Evaluation of earwax and hair cortisol concentration in relation to environmental stressors in Hanwoo cattle.

Global warming causes heat stress in livestock, impairing their health, welfare, and productivity. In bovines, chronic stress elevates cortisol levels; however, this response often goes undetected due to the lack of practical biomatrices for accurate assessment. Common biomatrices such as blood require repeated sampling that may affect measurement accuracy. This study compared earwax cortisol concentration (ECC) and hair cortisol concentration (HCC) in native Korean cattle (Hanwoo) to evaluate their usefulness in reflecting chronic environmental stress. A total of 68 earwax samples (left ear) and 68 hair samples (forehead) were collected monthly from April to July 2022 from 20 healthy cattle (mean age 30.2 ± 18.3 months). Cortisol was quantified using enzyme immunoassay kits. The temperature-humidity index (THI) reached 77 in July, indicating mild heat stress. A general linear model with Tukey's test, and Pearson correlation analysis were used to assess relationships among ECC, HCC, and THI. Average ECC and HCC were 29.02 ± 11.5 and 6.8 ± 4.9 pg/mg, respectively. ECC was higher in July than in April and June (p < 0.05) and in May than in June (p < 0.05). HCC was higher in July than in April, May, and June (p < 0.05). HCC showed differences among age groups (up to 12, 12-25, and above 25 months; p < 0.05), while ECC showed no significant age-related differences. ECC and HCC were moderately correlated (r = 0.3, p < 0.05), and both were correlated with THI (r = 0.3 and r = 0.2, respectively; p < 0.05). Overall, earwax and hair cortisol are practical biomarkers for assessing chronic heat stress, although variations in earwax production may affect the reliability of ECC.