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In the course of predicted climate change, the welfare of dairy cows and heat load to which they are exposed have become increasingly important even under moderate climate conditions. The objective of this study was to investigate the cow individual activity response to heat load in terms of the heat load duration and intensity in lactating, high-yielding Holstein-Friesian cows in a moderate climate zone.

The study was conducted from June 2015 to April 2017 in a naturally ventilated barn in Brandenburg, Germany. The determined temperature-humidity index (THI) inside the barn was used to define the heat load. The heat load was characterized by the average daily THI as well as the duration and intensity of the defined THI levels. In addition to the heat load on the measurement day, we studied the cow individual activity response to the heat load accumulated over the three days preceding the measurement day. The activity of the cows (n = 196) was measured by accelerometers and described the resting behavior and the number of steps per cow and day. The analysis models included autocorrelations in time series as well as individual cow factors.

An increase in the duration and intensity of heat load on the measurement day led to a decrease in the lying time and an increase in the number of steps. The cows showed a reduced activity response to heat load when there was additional heat load accumulation over the three days preceding the measurement day. The cows in an advanced stage of lactation were more sensitive to heat load than cows in the early lactation stage. Multiparous cows showed less pronounced activity responses than primiparous cows. Heat load accumulation and individual cow-related factors should be considered in prediction models for the sensitive animal-specific recognition of heat load on the basis of activity responses.