What causes heat stress?

Cows produce a lot of body heat due to high metabolic performance; with increasing milk yield, body heat production increases (approx. 31 percent of the energy absorbed is converted into heat). The dairy cow feels most comfortable in a range of four to 16 degrees. Here the cow does not have to regulate its body temperature.

At more than 16 degrees, the cow releases body heat via evaporation through increased breathing or sweating.

The relative humidity of the environment also has a limiting effect. The cow can no longer give off its body heat to a sufficient extent because the air temperature and/or relative humidity in its environment is too high.

How does heat stress and its consequences manifest themselves?

• The cow tries to give off heat by increased breathing (evaporative cooling) → Respiratory frequency rises far above physiological normal range (danger of respiratory alkalosis: this leads to a disturbance of the acid-base ratio, which can result in increased susceptibility to disease).
• In order to reduce the own body heat, the cow eats less; thus the milk yield also decreases
• The cow drinks more: for each temperature increase of one degree, about 1.2 kg more water is absorbed
• Body temperature increases overall (hyperthermia)
• Due to increased body temperature and stress-related increase in blood cortisol concentration → Reduced reproductive conduction (abnormal cycles, reduced fertility, increased embryonic mortality)
• Change in behaviour → active search for "more pleasant" places, reduced lying times, as less body surface is available for heat dissipation when lying down

Sources:

https://en.engormix.com/dairy-cattle/articles/live-yeast-dairy-production-t35335.htm

https://www.lfl.bayern.de/ilt/tierhaltung/rinder/106802/index.php

https://www.youtube.com/watch?time_continue=6&v=DpkCHGwDbfE&feature=emb_logo