Recent Advances on Early Detection of Heat Strain in Dairy Cows Using Animal-Based Indicators: A Review

SIMPLE SUMMARY: In the dairy industry, heat stress and its induced heat strain result in huge economic loss every year. To better manage heat strain in dairy cows, it is more sensible to advance the detection by using more sensitive indicators so that cooling measures can be implemented in time. With the development of sensor technologies and wireless transmission technologies, body surface temperature and respiration rate can be measured automatically through wearable devices. Lots of efforts have been made recently to develop meaningful thresholds on both physiological and environmental sides. However, the existing thresholds should be used carefully considering the differences in experimental conditions and animal information. Further studies are required to evaluate and customize thresholds based on different influencing factors. A comprehensive early detection system for heat strain based on both animal- and environment-based indicators is expected. ABSTRACT: In pursuit of precision livestock farming, the real-time measurement for heat strain-related data has been more and more valued. Efforts have been made recently to use more sensitive physiological indicators with the hope to better inform decision-making in heat abatement in dairy farms. To get an insight into the early detection of heat strain in dairy cows, the present review focuses on the recent efforts developing early detection methods of heat strain in dairy cows based on body temperatures and respiratory dynamics. For every candidate animal-based indicator, state-of-the-art measurement methods and existing thresholds were summarized. Body surface temperature and respiration rate were concluded to be the best early indicators of heat strain due to their high feasibility of measurement and sensitivity to heat stress. Future studies should customize heat strain thresholds according to different internal and external factors that have an impact on the sensitivity to heat stress. Wearable devices are most promising to achieve real-time measurement in practical dairy farms. Combined with internet of things technologies, a comprehensive strategy based on both animal- and environment-based indicators is expected to increase the precision of early detection of heat strain in dairy cows.