Heat Stress - How common is it?

Jun 17, 2020

Independent veterinary consultant Dr. Tom Chamberlain explains why heat stress is a significant issue in dairy herds.
Heat Stress is a growing issue for cattle, especially high yielding dairy cows. The temperature at which cows start to suffer is affected by their genetics and the environment they were born and raised in. Recent research has shown that cows in northern Europe can start to feel stressed at much lower temperatures – anything higher than an average daily temperature of around 18°C.
The extent of heat stress is influenced by both the air temperature and the humidity, but our cow sheds are continually humid from wet passageways, etc. so the average air temperature is the dominant factor.

Which cows are affected?

A lot of heat is generated as cows convert what they eat into milk and the higher the yields the higher the risk of heat stress.
Dry cows can also be affected as they are on high fibre diets that release a lot of heat as they are digested.
How can I tell if my cows are affected?
An early sign of heat stress is that cows stand up more as this helps them better lose heat. Rumen health and feed intake suffer and milk production declines as these deteriorate. The extra standing can lead to lameness problems and with time fertility is also affected. The problems are worse when overnight temperatures are also high as then the cow does not have an opportunity to lose her excess body heat.

What losses am I getting?

When we measured heat stress over a range of British dairy farms in the summer of 2014 the production losses ranged from 40 to 100 litres/cow over the three hottest months.

What can I do about it?

Cows lose heat through their skin so avoid over crowding them – especially in the collecting yard – to allow air to circulate. Winds over the cow’s back help and wind speeds of 4 mph are usually enough to mitigate the effects of heat stress in British dairy herds. Where possible winds should be directed over the cubicles and the feed areas as this is where cows spend most of their time. Misting with water only works when the humidity is low which is not common in the UK.
Hot cows will drool and pant more so make sure there is ample, preferably, cool water available and the diet is formulated with adequate minerals to make up for additional losses in the saliva.
More heat is generated when fibrous feeds are digested so lowering the fibre (NDF) content of the ration will help. However, this puts an already heat stressed rumen under even more pressure. Feeding supplements that are well-documented to support rumen function such as the proven rumen modifier, Saccharomyces cerevisiae CNCM I-1077 in these situations will help.

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