Cortisol is an indicator of an animal's endocrine response to environmental changes. This study was designed to examine changes in hair cortisol levels produced in response to a change from indoor winter to summer grazing conditions in dairy cows. The study population comprised 83 dairy cows from three farms. Hair samples were obtained using clippers from the animal's forehead at the time points: 7 (P1), 40 (P2) and 70 days (P3) after the start of summer grazing. At these same time points, milk yield, milk fat and somatic cell count (SCC) and body condition score (BCS) were determined in each animal. No effects of farm were detected on these factors. However, hair cortisol levels increased from P1 to P2 (P<0.01) and then remained constant from P2 to P3 (P>0.05). Transfer from the valley farm to summer pastures and the change in environment and nutrition modified hair cortisol concentrations (estimated marginal means +/- standard error), which were 2.1 +/- 0.10, 2.9 +/- 0.17 and 2.6 +/- 0.13 pg/mg for P1, P2 and P3, respectively. In addition, during summer grazing we observed a reduction in milk yield, an increase in SCC and general worsening of BCS. No significant correlation was detected between hair cortisol level and the other factors considered except for fat-corrected milk (r=-0.144, P=0.025). The rise in hair cortisol levels observed during the first month of summer grazing may reflect hypothalamic-pituitary-adrenal axis responses related to the change from winter indoor to high mountain conditions, though the low cortisol levels detected suggest minimal involvement of this axis along with the well-being of the animals.
