Wind speed and solar radiation adjustments for the temperature-humidity index

The Livestock Weather Safety Index is commonly used as an indicator of heat stress for livestock and is based on the temperature-humidity index (THI; THI = 8*Ta + ((relative humidity/100)*(Ta - 14.3)) + 46.4, where Ta = temperature, ^oC). However, wind speed (WSPD) and solar radiation (RAD) are known to influence heat stress but are not accounted for in the THI. Accurate adjustment to the THI equation based on WSPD and RAD are essential in determining level of heat stress. To obtain these adjustments, data were utilized from three summer studies utilizing confined feedlot cattle. Individual panting scores (PS; 0 = no panting to 4 = severe panting) were obtained from cattle between 1400 and 1700 h and used as a visual assessment of heat stress. These data were combined into one data set and included over 4,000 observations. Weather stations, located at the research facility where cattle were confined, recorded THI and WSPD (m/s) and solar radiation (kcal/m²). Temperature-humidity index averaged 77.9 ± 5.4 (range 62.4 to 86.1) at the time PS were determined. A regression equation was developed using hourly values for THI, WSPD, and RAD to predict PS (PS = -7.563 + (0.121 * THI) – (0.241 * WSPD) + (0.00095 * RAD)) between 1400 and 1700 h. The ratio of WSPD to THI and RAD to THI were -1.992 and 0.0079, respectively, and represent the adjustments to the THI for WSPD and RAD. For instance, for each 1 m/s (2.24 mph) increase in WSPD, THI is reduced 1.992 units, and for each 100 kcal/m² decrease in RAD, THI is reduced 0.79 units. On the basis of these ratios and average 1400 to 1700 hourly data, the THI adjusted (THWRI) for WSPD and RAD would be equal to [4.51 + THI – (1.992*WSPD) + (0.0079*RAD)]. Three separate cattle studies, comparable in size, type of cattle, and number of observations to the three original studies, were utilized to assess the relationship between the adjusted THI and PS and compare that to the relationship between the original THI and PS. Actual mean PS in these studies were 1.1 versus the predicted mean of 1.2. The mean R-square between THI and mean PS was 0.39, with correlation coefficients ranging between 0.47 and 0.87; while the mean R-square between the adjusted THI and mean PS was 0.50 with correlation coefficients ranging between 0.64 and 0.80. These corrections would be most appropriate to use, within a day, to predict THI during the afternoon hours using hourly data or current conditions. As real-time conditions change immediate adjustments in THI can be taken into account while implementing management strategies to mitigate heat stress on livestock. Although knowledge of THI alone is beneficial in determining the potential for heat stress, adjustments for WSPD and RAD are essential to more accurately assess and predict animal discomfort.