Heat Stress in Dairy Cows


Given the record high temperatures recently, it’s no surprise that animals are struggling to cope with the heat as well as ourselves.  Temperatures have been climbing again, and we may still have the hotter conditions in July and August.  Heat stress may be something that will become more common in the UK with hotter summers as a result of global warming.  There is also now a greater proportion of the UK dairy herd being housed all year round in sub-optimal buildings.  Breeding also plays a part with today’s dairy cows being bred for higher production which results in higher metabolic rates that generates more heat.  Cows in the UK are also not acclimatised to high temperatures and humidity as those in hotter countries.

Heat stress affects the metabolic and immune system of dairy cows when the core body temperature is raised above its normal range.  It is the result of high temperatures coupled with high relative humidity.  We can monitor if cows are likely to be at risk by measuring the temperature and relative humidity to get a Temperature Humidity Index (THI).  Heat Stress is believed to occur when a THI of 72 is reached.  This is when cows are unable to dissipate heat quickly enough to keep the core body temperature down.

Visual signs that a cow is suffering from heat stress include; panting, standing for longer periods, sweating, diet sorting and decreased frequency of eating.  These factors then result in depressed feed intakes which in turn means that the cow is not able to consume enough nutrients to meet her energy and protein requirements.  Saliva production is also suppressed, reducing the buffering capacity to lower rumen pH.  In terms of performance, this could result in lower milk yields, poorer display of heat, loss of embryos and a compromised uterine environment.

So how can we tackle heat stress?  This can be split in to two categories – environment & management or nutrition.

Environment & Management

  • provide shade for cows grazing outdoors
  • for housed cows – ensure adequate air flow, either through building modification or installing fans.
  • avoid overcrowding in collecting yards.
  • hosing down individual cows that are in a critical state of distress.



  • Water! Ensure adequate availability and cleanliness.  Cows drink between 10 – 20% more in hot weather.
  • Increase the nutrient density of the diet to compensate for the lower dry matter intakes.  If increasing concentrate inclusion, be careful of acidosis.
  • Reduce the fibre content – digestion of fibre creates more heat than concentrates.  *However, a minimum level of high quality fibre in the diet is still essential.
  • Increase the by-pass protein content of the diet, as rumen function may be impaired.
  • Review the mineral content of the diet.  Cows will lose more minerals when drooling and sweating – sodium in particular.
  • Feeding rumen enhancers, such as Actisaf live yeast, Carrs Billington’s TechTonic, AcidBuf or sodium bicarbonate.  These will all help maintain healthy rumen function.


Lowri Davies
Ruminant Technical Specialist