Heat stress occurs when animals are exposed to environmental temperatures in excess of
25°C (the upper critical temperature), particularly in combination with high relative humidity
or sunshine. High humidity makes the sweating mechanism relatively ineffective, thereby
making cattle unable to maintain their core body temperature. Affected cows attempt to
reduce heat load by reducing exercise, feed intake and lactation. They actively seek shade
and wet areas. As their body temperature rises animals become agitated and distressed,
have laboured open-mouth breathing and eventually collapse, convulse and die. Heat
stress that is not life-threatening leads to reduced milk production and impaired
reproductive performance, and may predispose amongst others to subclinical acidosis.
Treatment of severely affecled animals is by COOling with cold water and/or fans. Prevention
is by providing good-quality drinking water and shade (natural or artificial), and the use Of
water sprinklers and/or fans. Changes to the diet (i.e. high energy density and low protein)
are also beneficial and often implemented. However, there may be some potential risks
associated with the nutritional management of heat stress in dairy cattle; i.e. the animals
are at increased risk of developing subacute rumen acidosis, with ensuing
laminitis/lameness, and displaced abomasum. The first part of this paper provides a brief
review of heat stress in dairy cattle. The second part discusses how increasing the energy
density of the diet (i.e. increasing the grain/forage ratio), as part of the nutritional
management of heat stress, may put the cows at greater risk of the above mentioned
digestive disorders.