Coat color affects cortisol and serotonin levels in the serum and hairs of Holstein dairy cows exposed to cold winter.

Previous studies showed that coat color significantly affects hair cortisol levels but not serum cortisol and serotonin levels in heifers and dairy cows under heat stress conditions. As a follow-up study, we tested whether both serum and hair cortisol and serotonin levels in winter-housed dairy cows were affected by coat color (black vs white). Twenty multiparous high-yielding dairy cows (DIM = 110 ± 25, milk yield = 35 ± 2.1 kg) were assigned to one of the following groups: 1) black coat color (BCC; over 85% of coat black, n = 14) and 2) white coat color (WCC; over 85% of coat white, n = 6). The experimental period lasted 60 d (from December to February), during which the animals were kept in a building with free stalls that had dry river sand bedding. Blood was harvested for 3 consecutive days at the end of the study. Hair was harvested from the forehead of each individual at the beginning and on the end-day of the study. Cortisol and serotonin hormones were measured in serum and hair. Data were analyzed using t-test. The results revealed that cows with BCC had lower hair cortisol and higher hair serotonin levels than those with WCC (P < 0.05). No differences in serum cortisol levels were observed between cows with WCC and BCC (P > 0.05). The serum serotonin level was lower in cows with WCC compared to the BCC group (P < 0.05). Taken together, coat color significantly affected stress levels indicated by higher hair cortisol and lower hair serotonin levels in WCC cows in addition to lower serum serotonin levels in the corresponding animals. This feature of coat color should be considered as a management tool, particularly in countries with long winters.