Identification of Chronic Stress Biomarkers in Dairy Cows
Abstract Stress in dairy herds can occur from multiple sources. When stress becomes chronic because of a long duration and inability of animals to adapt, it is likely to affect the emotional state, health, immunity, fertility and milk production of cows. While assessing chronic stress in herds would be beneficial, no real consensus has emerged from the literature regarding the indicators of interest. The goal of this study was to compare and evaluate potential biomarkers for chronic stress after inducing stress over a 4-week period through severe overstocking, restricted access to feed and punctual unusual events. A total of 30 cows were involved in the experiment and two similar groups were constituted. Over a 4-week period, the 15 cows of the stress group were housed in overstocked conditions, with 4.6 m² per cow, including resting and feeding areas. In this area, only 7 individual places at the feeding area were available for the 15 cows to generate competition for feed access. Once weekly and during 2 hours, an additional stress was induced by moving cows to an unfamiliar barn and diffusion of stressing noises (dog barking). Meanwhile, the 15 cows of the control group stayed in the original barn, with more than 10 m² per cow and more individual places at the feeding area than cow number. On a weekly basis, several variables considered as potential biomarkers for chronic stress were recorded. Collected data were analyzed using single trait linear repeated mixed models. No differences were observed regarding milk yield, bodyweight of cows or body condition score but the milk loss was more pronounced in the stress group. Whereas no differences were observed in terms of activity between the groups, the standard deviation of activity per cow per week was higher and the rumination of cows was lower in the stress group. The heart rate was lower in the stress group and showed more heterogeneity at the end of the stress period. No differences were observed regarding salivary cortisol, blood glucose, β-endorphin, thyroxine and leucocyte profile. A higher level of hair cortisol and blood fructosamine were observed in the stress group at the end of the stress period. Among the variables highlighted, milk losses and fructosamine were not specific biomarkers for stress while heart rate variables were difficult to collect in practice, consequently the hair cortisol content seemed to be the most promising indicator to assess chronic stress at a large scale.
