Genetic and physiological factors affecting thermoregulation and resistance to body cooling in newborn lambs

Abstract

Lamb mortality is a serious source of economic loss to the sheep industry. Surveys have shown that up to 50% of the post-natal losses can be due either directly or indirectly to cold exposure. This thesis examines various ways of improving thermoregulation in the newborn lamb. Emphasis was placed on the need to develop methods which required little or no capital expenditure. This was essential if the most promising techniques were to be adopted in the less favoured hill environments.

The effectiveness of the various treatments was measured by subjecting each newborn lamb to a standard cold exposure in a progressively cooled water bath. Various components of heat production and heat loss were recorded during this test. The cold resistance of each lamb was defined as the time taken for rectal temperature to fall by approximately 4C. The capacity for non-shivering thermogenesis (NST) was also measured in some lambs by recording the calorigenic response to noradrenaline administration at a therinoneutral temperature.

Shearing of the pregnant ewe 4 weeks before parturition increased the cold resistance of the newborn lambs. Components of both heat production and heat loss were influenced by this treatment.

Short periods of acute cold exposure imposed daily for the last 12 days of pregnancy enhanced NST in the newborn lamb. Further work showed that all these results were influenced by ewe age, breed, foetal number and year effects. The implications of this, and possible mechanisms, were discussed.

Genetic selection for cold resistance in Scottish Blackface lambs started at ABRO in 1980. Preliminary results of this experiment are reported here. The response was asymmetrical. Selection for high cold resistance produced considerably more progress than selection for low cold resistance. Heritability of cold resistance estimated from line divergence was 0.17+0.09. Heritability for increasing cold resistance was 0.27+0.13 and for decreasing cold resistance it was 0.01+0.16.

The large number of lambs tested during this experiment (594) allowed a detailed investigation of various components which influenced water bath performance. Single lambs had a greater we ight-specif ic heat production and reduced heat loss compared to twins. Female lambs were better able to thermoregulate than males. Cold resistance tended to decline with age but increase with liveweight.

The genetic and phenotypic relationships between cold resistance and components of heat production and heat loss were analysed. The implications of these findings with regard to thermoregulation and hence lamb mortality in the field situation were discussed.