Physiological and Behavioral Reactions of Simmental Dairy Cows to Increasing Heat Load on Pasture

Abstract

Climate change is accompanied by temperatures exceeding the thermal comfort zone of dairy cows, resulting in numerous consequences for production and welfare. Early detection of heat load enables taking countermeasures and can be realized using monitoring systems. We aimed at investigating heat load-induced changes in the behavior and physiology of grazing Simmental cows. Data were collected on five (round 1; r1) and eight (round 2; r2) consecutive days on a dairy farm combining a freestall barn with permanent access to pasture. Weather data, respiration rate, milk yield, milk composition, and fecal cortisol of eleven (r1) and thirteen (r2) cows were collected. The behavior of five animals was recorded with collar-based monitoring systems. Previously reported thresholds of different weather indices were exceeded on two days in r1 and on four days in r2. Under heat load, respiration rate and somatic cell count increased. Fecal cortisol did not change in r1 but increased steadily in r2. Grazing time decreased in r1. Rumination mainly occurred at night in both rounds, and its synchrony decreased in r2. Although limited by sample size, our results give a first impression on heat load induced physiological and behavioral reactions of Simmental dairy cows on pasture.