Hair cortisol concentration in postpartum dairy cows and its association with parameters of milk production.

Hair cortisol concentration (HCC) is considered as an indicator for a minimally invasive assessment of long-term stress. In dairy cows, in addition to stress influences, changing physiological conditions during gestation and lactation (eg, due to varying energy requirements or fluctuating milk yield) may affect HCCs. Thus, the aim of our study was to investigate HCCs of dairy cows during different stages of lactation and to determine the relationship between milk production traits and hair cortisol levels. Samples of natural hair and regrown hair were collected from 41 multiparous Holstein Friesian cows at 100-d intervals from parturition to 300 d postpartum. All samples were analyzed for cortisol concentration and the association of HCC with milk productions traits was evaluated. Our results show that cortisol concentration in natural hair increased after parturition and was highest 200 d postpartum. Cumulative milk yield from parturition to 300 d showed moderate and positive correlation with HCC in natural hair at 300 d. There was a positive correlation between urea concentration in milk and cortisol levels in regrown hair at 200 d, and between somatic cell count in milk and HCC in natural and regrown hairs 200 d postpartum. Together, these findings suggest that physiological loads during lactation, eg, caused by metabolic stress and/or inflammation, may be associated with increased HCC levels. In addition, the results on hair color confirm previous findings in cattle that black hair has higher cortisol concentrations than white hair. Black hair therefore appears to be more suitable for hair cortisol analysis as it provides higher protection against photodegradation.