Genetic and physiological aspects of resistance to hypothermia in relation to neonatal lamb survival
Abstract
Every year many thousands of newborn lambs die of cold exposure.
This thesis is concerned with some of the physiological and genetic
factors involved in the resistance of neonatal lambs of ten different
breeds to hypothermia with particular reference to their heat
production capabilities.
The project was a development-of preliminary work carried out at
the Animal Breeding Research Organisation, Edinburgh during the year
immediately preceding this study. An attempt was made to reproduce
in the laboratory, a standard cold stress comparable to that
experienced by the newborn lamb born into cold, wet and windy
conditions. A suitable test was evolved using water as the cooling
medium to reduce body temperature and measuring heat production (in
the form of oxygen consumption) using an indirect open-circuit
calorimeter.
265 lambs of ten different breeds were tested. Significant
breed differences were found for resistance to body cooling and base
metabolic rate. Peak metabolic rate was significantly affected by
weight.
The results of the experimentation (1976-1979) give an insight
into the variation existing, both within and between breeds, and into the
traits likely to be useful in the selection of individuals and breeds
most suited to survive severe environmental conditions during the
neonatal period.
An attempt was made to separate the effects of factors such as
weight, skin thickness, age, sex and litter size.
In particular, observations during cooling tests indicate the
possible importance of tissue insulation, recorded in the form of
skin thickness, in aiding the neonate to maintain body temperature in
a cooling environment where fleece insulation is low.
It was thought that the insulative properties of the birthcoat
were unlikely to be expressed under waterbath conditions and no
significant effects of birthcoat were found despite there being
significant breed variation in this trait. To demonstrate birthcoat
effects a wind tunnel procedure was used with artificial wind and
rain providing a cold stress. Use of this equipment allowed
separation of the effects of birthcoat extremes within the Welsh
Mountain breed.
The effect of fasting lambs from birth (4hrs) and pre-partum
cold exposure of the dam were investigated for effects on the
newborn's metabolic response to cold and cold resistance capabilities
in the Scottish Blackface breed. The fasted lambs showed enhanced
cold resistance possibly as a result of fat metabolism being
initiated prior to the applied cold stress. Cold exposure of
pre-partum ewes increased gestation length and the birth weight of
their lambs. Base metabolic rate was lower for fasted lambs and peak
metabolic rate enhanced in lambs from cold stressed ewes.
The contribution of non-shivering thermogenesis was investigated
and emphasis placed on fat metabolism in early post natal life when
brown adipose tissue may be particularly relevant in cold
thermogenesis. Metabolic responses to injected noradrenaline were
investigated and a comparison made between maximum metabolic response
to cold and catecholamine stimulation in the Cheviot breed.
Some methods of rewarming were also studied with passive
techniques showing possible field application.
The possibilities for selection of more cold resistant types of
sheep are discussed.