Effect of artificial shade on saliva cortisol concentrations of heat-stressed dairy calves

Responses to heat stress have not been evaluated in dairy cattle by noninvasive techniques such as analysis of saliva cortisol concentrations. The aim of the present study was the assessment of saliva cortisol levels in Holstein bull calves with (n = 8) or without supplemental shade (n = 8) in response to acute heat stress. Measurements were carried out during a 5-d period [temperature, average/max (°C); day 1 (control, all calves shaded): 22.9/29.4, day 2 (heat stress day): 28.3/38.8, day 3: 26.2/33.5, day 4: 23.7/28.7, and day 5: 21.2/24.7]. The level of thermal stress was characterized with a temperature-humidity index (THI). Saliva cortisol levels did not differ between groups during the control day. On the heat stress day, saliva cortisol levels increased from 8:00 to 12:00 by 51% and 342% in shaded and nonshaded calves, respectively, and nonshaded calves showed higher cortisol concentrations at 12:00, 16:00, and 24:00. Saliva cortisol levels peaked at 12:00 on day 3 in both groups. On days 4 and 5, saliva cortisol did not show significant daytime elevations in either group; however, group differences remained significant until 20:00 on day 4. Based on our results, measurement of saliva cortisol concentrations is a promising approach to detect acute heat stress in dairy calves, which could be reduced by artificial shading.