Heart rate variability: a noninvasive approach to measure stress in calves and cows.

The aim of this study was to evaluate the usefulness of heart rate variability (HRV) and its specific parameters as a new approach to assess stress load in cattle. We recorded HRV in 52 calves in three groups and in 31 cows in two groups. In calves we divided Group 1 with no obvious stress load (n=18), Group 2 with external stress load (n=17), and Group 3 with internal stress load from sickness (n=17). In cattle we divided lactating cows (n=21) and nonlactating cows (n=10). HRV parameters were analyzed in the time domain and in the frequency domain. Moreover, we applied Recurrence Quantification Analysis (RQA) to quantify nonlinear components of HRV. In calves, linear HRV parameter decreased from Group 1 to Group 3 (P<.05). However, not a single parameter showed significant differences regarding all three groups. The value of all nonlinear measurements increased at the same time (P<.05). The only parameter that exhibited significant differences between all three groups was the longest diagonal line segment in the recurrence plot (L(MAX)) which is inversely related to the Lyapunov exponent. We did not find differences concerning the linear HRV parameters between the two groups in the cows. The nonlinear parameter Determinism showed significant higher values in lactating cows compared to nonlactating cows. The importance of particular HRV-parameters was tested by applying a discriminant analysis approach. In calves and cattle nonlinear parameters were most important to indicate the level of stress load on the animals. Based on the results we assume HRV to be a valuable physiological indicator for stress load in animals. Whereas linear parameters of HRV are supposed to be useful to separate qualitative different level of stress, nonlinear components of HRV distinguish quantitative different challenges for the animals.