Dairy cattle are unable to dissipate their heat load efficiently. Their sweating mechanism is poor and they rely on respiration to cool themselves. A further disadvantage is the fermentation process within the rumen generates additional heat that cattle need to disperse. As they cannot get rid of heat effectively they accumulate a heat load during the day and dissipate heat at night when it is cooler.
When cows cannot dissipate their body heat, in addition to depressed feed consumption, heat stress has also been shown to have
an effect on milk production and composition including milk protein
and butterfat content.
Heat stress has a dramatic impact on feed consumption and milk production.
It is not only related to ambient temperature but also associated with humidity
and air movement. When the humidity increases the cows evapo-transpiration
is reduced and the animal cannot cool itself. This inability to cool itself
increases the core body temperature and greatly depresses feed intake.
Dairy Cattle Temperature Humidity Index
Temperature alone is not a good way of measuring heat stress. Various
indexes have been developed which take into account such factors as
ambient temperature, relative humidity and evaporation rate. These are
known as THI (Temperature Humidity Index).
Dairy Cattle Zone of Comfort
As this stress adaptation develops, diversionary activity within cells and
in their membranes escalates, to protect their structures and productive
machinery from heat damage. Cellular metabolism is disturbed and
membranes close down some of their operations altogether.
Visible signs of Heat Stress in Dairy Cattle
Invisible signs of heat stress in Dairy Cattle