The effects of litter size, sex, age, body weight, dam age and genetic selection for cold resistance on the physiological responses to cold exposure of scottish blackface lambs in a progressively cooled water bath

Thermogenesis and physiological maturity in neonatal lambs: a unifying concept in lamb survival

Lamb mortality represents reproductive wastage and an animal welfare concern. While lambs are thought to be at a thermogenic advantage following birth in comparison to other species, death from exposure can still be a major contributor to lamb mortality, largely because of the inclement conditions often prevailing at lambing. For this reason, thermogenesis has been studied extensively in neonatal lambs. Heat is produced in the neonatal lamb by shivering and non-shivering thermogenesis. The latter is heat generated by metabolism of brown adipose tissue (BAT) found largely in the thorax and peri-renal areas of the newborn lamb. Brown adipose tissue differs from normal adipose tissue in that it contains densely packed mitochondria, a high cytochrome c content and a vast vascular network. Heat is generated in BAT by uncoupling of the proton conductance mechanism from ATP production, resulting in heat production instead of stored energy. The ability of lambs to resist cooling differs among individuals and this is likely to be due to both genetic and phenotypic factors. The heritability of cold resistance is moderate-to-high and polymorphic gene markers associated with energy homeostasis and cold-related mortality have been identified. Additionally, several aspects of the phenotype of the lamb have been associated with cold resistance. Most relate to properties of the coat, skin and bodyweight, the latter being particularly important, presumably through effects on surface area to volume ratios and subsequent heat loss. The ability of the neonate to achieve the transition from intra- to extra-uterine life has been termed physiological maturity and is associated with the ability to activate appropriate neuro-endocrinological and behavioural changes that are consistent with homeostasis of energy metabolism. Ways to alter physiological maturity of the lamb, such as nutrition, pharmacology and genetic selection, have been identified, and while these show promising results with regards to thermoregulation, a key limitation of their application has been the lack of a repeatable, representative model of neonatal cold stress. An estimation of the non-shivering component potential of a lamb’s ability to thermoregulate can be derived from norepinephrine challenges, but more useful models of real-world cold stress are climate chambers or controlled water bath tests. Further use of repeatable test models such as these with appropriate neuroendocrine and metabolic metrics will identify key components and markers of physiological maturity associated with lamb thermogenesis and survival.

An improved water-bath test to study effects of age and previous sucking on metabolic rate and resistance to cold in newborn lambs

Different procedures for measuring cold resistance and metabolic rate of newborn lambs were evaluated by varying the extent of induced hypothermia, the rate of cooling and the method of rewarming. Relatively fast cooling followed by a simple self-rewarming procedure proved harmless and satisfactory.

The effect of age, from birth up to 2 weeks, on thermoregulation was studied. There was no difference in cold resistance between 0·5 h and 30 h after birth, and between 1 day and 2 weeks after birth, despite a large increase in insulation, body weight and coat depth over this period. Weight-specific resting metabolic rate and cold-induced peak metabolic rate similarly did not change significantly in the first 30 h, although resting metabolic rate tended to be lower at birth than at 30 h of age. Peak metabolic rate decreased significantly between 1 day and 2 weeks of age.

The effect of fasting, for 3 to 4 h after birth, on thermoregulation was also studied. Cold resistance and peak metabolic rate were not significantly affected by fasting. Recovery from hypothermia was slightly slower in fasted lambs.

These results may reflect the newborn lamb's initial reliance on heat production derived from brown fat and non-shivering thermogenesis. Older lambs, which benefit from better insulation, rely more upon shivering. Fasted lambs showed a tendency to rely more on insulation and slightly less on heat production than suckled lambs.

Maintenance of body temperature in the neonatal lamb: effects of breed, birth weight, and litter size

To survive, the newborn lamb must be able to maintain body temperature and to stand and move to the udder to suck colostrum to fuel heat production. The objective of this study was to investigate whether neonatal lambs showing slow behavioral progress to standing and sucking also have an impaired ability to maintain body temperature. The time taken to stand and suck after birth, rectal temperatures, and blood samples were collected from 115 newborn single, twin, and triplet lambs of 2 breeds, Scottish Blackface and Suffolk, which are known to show variations in their neonatal behavioral progress. Blood samples were assayed for thyroid hormones, known to be involved in heat production, and cortisol, which plays a role in tissue maturation before birth. In addition, colostrum samples were collected from the 56 ewes that gave birth to the lambs, and assayed for protein, fat, and vitamin contents. Heavy lambs (more than 1 SD above the breed mean), Blackface lambs, and singleton or twin lambs were quicker to stand and suck from their mothers than lightweight (more than 1 SD below the breed mean), Suffolk, or triplet lambs. Low birth weight lambs also had lower rectal temperatures than heavier lambs (P < 0.01), as did Suffolk compared with Blackface lambs (P < 0.001). Lambs that were slow to suck after birth had lower rectal temperatures than quick lambs, and this difference persisted for at least 3 d after birth. Within breed, heavy lambs had greater plasma triiodothyronine and thyroxine immediately after birth than light lambs. Blackface lambs had greater plasma triiodothyronine and thyroxine than Suffolk lambs but tended to have less cortisol. Colostrum produced by Blackface ewes had a greater fat content than that of Suffolk ewes (P < 0.001). Thus, lambs that are behaviorally slow at birth are also less able to maintain their body temperature after birth. Although part of their lower body temperature might be attributable to behavioral influences on thermoregulation, the data also suggest that physiological differences exist between these animals. These differences may be related to different degrees of maturity at birth.

Cold thermoregulation in the newborn calf

During the fetal to neonatal transition, the newborn calf encounters severe thermolysis due to an abrupt change in thermal environment that is compounded by evaporation of fetal fluids and severe weather conditions. Maintenance of homeothermy during the neonatal period necessitates an acute and sustained thermogenic response by the newborn calf. It is now widely accepted that this thermogenic response is derived from both shivering thermogenesis in muscle tissue and nonshivering thermogenesis in brown adipose tissue (BAT). It is critical that newborn calves possess functional BAT during the neonatal period. This article focuses on the pre- and postnatal factors that influence nonshivering thermogenesis of BAT in the neonatal calf.

Description of the effects of a single gene which inhibits the normal metabolic response of newborn lambs to exogenous noradrenaline

A group of newborn Scottish Blackface lambs previously selected for low cold resistance failed to show the normal metabolic response to subcutaneous injection of exogenous noradrenaline. A subsequent study showed this failure to be due to a single major gene. Further studies on the thermogenic and physiological responses and on the inheritance of the trait, are described here. Two intercross matings between non-responders produced responder lambs indicating that the gene is not recessive. The increases in metabolic rate and rectal temperature after injection of noradrenaline were much greater in responder than in non-responder lambs (P less than 0.001), as were the increases in heart rate and respiration rate (P less than 0.001). Dissected brown adipose tissue from non-responders was apparently normal and indistinguishable from brown adipose tissue from responders. Although there was no apparent effect of the gene on birthweight or survival, survival under normal field conditions may be affected.

Breed and environmental influences on weaner lamb production in Zimbabwe

Performance of the Dorper and Mutton Merino breeds and their crosses was assessed from 1,305 ewe and 1,453 lamb records from six lambing seasons. Dorper and Mutton Merino were mated in all combinations under an accelerated lambing system and crossbred females were mated to Suffolk rams. The Dorper and Mutton Merino breeds were not significantly different for fertility, pre-weaning lamb growth and survival. Suffolk sired lambs from crossbred ewes grew rapidly and were heaviest at birth and at weaning. They also survived better than lambs from the other breeding groups and this contributed to the higher weaning rate (P less than 0.05) in crossbred ewes. Weight of weaned lambs per ewe exposed to the ram per season was 12% higher (P less than 0.05) in the Dorper than the Mutton Merino ewes. However lifetime production was 28% higher (P less than 0.01) in Mutton Merino ewes due to significantly superior longevity. Crossbred ewes were highly productive. Individual heterosis was not significant but estimates were positive for most traits. Growth rate and survival of lambs increased as age of dam increased to 48 and 40 months respectively. Lambs born to previously non-lactating ewes were heavier and survived better than lambs born to ewes lactating in the previous season. Lambs born and reared as singles were 25% heavier at birth (P less than 0.001) and 52% heavier at weaning (P less than 0.001) than those born and reared as twins. Male lambs grew faster and were 7% heavier at weaning than females (P less than 0.001).