Heat stress is a growing threat to the welfare, health and productivity of dairy cows, especially on the pasture. Commercial farms need simple indicators that enable farmers to detect heat stress at an early stage.
How can farmers detect that cows on pasture are getting too hot?
Heat stress leads to milk yield losses and jeopardises the health and wellbeing of the animals. Farmers should therefore reliably monitor grazing dairy cows in order to be able to react in a timely manner to heat stress. Heat-stressed cows exhibit different behavioural responses. Like all mammalian species, for example, cows reduce their feed intake to minimise heat production during digestion. Cows also adapt their behaviour by seeking out shade or cooler areas. Particularly in intensive grazing systems, however, shade is frequently unavailable and the animals are exposed to the sun. A research project examined the question of what behavioural patterns alert farmers when the cows become too hot in these conditions.
Heat-stressed cows stand closer together and hang around the trough
In an experiment with 38 dairy cows conducted at Agroscope’s Posieux site it was shown that the comprehensive climate index (CCI), which takes into account the ambient temperature, relative humidity, wind speed and solar radiation, is closely correlated with the body temperature of the cows. However, individual reactions to heat stress can vary considerably. Consequently, climate indices alone cannot reflect the actual heat stress that an individual cow is exposed to. As expected, cows with raised body temperatures consumed less forage and spent less time lying down. However, cows with heat stress were also more frequently observed near the trough, and decreased their distance to other cows. These behaviour patterns are relatively easy to spot on the pasture.
Study with commercial farms confirms heat-induced behaviour patterns in dairy cows
The behaviour of dairy cows on pasture in terms of heat load was then investigated under practical conditions on four commercial dairy farms in Switzerland with herd sizes of between 20 and 57 cows. Here too, very similar behaviour patterns to those in the experiment were observed. On days with a high temperature-humidity index (THI) the cows were seen more frequently near the trough in the morning than on days with a lower index; but not in the afternoon, when they would stand close together or in the shade. On days with a high THI, the cows spent less time lying down and increased their movement activity towards midday. Over the course of the day, they reduced their distances to each other. The hotter it was, the more they stood close together in groups. Consequently, deviations from the animals’ usual behaviour over the course of the day associated with less time spent lying, more movement and smaller distances between animals can be used to monitor heat stress in grazing dairy cows.
