Responses of beef calves to long-term heat stress exposure by evaluating growth performance, physiological, blood and behavioral parameters.

The objective of this study was to explore the responses of beef calves to long-term heat stress (HS) exposure at various levels in comparison with the animals under thermoneutral conditions by evaluating growth performance, physiological, blood, and behavioural parameters. Data were collected from sixteen beef calves (BW: 136.9 ± 6.23 kg; age: 169.6 ± 4.60 d) kept at four stress levels of designated temperature humidity index (THI): threshold (22-24 °C, 60%; THI = 70 to 73), mild (26-28 °C, 60%; THI = 74 to 76), moderate (29-31 °C, 80%; THI = 81 to 83), and severe (32-34 °C, 80%; THI = 89 to 91) stress levels in climatic controlled chambers. Feed and water intake were recorded daily, and body weight was measured once a week. Blood was sampled every three days to analyse metabolite parameters. Dry matter intake (DMI) (p = 0.069, tendency) and blood glucose levels (p = 0.028) were decreased after sudden exposure to HS conditions (severe THI levels). Also, blood cortisol (p = 0.002), glutamic-oxaloacetic transaminase (GOT) (p = 0.009), blood urea nitrogen (BUN) (p = 0.004) and standing time (p = 0.009) were increased in moderate and severe THI levels compared with threshold after exposure to HS conditions. However, in the severe THI group, blood cortisol (p < 0.05), glucose (p < 0.05), GOT (p < 0.05) and BUN (p < 0.05) levels were returned to normal range after 6-13 days of continuous HS exposure. In conclusion, DMI, blood cortisol, GOT, glucose, BUN, and standing time were closely associated with long-term HS condition in beef calves. In addition, calves exposed to HS modulated their physiological responses that resulted in the regulation of the pertinent blood metabolites in the blood to maintain homeostasis during the long-term HS.