Constant traffic noise affects the behavior and physiology of naïve horses at pasture

Introduction: Horses are frequently kept on pastures near major roads, exposing them to high levels of traffic noise. In humans, prolonged traffic noise exposure is associated with physiological stress, sleep disruption, and adverse health effects. Despite horses’ high auditory sensitivity, comparable data on the behavioral and welfare implications of continuous traffic noise are scarce. This study investigated short-term behavioral and physiological responses of horses naïve to constant motorway noise in their home environment. Methods: Twenty horses housed in four stable groups across three facilities were exposed to recorded motorway noise for 1–2 weeks. Emitted noise levels ranged from 54 to 71 dB, corresponding to distances of approximately 500 to 10 m from a four-lane motorway, and were played during daytime for up to 11 h/day using portable outdoor loudspeakers. Control periods without motorway noise (average ambient noise level: 38 dB) before and between exposure days served as baseline, intermediate washout, and final control phases. Behavioral parameters were recorded throughout the day using focal and scan samplings. Heart rate was continuously monitored as a physiological indicator of stress. Results: During noise exposure, horses reduced foraging behavior from a median of 81% at baseline to 41% during the first exposure phase (p<0.001), while vigilance increased from nine to 54 instances per day (p<0.001). Horses also avoided the vicinity of the noise source (p<0.001). Behavioral effects were pronounced at higher noise levels, with clear avoidance responses suggesting aversion as an active coping strategy. Heart rate increased within the first exposure phase, rising from an average of 57.60 bpm to 66.76 bpm (p<0.001), suggesting an acute stress response. Discussion: Although some behavioral habituation was observed over time, spatial avoidance of the noise source persisted, and elevated heart rates indicated continued physiological arousal. Even noise levels of a 500 m distant motorway (≥54 dB) altered behavior and physiology. These findings demonstrate that traffic noise represents a relevant environmental stressor for horses and may compromise welfare when exposure is frequent or prolonged. The study highlights the need to consider noise in equine management and supports mitigation strategies such as providing large pastures with quiet retreat zones or installing noise barriers.