The behavioural and physiological responses of naive dairy heifers to noise that was pre-recorded from a commercial milking facility were examined. Thirty-two dairy heifers were individually moved through an indoor raceway three times daily over a period of 15 days. This period consisted of a pre-treatment, treatment and post-treatment phase, each of 5 days duration. Half (16) of the animals were exposed to the noise stimulus during the treatment phase and half were not exposed to the noise and were used as controls. Heart rate (HR), movement behaviour (transit times and latency to enter the raceway), and animal handling parameters (number of stops and handler interventions required) were recorded throughout the 15 days, and cortisol concentrations were measured on days 6 and 10. Data from the treatment phase were analysed as percentage change from pretreatment measurements for these variables (excluding cortisol). Overall in the treatment phase, exposure to noise resulted in greater (p < 0.05) increases in HR (on day I of noise exposure only) and faster (p < 0.05) transit times (on days 1-4 of noise exposure) compared to control heifers. There were no significant effects of noise on latency to enter the raceway, cortisol concentration or animal handling parameters. It was concluded that dairy heifers' reacted to the noise stimulus with responses consistent with an escape-based strategy (i.e., faster movement) but that this was not accompanied by a corresponding increase in physiological responses except during the initial exposure to noise. A second experiment assessed the relevance of including an adaptation phase (the pre-treatment phase in this experiment) in the methodology. Sixteen new heifers were exposed to the same raceway used in Experiment 1 for only 5 days with no prior exposure, with eight animals exposed to the same noise stimulus and eight control animals. The same variables were measured as in Experiment 1, and results showed no main effect of noise on any of the measured variables. Comparison of the pre-treatment phase of Experiment 1 and the control heifers in Experiment 2 revealed no significant differences between the two groups, indicating that there were no pre-existing differences between animals in the two experiments. However, when compared with the treatment phase of Experiment 1, noise-exposed heifers in Experiment 2 had higher HR, stopped more often, required more handler intervention, and recorded higher cortisol concentrations. These findings suggest that the absence of the pre-treatment phase resulted in a greater level of reactivity in noise-exposed heifers in Experiment 2 compared to those in Experiment 1, which appeared to mask effects of noise exposure on the measured variables. This result has implications for experimental design when assessing effects of environmental stimuli on animal responses. 2006 Elsevier B.V All rights reserved.
