25y» to 66^. Individual repeatability of performance was high. The cold resistance of all breeds was increased by previous exposure to +8°C, while an increase in cold resistance as a result of acute cold exposure alone was clearly shown by the Blackface sheep. These findings indioated that sheep could acclimatize to cold. Blackface sheep had a much greater capacity for acclimatization than either Southdown or Welsh sheep. The mean rates of decline of rectal temperature were 74$, 19$ and 32$ slower respectively after acclimatization. In general both high and low plane sheep showed similar amounts of acclimatization. There was some inconclusive evidence for breed x nutritional interactions in ability