Regional heat stress maps for grazing dairy cows in New Zealand under climate change

Context For dairy cows housed indoors, ambient temperature and relative humidity are key drivers of heat stress, whereas for cows kept outdoors, solar radiation and wind speed are also important. Solar radiation directly increases the heat load on cows, whereas wind speed affects their ability to dissipate heat through convection and evaporation. Aim We aimed to determine whether climate-driven changes in these variables affect heat stress risk where cows are outdoors during summer and shoulder seasons, particularly in pasture-based farming systems such as in New Zealand. Understanding outdoor-specific factors is crucial for accurately assessing and mitigating heat stress in grazing dairy cattle, because their management needs differ substantially from those in housed systems. Methods Using daily climate projection data from 2006 to 2098, peak daily values of the temperature–humidity index (THI) of Thom and the grazing heat-load index (GHLI) of Bryant were calculated and used to map predicted changes in both the annual (June–May) number of days with heat stress risk and also the annual accumulated heat stress exposure (the sum of effective degrees Celsius above the threshold) for dairy production regions of New Zealand. Key results The results illustrated the limitations of using THI in the context of outdoor use, where solar radiation and wind speed are shown to be more important than relative humidity. The GHLI predicted that the risk of heat stress is already high in the Waikato (69 days), Bay of Plenty (69 days) and Canterbury (80 days) regions in the 2020s. Canterbury was also notable for having high heat stress exposure within day compared with other regions (i.e. heat stress days were particularly intense), attributable to the combined effect of high air temperatures, high solar radiation and low wind speeds. Conclusions According to climate projections, regions already experiencing high numbers of heat stress risk days and heat stress exposure in the 2020s will experience the greatest increases in heat stress risk to the 2050s. However, dramatic increases in the number of heat stress days are not anticipated. Implications This allows research and development to focus on mitigation practices in these regions where dairy farming systems must adapt to a changing climate. Mitigation strategies may include provision of shade, access to sprinklers, genetic selection for heat stress resilience, modifying feeding regimes to reduce heat load, or development of new solutions and technologies.