Heat Stress Effects on Physiological and Blood Parameters, and Behavior in Early Fattening Stage of Beef Steers

SIMPLE SUMMARY: This study explores the effects of heat stress on physiological, blood, and behavioral parameters in beef steers. We found that heat stress increases water intake, heart rate, rectal temperature, blood cortisol, heat shock protein 70, and time spent standing. However, there was no difference in intake parameters such as dry matter intake, blood glucose, and non-esterified fatty acids (NEFA). The findings indicated that physiological, blood, and behavioral parameters can be used as indicators of heat stress in beef steers and that heat resistance in beef steers is stronger than that of beef calves. ABSTRACT: This study was conducted to investigate the effect of heat stress (HS) on physiological, blood, and behavioral parameters, according to the temperature–humidity index (THI), in beef steers. Twelve Korean native beef steers (342.7 ± 13.81 days old, body weight (BW) of 333.0 ± 18.53 kg) were used in this experiment. Beef steers were randomly distributed into three homogenized groups (four beef steers each) for 14 days, namely, threshold (THI = 64–71), mild–moderate (THI = 72–79), and severe (THI = 80–87). Feed and water intake were recorded daily. Physiological parameters, including heart rate and rectal temperature, and behavioral patterns (standing and lying down) were measured weekly. Blood was sampled every week to analyze hormones, heat shock protein (HSP) levels, metabolites, and hematological parameters. All data were analyzed using repeated-measures analysis. Beef steers exposed to severe THI had significantly increased (p < 0.001) water intake, heart rate, and rectal temperature compared to the threshold and mild–moderate THI beef steers. Additionally, increased blood cortisol (p < 0.001), HSP70 (p < 0.001), blood urea nitrogen (BUN) (p = 0.014), and time spent standing (p < 0.001) were observed in beef steers after exposure to severe THI compared to beef steers in the threshold and mild–moderate THI groups. However, dry matter intake, blood glucose, and non-esterified fatty acids were not different among the THI groups. In conclusion, heart rate, rectal temperature, blood cortisol, HSP70, BUN, and time spent standing were closely associated with severe HS conditions in beef steers. These phenomena indicated that beef steers exposed to HS modulated their behavior and blood parameters, as well as their physiological response, to maintain homeostasis.