Dynamic Response Indicators of Heat Stress in Shaded and Non-shaded Feedlot Cattle, Part 2: Predictive Relationships

Summer heat provides stressful conditions for Bos taurus feeder cattle; in extreme instances these conditions can be fatal. One management option is to provide shade structures for feedlot animals. This study was conducted during the summer of 2001 to compare physiological responses of cattle with shade access or no-shade access. Eight steers were selected from a group of 12, and assigned individual pens. Four pens were fitted with shade structures that allowed the steers to choose shade; the remainder had no shade option. The animals were rotated through pen assignments during the summer season. Continuous measures of respiration rate and body temperature were recorded as response variables to the shade treatments. Environmental conditions were monitored for the experimental period. Daytime means and standard errors were 86·0±0·39 breaths min−1 for respiration rate shade, and were significantly lower (probability P<0·05) than respiration rate No-shade of 102·3±0·36 breaths min−1. Linear regression fit for daytime data showed the slope for No-shade to be 4·5±0·15 breaths min−1 and 1·5±0·11 breaths min−1 for Shade. Thresholds for humidity and treatment of Shade and No-shade were determined to exist between 25 and 30 °C. Linear regression equations were developed for respiration rate including effects of temperature, humidity, wind speed, and solar radiation for animals in either Shade or No-shade feedlot pens.