Non-Invasive Physiological Indicators of Heat Stress in Cattle

SIMPLE SUMMARY: Heat stress is a common cause of poor welfare in cattle and is expected to increase with climate change. The biggest challenges are to feedlot and dairy cattle in hot climates but determining whether cattle are heat stressed can be difficult due to the invasive and time-consuming nature of established methods of measurement, such as rectal temperature, and the lack of specificity of some measures, such as cortisol in plasma or faeces. We review the various methods of non-invasively measuring heat stress in cattle and highlight the value of infrared thermographic imaging for the accurate and rapid determination of external body surface temperatures. Further research options to establish thresholds and optimum methodology for these non-invasive measures and their use in different cattle types are identified. Their use promises to accelerate response times to cattle experiencing heat stress under intensive management conditions. ABSTRACT: Cattle are susceptible to heat stress, especially those kept on high levels of nutrition for the purpose of maximising growth rates, which leads to a significant heat increment in their bodies. Consequences include compromised health and productivity and mortalities during extreme events, as well as serious economic loss. Some measures of heat stress, such as plasma cortisol and temperature in the rectum, vagina, or rumen, are invasive and therefore unlikely to be used on farms. These may cause additional stress to the animal due to handling, and that stress in itself can confound the measure. Consequently, it is desirable to find non-invasive alternatives. Panting score (PS), cortisol metabolites in faeces, milk, or hair, and the infrared temperature of external body surfaces are all potentially useful. Respiratory indicators are difficult and time consuming to record accurately, and cortisol metabolites are expensive and technically difficult to analyse. Infrared temperature appears to offer the best solution but requires further research to determine the thresholds that define when corrective actions are required to ensure optimal health and productivity. Research in this area has the potential to ultimately improve the welfare and profitability of cattle farming.