Heart rate and diurnal rhythmicity in dairy calves at different ages

Long term heart rate recordings were taken at 5, 15, 40 and 60 days of age using a Polar Sport Tester on dairy calves, of the German Holstein Friesian and German Red Pied breeds, used for rearing. These observations were used to establish the number and duration of activity and rest periods. For the se periods, characteristic heart rate values were calculated (HFA and HFR) and analysed for three hour daytime periods. The HFA exhibited substantial inter-animal wariation and a high individual specificity. The mean HFA and HFR values were significantly different between the ages. The HFA and HFR r ecorded for the daytime periods changed regularly, but mean values were not significantly different between daytime periods. There were minor cyclical variations of the HFA around the four daytime periods when milk replacer w as provided which overlapped the circadian rhythm. Variation of the HFR wit hin the course of a day followed a circadian rhythm.