Understanding heat stress in pasture‑based dairy cows: Challenges and innovations from thermal indices to multivariate sensor technologies
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https://orcid.org/0000-0002-3215-5379
Abstract
Heat stress is an increasingly critical challenge for modern dairy production, particularly in pasture‑based systems where cows are directly exposed to solar radiation, humidity, wind, and high temperatures. This review synthesises the current evidence on environmental thermal indices, physiological responses, behavioural adaptations, and sensor-based technologies relevant to heat stress assessment in grazing dairy cattle. A narrative review of peer-reviewed and grey literature (2015–2026) was conducted across major scientific databases, focusing on studies that developed or validated thermal indices, compared them with animal-based indicators, or quantified their impacts on performance and health. The findings show that the Temperature Humidity Index (THI), although widely used, has limited accuracy in outdoor conditions owing to its exclusion of solar radiation and wind. Updated indices, including the adjusted THI, Heat Load Index (HLI), Comprehensive Climate Index (CCI), Dairy Heat Load Index (DHLI), and Grazing Heat Load Index (GHLI), demonstrate better alignment with physiological responses under grazing conditions. Early and sensitive indicators, such as respiratory rate (RR), panting, and rising body temperature (BT), provide direct insight into the loss of thermal balance, whereas reductions in rumination, shifts in feeding behaviour, and increased shade-seeking reflect behavioural adaptation. Emerging evidence also highlights heat-induced dysbiosis in the rumen and intestinal microbiome, with implications for metabolism, immunity, and reproductive function. Sensor-based technologies, including rumen boluses, infrared thermography, accelerometers, and subcutaneous or vaginal temperature devices, enhance real-time monitoring and support early detection, particularly in extensive outdoor environments. Overall, integrating multivariable climate indices with animal-centred physiological and behavioural monitoring provides a more accurate and responsive framework for heat stress detection, risk assessment, and management in pasture-based dairy systems, supporting productivity, welfare, and long-term system resilience.
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