Rethinking Heat Stress Assessment in Dairy Practice: From THI to HLI–AHL

Rising global temperatures and increasingly frequent heat waves are extending the duration of thermal stress in dairy systems, making heat management a year-on-year clinical priority¹. In field practice, the Temperature–Humidity Index (THI) has long been used to assess heat stress; however, it reflects only the intensity of heat exposure and fails to capture how long animals remain under stress. This limitation is critical because prolonged exposure to moderate heat can be just as harmful as short periods of extreme heat. 

Understanding Heat Load in Real Farm Conditions 

The Heat Load Index (HLI) offers a more comprehensive alternative by incorporating additional environmental variables such as solar radiation and wind speed alongside temperature and humidity. When combined with Accumulated Heat Load (AHL), it provides a dynamic measure of how much heat an animal absorbs over time. This dual consideration of intensity and duration allows for a more realistic assessment of the thermal burden experienced by dairy cows, particularly in open or semi-open housing systems where environmental exposure is variable². 

Clinical Relevance for Field Veterinarians 

The practical value of HLI–AHL lies in its ability to identify when cows are actually accumulating heat rather than simply being exposed to it. Evidence indicates that once AHL rises above zero, cows begin to experience physiological strain that can predispose them to production losses and disease. This is especially relevant for high-yielding animals, which generate more metabolic heat and are therefore less capable of coping with environmental stressors^1,2. 

Seasonal patterns further reinforce the need for proactive monitoring. Heat load typically begins to accumulate from late spring and persists through early autumn, with peak stress often observed in mid-summer. However, early-season heat events tend to have a disproportionate impact, likely because animals are not yet acclimatized. Recognizing this window allows veterinarians to intervene earlier rather than responding only during peak summer^1,2. 

Implications for Practice 

Incorporating HLI–AHL into herd monitoring shifts the focus from reactive to preventive care. Rather than relying solely on temperature thresholds, veterinarians can evaluate cumulative heat exposure and anticipate health risks before they manifest clinically. This approach is particularly valuable in large dairy operations where subtle environmental changes can translate into significant herd-level effects. 

Conclusion 

Heat stress assessment in dairy practice is evolving. Moving beyond THI to models such as HLI–AHL enables a more accurate understanding of thermal load and its biological impact. For field veterinarians, this translates into better prediction of risk periods, improved herd management decisions, and ultimately enhanced animal welfare and productivity.  

References 

1. Masson-Delmotte V, Zhai P, Pirani A, Connors SL, Péan C, Berger S, Caud N, Chen Y, Goldfarb L, Gomis MI, Huang M. Summary for policymakers. Climate change 2021: The physical science basis. Contribution of working group I to the sixth assessment report of the intergovernmental panel on climate change. 2023 Sep 25:3-2. https://pure.iiasa.ac.at/id/eprint/19094/1/IPCC_AR6_WGI_SPM.pdf   

2. Vitali A, Felici A, Lees AM, Giacinti G, Maresca C, Bernabucci U, Gaughan JB, Nardone A, Lacetera N. Heat load increases the risk of clinical mastitis in dairy cattle. Journal of dairy science. 2020 Sep 1;103(9):8378-87. https://www.sciencedirect.com/science/article/pii/S0022030220304586#cesec80