Metabolic maturity and vigour in neonatal lambs, and subsequent impacts on thermoregulation and survival.

Abstract

Lamb mortality in Australia averages approximately 20%, representing a major constraint to the profitability of sheep enterprises and compromised animal welfare. Most postpartum lamb loss occurs within the first three days of life and is largely caused by starvation, exposure to cold conditions and mismothering from the ewe. In this thesis we developed an over-arching hypothesis that differences in metabolic or physiological maturity exist between lambs, and that these differences relate to early postnatal vigour and survival, particularly during exposure to cold conditions. To test this hypothesis, behaviour associated with initial vigour was quantified in breeds of sheep which differ widely in neonatal survival and more specifically, risk of hypothermia. Pre-suckling blood samples were collected from these animals in order to identify potential markers of maturity chosen to represent the hypothalamic- pituitary- adrenal (HPA) axis, renal function and energy metabolism. A number of metabolite and endocrine shifts were identified in those that were quicker to reach the udder of the ewe and begin sucking. Namely, creatine, non-esterified fatty acids, leptin and ghrelin concentrations were elevated, implying these individuals may be better able to regulate energy mobilisation soon after birth. Lamb vigour was also strongly associated with rectal temperature at birth, indicating an association between maturity, post-natal behaviour and thermogenesis. A controlled water bath testing system was then used to experimentally induce mild hypothermia in the lambs, and metrics of thermoregulation included time taken for core body temperature to reach 35°C (cold resistance), and time to restore core temperature to 39°C (cold recovery). Lambs that were slow to stand and reach the udder had impaired cold resistance. None of the physiological measures (circulating metabolite and hormone concentrations at birth) were related to performance in the water bath. Somewhat surprisingly, those lambs identified as being more mature, as assessed by speed to perform peri-natal behaviours and physiological blood measures, experienced a delay in cold recovery when compared to those with lower vigour and maturity. We proposed that this may be due to a reduced ability to perform non-shivering thermogenesis in more mature individuals (as is observed to occur with age), but this need further exploration. Given these strong relationships between maturity, peri-natal vigour and thermogenesis, an attempt to alter the metabolic maturity of newborn lambs was made. Peri-conception nutrition was shown previously to influence fetal HPA axis activation (responsible for the maturation of a suite of fetal systems) hence differing nutritional treatments (0.7, 1.0 and 1.5 maintenance energy requirement) were applied to the ewes at this time. No effect of peri-conception nutritional manipulation on lamb survival was observed, but lambs from ewes fed a restricted diet around conception exhibited a decreased crown-rump length when compared to those from the high energy treatment. Whilst this had no effect on survival, under more inclement conditions this finding may increase risk of hypothermia through effects on surface area dependent heat loss. In summary, the findings presented in this thesis provide strong evidence that the metabolic maturity of lambs at birth is related to initial vigour and thermoregulatory ability, two traits that are closely linked with survivability. The attempt to reduce lamb mortality through altering HPA axis activation by nutritional means had limited effects on lamb phenotype. Consequently, peri-conception nutrition failed to influence lamb survival. Future investigations should target other means by which metabolic maturity at birth can be enhanced in order to improve lamb survival.