How Do Grazing Dairy Cows Respond Physiologically to Heat Stress?

Even in temperate climate zones, an increase in the ambient temperature and solar radiation can cause heat stress in grazing dairy cows. Agroscope studied the physiological changes in cows caused by increasing heat load.

Society’s desire for sustainable animal production that takes account of natural animal behaviour is constantly increasing. Pasture-based milk production systems would seem to be ideal for fulfilling this desire. Not only is pasture-based milk production cost-efficient, but it also helps reduce feed-food competition. However, grazing cows are particularly susceptible to heat stress, since they are exposed both to the increasing ambient temperature and to direct solar radiation.

Body temperature, blood and milk of grazing dairy cows monitored

Over two consecutive summers, 38 lactating Holstein dairy cows were studied in a full-time grazing system. Data were collected during ten experimental periods of up to three consecutive days with temperatures in the 13-to-24 °C range. Sensors were used to monitor the vaginal temperatures of the cows as a measure of body temperature as well as to monitor heart rate. Blood and milk samples were collected at afternoon milking. Fat, protein, lactose, urea nitrogen, cortisol, Na⁺, K⁺ und Cl^– concentrations were analysed in milk, and beta-hydroxybutyrate, glucose, non-esterified free fatty acids, urea nitrogen, thyroxine and triiodothyronine were analysed in blood.

Body temperature follows a daily pattern

The body temperatures of the cows changed markedly over the course of the day and rose with increasing heat load. Cows with a vaginal temperature over 39° C were assumed to be suffering from heat stress (fig. 1). Such cows exhibited an elevated average heart rate and increased plasma glucose and milk cortisol concentrations, as well as decreased plasma thyroxine and plasma triiodothyronine concentrations. Na+ concentrations in milk were lower and K+ concentrations in milk tended to be higher in cows with higher body temperatures. Urea nitrogen and protein concentrations in milk varied greatly, and inter-animal variance was high.

Na⁺ and cortisol levels in milk can serve as possible heat-stress indicators

In summary, it can be said that the changes observed in the milk and blood probably indicate the short-term physiological responses to moderate heat stress. In particular, milk cortisol and Na+ could be useful traits for the timely monitoring of heat stress in individual cows, since their inter-individual variances are relatively small and samples can be taken non-invasively.