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

Grass growth remains below last year

The latest data from Trouw International show that, thanks to recent rainfall, average GB grass growth did increase last week from 46kg DM/ha/day to 53. However, it remains below last year. Within our trading area the range in growth rates is from 62kg in Scotland to 49 in Yorkshire.

The average milk from grass is 12.4 litres, both energy and protein content have lifted as grassland responds to fertiliser washed in by the rain. Typical analysis is 11.8ME, 25% crude protein and 17% dry matter, although as usual there was a wide range and we would recommend contacting your local Carrs Billington nutritionist to take a sample of your grazing paddocks.

Protect your cows from heat stress this Summer


Grass growth falls but potential milk from forage rises.

The average GB grass growth rates, produced by Trouw Nutrition, fell to 59kg DM/ha/day last week. This is 10kg down on the previous week and is below the same time last year. Scotland and North West England saw growth rates of 67/68kg DM/ha/day and Yorkshire 54kg.

On the plus side, high temperatures increased the average dry matter of grass back up to 19%, which will have improved potential intakes. With a typical energy content of 11.8 ME this means, on average, grazing could support M + 14 litres although in many places this potential will have been offset by very high temperatures leading to heat stress in cows and reducing forage intakes.

Reasonable levels of fibre in grass and a low acid loading have reduced the risk of low butterfats but, as above, this will have been offset by potential heat stress.

In the USA and Canada, cows are housed to help protect them from heat stress in summer, the barns are equipped with fans which come on at 68 degrees F (20 degrees C) and many have water sprinklers in the feed passages which come on at 72 F (22 degrees C) Last week there will have been many cows in GB grazing outdoors in temperatures of 86 F (30 degrees C). I must emphasise that we are not saying cows should be kept indoors, just that when grazed outdoors in high temperatures, shade and good access to water are very important for cow health and welfare as well as production.