Hair cortisol as a biomarker of chronic stress in periparturient dairy cattle: associations with health status, lactation, and metabolic profiles.

Dairy cows experience long-term stress during the pre- and postpartum period, as during this period they face extreme challenges related to their health, productivity and overall welfare. Recently, hair cortisol has gained attention as one of the most promising and non-invasive biomarkers for evaluating stress. However, the exact mechanism of its deposition in the hair remains unknown. This review combines the utility of hair cortisol as a tool for evaluating chronic stress and its effect on health status, lactation, parity, body condition score (BCS), and metabolic parameters, specifically glucose, β-hydroxybutyrate (BHBA), and non-esterified fatty acids in dairy cows. Emphasis is given to the transition period around parturition, where hormonal fluctuation, negative energy balance, and immunosuppression increase the animal's susceptibility to stress. Methodologically, tail hair exhibits a higher cortisol concentration than other regions due to a faster growth rate, making it an ideal site for maintaining a proper sampling interval during the transition period. Furthermore, regrown hair contains higher cortisol levels than native hair during parturition. In addition, hair cortisol level significantly elevated during peak lactation in multiparous cows and in cows with oversized or twin calves. Notably, newborn calves have higher cortisol than their dam during birth. Cows diagnosed with a disease condition display higher cortisol levels than healthy cows during the postpartum period. During the transition period, Glucose was found to be positively correlated with hair cortisol level, whereas β-hydroxybutyrate (BHBA) was negatively correlated with hair cortisol in multiparous cows. Ultimately, evaluating hair cortisol concentration alongside associated production and metabolic factors during the periparturient period can provide a proper framework for improving and monitoring dairy cattle welfare. However, further research is required on hair cortisol concentration and its connection with various stressors of the transition period, sampling and analytical methods, and a standardized reference range so that it can have practical use at the farm level.