Heart rate variability in dairy cows—influences of breed and milking system K. Hagen a, * , J. Langbein b , C. Schmied a , D. Lexer a , S. Waiblinger a a University of Veterinary Medicine Vienna, Department of Veterinary Public Health and Food Science, Institute of Animal Husbandry and Animal Welfare, Veterina ¨rplatz 1, A-1210 Vienna, Austria b Research Institute for the Biology of Farm Animals, Research Unit Behavioural Physiology, Wilhelm-Stahl-Allee 2, D-18196 Dummerstorf, Germany Received 9 January 2005; received in revised form 16 March 2005; accepted 30 March 2005 Abstract Heart rate variability parameters in the time, frequency and nonlinear domains were investigated in two breeds of dairy cows (Austrian Simmental and Brown Swiss) milked either in an automatic milking system with partially forced cow traffic or in a herringbone milking parlour. Recordings were made of 24 cows (six of each breed and milking system) during lying, standing idle, and standing being milked, and analysed with linear mixed effects models taking the covariates time of day, live body weight, milk yield, stage of lactation and stage of pregnancy into account. Heart rate and nonlinear deterministic shares were higher, and heart rate variability in the time and frequency domains was lower, later in the day, in cows with higher body weight and in Simmental compared to Brown Swiss cows. Differences in the linear and nonlinear domains during lying indicated an increased level of chronic stress in cows in the automatic milking system with partially forced cow traffic, compared to cows milked in the herringbone milking parlour. No effects of milking system were found during milking, indicating that the stressor in the automatic milking system was not the milking process itself. D 2005 Elsevier Inc. All rights reserved. Keywords: Cattle; Dairy; Bovine; Heart rate variability; Automatic milking; Breeds 1. Introduction Measures of cardiovascular parameters including heart rate (HR) have a long tradition as indicators of health and welfare in many vertebrate species [1]. Interpretations have often been based on the assumption that HR reflects the activity of the sympathetic branch of the autonomic nervous system (ANS) and therefore is an indicator of the sympatho- adrenomedullary stress response [2]. However, the heart is also under parasympathetic control, and HR at every point in time is the net effect of the non-additive regulatory functions of the interacting antagonistic branches of the ANS [3,4]. As a consequence of the ongoing regulatory mechanisms, HR is never constant but varies from beat to beat even in the absence of physical or psychological stress. This beat-to- beat variability is referred to as HR variability (HRV) or heart period variability. Methods for HRV analysis on the basis of interbeat interval (IBI) measurements have been developed in cardiology and psychophysiology since the 1960s and allow for more detailed interpretation of cardiac activity in terms of ANS activity [5]. Frequency domain methods for HRV analysis quantify sympatho-vagal balance through identification of cyclic components of variability by polynomial or power spectral time series analysis. High-frequency cyclic components corresponding to the respiratory frequency of the organism, also known as respiratory sinus arrhythmia, reflect primarily vagal tone [6,7], whereas lower frequency components are influenced by both branches of the ANS [8]. Time domain methods of HRV analysis do not take the order of measurements into account and therefore do not identify cyclic or deterministic processes. They may nevertheless allow a distinction between predominantly vagally mediated 0031-9384/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.physbeh.2005.03.019 * Corresponding author. Tel.: +49 177 6007047; fax: +43 1 25077 4990. E-mail address: Kristin.Hagen@vu-wien.ac.at (K. Hagen). Physiology & Behavior 85 (2005) 195 – 204