Heat stress is defined as an increase of core body temperature above its normal range due to increased total heat load, exceeding the capacity for heat dissipation. Heat stress modifies feed intake, cow behaviour, maintenance requirements, and metabolic processes. This has tremendous consequences for feed efficiency, milk yields, reproductive efficiency, and disease incidence.
The combination of ambient temperature and humidity (THI) determines the level of heat stress.
Heat-stressed animals reduce their feed intake. This is presumably a survival strategy, especially in ruminants, as digesting and processing nutrients generates heat (i.e. thermic effects of feeding). It has traditionally been assumed that inadequate feed intake caused by the thermal load is responsible for decreased milk production (Fuquay, 1981; DeShazer et al., 2009).