Heat stress in feedlot cattle can cause decreases in feed intake and growth, and in extreme cases may result in death. Providing shade during hot weather has shown inconsistent results, reducing direct and indirect losses in some areas of the United States, but not in others. The objectives of this study were to evaluate the dynamic responses of feedlot cattle to environmental conditions with and without access to shade, and to determine the most appropriate physiological measurement for monitoring feedlot cattle during hot weather as a guide for improved management. Eight crossbred steers (initially weighing 294Á7710Á8 kg) were randomly assigned to one of eight individual pens, where one of two treatments were applied: shade access, or no-shade access. Respiration rate, daily feed intake, and core body temperature were collected, using automated systems during eight periods, for a total of 37 days. The data were analysed using four categories of daily maximum temperature humidity index (maximum I TH ) values (Normal for maximum I TH o74; alert for 74p maximum I TH o78; Danger for 78pmaximum I TH o84; Emergency for maximum I TH X84). Shade was found to impact the physiological responses in all I TH categories, with the largest impacts in the Danger and Emergency categories. Shade lowered respiration rate and core body temperature during the peak temperature hours of the day. It was concluded that respiration rate is the most appropriate indicator of thermal stress to monitor because it was consistently affected in all I TH categories, it is easy to monitor without the need for costly equipment, and there is little or no lag associated with it.

Heat stress in cattle causes decreases in feed intake and feed efficiency; in extreme cases, it can cause death. These losses amount to millions of dollars each year. A study was designed to determine severity of heat stress among four breeds of cattle. Throughout two summers, 256 feedlot heifers of four different breeds were observed. Respiration rates, panting scores, and surface temperatures were taken twice each day on 10 animals/breed for several weeks during the summers of 2002 and 2003. Twenty-four-hour behavior measurements were recorded for four heat-stress and four thermoneutral days. Results showed during the afternoon, Angus cattle (black) had the highest respiration rates, panting scores, and surface temperatures, followed by the MARC III (dark red), Gelbvieh (tan), and Charolais (white). Behavior data showed that heat stress increased drinking and standing behavior, and decreased eating, lying, and agonistic behaviors, and that dark-hided cattle adjusted their behavior more than light-hided cattle. Overall, it was found that breed of cattle with dark-hides were more affected by temperature changes and at peak temperatures than breeds of cattle with light-hides.

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 noshade 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 86Á070Á39 breaths min À1 for respiration rate shade, and were significantly lower (probability Po0Á05) than respiration rate Noshade of 102Á370Á36 breaths min À1 . Linear regression fit for daytime data showed the slope for No-shade to be 4Á570Á15 breaths min À1 and 1Á570Á11 breaths min À1 for Shade. Thresholds for humidity and treatment of Shade and No-shade were determined to exist between 25 and 30 1C. 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.

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