Submitted to: Biosystems Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 8, 2004
Publication Date: April 15, 2005
Citation: Brown Brandl, T.M., Eigenberg, R.A., Nienaber, J.A., Hahn, G.L. 2005. Dynamic response indicators of heat stress in shaded and non-shaded feedlot cattle: part 1. analysis of indicators. Biosystems Engineering 90(4):451-462. Interpretive Summary: To reduce stress in feedlot cattle, researchers have suggested low cost lightweight structures to provide shade. Access to shade has shown an increase in growth in warmer regions. This study was designed to quantify the reduction in stress level of feedlot cattle given access to shade. The impact of the access to shade under different weather conditions is also discussed. Animals given access to shade had lower body temperatures than unshaded animals at the hotter hours of the warmest days. Respiration rates of shaded cattle were lower at hotter hours of all days. It was found that respiration rate was a good indicator of stress and can be easily monitored without expensive equipment.
Technical Abstract: 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 294.7 +/- 0.8 kg) were randomly assigned to one of eight individual pens, where one of two treatments were applied: shade access (SA), or no shade access (NS). Respiration rate (RR), daily feed intake (DFI), and core body temperature (Tcore) were collected, using automated systems during eight periods, for a total of 37 days. The data were analyzed using four categories of daily maximum Temperature Humidity Index (THI) values (Normal: THImax <74; Alert: 74< THImax >78; Danger: 79<THImax >83; Emergency: THImax >84). Shade was found to impact the physiological responses in all THI categories, with the largest impacts in the Danger and Emergency categories. Shade lowered RR and Tcore during the peak temperature hours of the day. It was concluded that RR is the most appropriate indicator of thermal stress to monitor because it was consistently affected in all THI categories, it is easy to monitor without the need for costly equipment, and there is little or no lag associated with it.