Extended Abstract (88K)6A.1
Wind speed and solar radiation corrections for the temperature-humidity index
Terry L. Mader, University of Nebraska, Concord, NE; and S. Davis
Wind speed (WSPD) and solar radiation (RAD) are known factors contributing to the degree of heat stress feedlot cattle are subjected to. However, the most commonly used indicator of heat stress for feedlot cattle (Livestock Weather Safety Index) uses only the temperature-humidity index (THI) and does not account WSPD or RAD. Accurate adjustment to the THI equation based on WSPD and RAD are essential in determining level of heat stress. Therefore, visual assessments of heat stress based on panting scores (PS; 0=no panting, 4=severe panting) were collected at 1700 h during three previously conducted summer studies. The 1700 h time was selected, since it is typically near or at the hottest portion of the day and was shown to be the time cattle displayed the greatest level of heat stress. These data were combined into one data set and included 5520 observations. A weather station, located in the facility where cattle were confined, recorded THI and WSPD (m/s). Solar radiation (W/m2) was recorded .7 km West and 1.7 km North of the facility. Temperature-humidity index averaged 79.7 ± 5.2 (range 63.9 to 86.2) at the time PS were assigned. A regression equation (RE) was developed using hourly values for THI, WSPD, and RAD to predict PS (PS=-6.317 + (0.097 * THI) – (0.233 * WSPD) + (0.0026 * RAD)) at 1700 h. Using this equation, mean THI was entered into the equation and held constant, while WSPD and RAD were adjusted using mean values ± 1 std. err. The RE equation was then solved for THI using each calculated PS and mean values for WSPD and RAD. The calculated THI was then regressed against its corresponding WSPD and RAD value. The slopes of these lines (-2.400 and 0.027 for WSPD and RAD, respectively) 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 2.4 units, and for each 100 W/m2 decrease in RAD THI is reduced 2.7 units. These corrections would be most appropriate to use, within a day, to predict THI during the day using hourly data or current conditions. As real-time conditions change immediate adjustments in THI can be made using these corrections. To predict THI for a future weather event or day then daily averages could and may be more appropriate to use, whereby adjustments to THI would be based on projected average daily conditions. Adjustments in THI based on daily averages were for each 1 m/s increase in WSPD THI is reduced 3.14 units, and for each 100 W/m2 decrease in RAD THI is reduced 1.46 units. Although, knowledge of THI alone is beneficial in determining the potential for heat stress, accurate adjustments for WSPD and RAD are essential to more accurately represent and predict the degree of animal comfort.
Session 6A, Emerging Technologies: Animal Stress
Tuesday, 29 October 2002, 3:00 PM-3:46 PM
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