<p>Numerous climatic indices have been utilized to predict the efect of hot, and cold, climatic conditions on animal production and welfare. To date, the dairy industry has relied extensively on the Temperature Humidity Index (THI) to predict adverse climatic conditions" however, neither solar radiation nor air movement is accounted for in the THI equation. The Dairy Heat Load Index (DHLI) was initially developed as an alternative climate index. In its current format, the DHLI does not account for the efects of heat load mitigation strategies, such as shade, which decreases the negative efects of hot climatic conditions on lactating cows. Therefore, this experiment aimed to determine the efectiveness of the DHLI as a predictor of heat load responses in both shaded and unshaded cows, as compared with the THI. Forty lactating Holstein Friesian (n=40) cows were selected and paired based on live weight, milk yield, and days in milk. One cow from each pair was randomly allocated to one of two treatments: shaded (n=20) or unshaded (n=20). Cows were given 7 days to acclimate prior to the commencement of data collection. After 28 days, cows were transitioned into the alternate treatment in a crossover design and given 7 days to acclimate prior to data collection. Behavioral observations (0800, 1200, 1400, and 1800 h daily), daily milk yield (kg), milk composition (various days), and vaginal temperature (T_VAG, °C" 5 pairs/week, over a 4-week rotation) were recorded. Overall, data from this experiment indicated that the DHLI was a better predictor of standing and feeding behaviors in unshaded cows and drinking behaviors in shaded cows. Conversely, the THI was a better predictor of standing behavior and shade usage in shaded cows. Furthermore, the THI was a better predictor of mean panting score (MPS) in shaded cows, whereas the DHLI performed better in unshaded cows. Additionally the DHLI was a better predictor of T_VAG in these cows. Finally, when evaluating the 7-day average of each climatic index, the DHLI was a better predictor of change in milk yield. Incorporation of additional animal and management factors is required if the DHLI is to become an efective heat load management tool.</p>