Dehydration as a risk factor for calf mortality in northern Australia

Abstract

It is estimated that approximately one million calves die in Australia every year, with the majority unexplained. This thesis reviews losses associated with dehydration-mediated mortality during neonatal life, when there is highest risk of mortality, and reports three studies of neonatal calf dehydration. For 12 neonatal Brahman calves at Spyglass research site (northern forest land type, Queensland), variation in hydration status was investigated (Experiment 1). Change in live-weight from birth was used as the primary measure of hydration. Half of this cohort had low live weight gain between birth and day 3 of life (p = 0.01) compared to calves growing at 0.95±0.24 kg/day from birth. Low growth and associated reduced plasma protein (p = 0.04) in 6 calves indicated reduced colostrum, milk uptake and therefore reduced body fluid replenishment compared to other calves. This positions them at high risk of mortality under adverse environmental conditions. Urea space was tested as a measure of body water; however, percentage body water estimates were unrealistically elevated (> 80% of live weight) and therefore not representative of actual calf percentage body water. Descriptive analysis of risk factors for calf mortality was conducted for 478 and 378 tropically adapted newborn calves, respectively at Spyglass and Brian Pastures (southern forest land type, central Queensland) research sites (Experiment 2). The incidence risk of neonatal mortality in the southern forest of Queensland and northern forest of Queensland was 3.1% and 4.7%, respectively. Some risk factors and the percentage of calves exposed to high mortality risk included: dam body condition score ≤2 (8%), birth weight ≤28 kg (18%), low birth vigour (2%), large udder size (1%), at least two large teats (2%) and poor maternal protectiveness (5%). The low proportion of cows in poor body condition and good nutritional management limited the opportunity for dehydration-mediated neonatal mortality. There were no cases of prolonged heat stress around calving at the level previously associated with foetal and calf mortality. Experiment 2 indicates that low calf mortality may be achieved in any country type, provided that a low proportion of calves are exposed to high risk of mortality. A simple mechanistic model was developed to describe milk uptake volumes and ambient temperature (heat stress) impacts on neonatal hydration. This model provided clarification for the possible risk of dehydration mediated mortality across breeding herds of northern Australia. This thesis demonstrated variation in milk delivery, modelled that low milk delivery is associated with dehydration-mediated mortality and demonstrated that risk of mortality is low at a herd level when the risk factors associated with reduced milk delivery have low frequency.