When, like much of the farming community, you live, work and breathe livestock, seeing an animal in distress can be worrying for all. So for UK and Irish dairy farmers in a climate that experiences increasing extremes, heat stress in your cows is a very real and growing concern. 

So what is heat stress, and why is it such an issue? 

If you’re farming in the UK or Ireland, you will have noticed our weather becoming more extreme, with some areas being more affected than others. Lately, we’ve been experiencing particularly warm, if not hot, springs and scorching summers, with very little rain. And the repercussions are twofold: the direct implications of heat on the cow and the impact on the availability of forage and feed intake. 

Due to the relatively high humidity felt in the UK, the impact of heat on a cow can begin to occur at much lower temperatures than you might think. So it’s not uncommon for dairy farmers to run into problems in the low- to mid-20oC‘s – let alone when the mercury rises to 30oC plus. 

It seems cows aren’t much different to ourselves when it comes to time spent in the heat of the sun. They can become dehydrated, lethargic and lacking much of an appetite by the end of the day. A reduced appetite will also mean reduced dry matter intake, which is important to rectify as while the season progresses, dry matter and grasses will become less readily available. 


What are the effects of heat stress on our dairy herds? 

On a sliding scale, depending on the severity of the situation, cows will experience the general discomfort that we also experience in the heat. They will find difficulty regulating their body temperature and breathing rate, and their panting will increase as they attempt to lose heat.  This can increase a cows’ maintenance requirements by up to 25%.  

As a result of reduced energy available for production, dairy farmers will also see a decline in productivity, with milk yields dropping by anything from 0.5 – 4 litres and fertility rates reducing by 5%. While those temperatures in the low to mid-20s might only result in a 0.5-1 litre drop in milk production, the cumulative impact over three months is really quite substantial. In hotter climates the effects on milk production and fertility can be considerably more extreme. 

Secondary issues arising from heat stress can include Ketosis, increased “days open”, and even a decrease in colostrum quality. 


How to recognise heat stress 

Behavioural changes and fluctuations in productivity are key signs of heat stress in your dairy cows. With this in mind, you should look out for particular signs, such as: 


  • Higher than usual breathing rate. 
  • Open-mouth breathing/panting progressing to laboured breathing. 
  • Drooling progressing to foaming at the mouth. 
  • Restlessness/reluctance to lie down. 
  • Tongue protruding. 
  • Lying down/unresponsiveness. 



  • Reduced milk yields. 
  • Reduced fertility. 

 During prolonged high temperatures (20oC+), it is essential to keep a closer than usual eye on your dairy herd. Heat stress can quickly turn into heatstroke, whereby cows cannot stand and can become unresponsive. While completely reversible, it is often fatal if left untreated. 


How can you prevent and treat heat stress in your dairy cow? 

As with many situations regarding the management of livestock, in the case of heat stress, prevention is definitely better than cure. There are a number of simple steps when it comes to the husbandry of your dairy herd, as well as dietary adjustments that you can make to reduce the occurrence and severity of heat stress. 


Increasing the energy density in the diet is vital for protecting your herd during periods where the consumption of dry matter is in decline, but it is essential to do this in the right way. 

Simply swapping lower-energy forage for highly fermentable cereals will indeed increase the energy density on paper. However, the reality is that the increased starch will lead to raised acidity in an already struggling rumen. This could result in acidosis, which may then lead to lameness and reduced milk fat production. 

Instead, high-quality forage should be offered to the highest yielding cows under the most stress, while highly digestible fibre in the form of sugarbeet pulp will help maintain production. 

Furthermore, Dr Richard Kirkland, Global Technical Manager for Volac Wilmar Feed Ingredients, recommends, “Supplementing diets with energy-dense nutrients like rumen-protected fats is the most effective way to help meet energy requirements while not adding to the acid load in the rumen like starchy sources of energy.” 

 Fat is considered a ’cool‘ ingredient - generating very little internal heat production during digestion and metabolism. However, dairy farmers should consider the most appropriate type of fat supplement based on the fatty acid profile to best suit their needs and targets. 

Richard goes on to explain, “Fats are highly energetic, but it is the fatty acid make-up of the fat that will determine how the animal responds to the additional energy supplied.” 

To summarise: 

Oleic acid (C18:1) helps improve body condition, development of eggs and embryos and total diet digestibility. 

Palmitic acid (C16:0) is effective at promoting milk fat production and may be of interest under heat stress conditions. However, it can also be responsible for partitioning nutrients away from body fat, resulting in a reduced body condition score. 


There are many steps you can take in the management of your dairy herd to minimise the impact and occurrence of heat stress in your cows. These include: 

  •  Increased provision of clean, cool drinking water – Just like us, our cows can go through gallons of H20 on a hot summer’s day.  Position your water sources at the exits to your milking parlour and in shaded areas across your fields to help keep cows and water cool. 
  • Improved ventilation in your housing – just as we fling open every available window and door in the house—and may even stick on a fan—ventilation is key. Removing the ridge of the roof of your barn can be a very effective way of allowing the moisture created by evaporative cooling to escape. The installation of fans to keep the air moving is another worthy consideration. 

 Did you know: According to studies in the US, airflows as low as 10 km per hour can reduce respiration rates in heat-stressed animals by as much as 50%.* 

  • Feeding at cooler times of day - digestion is heating and usually peaks a few hours after feeding, so providing feed in the late afternoon means that the ambient temperature should have cooled before the digestive heat peaks. 
  • Milking in the early morning and late evening – The last thing anyone wants is to be bunched up, queuing or herded in the heat of the day, and your cows are no exception. Keep movement minimal during the heat of the day. 

*source: https://nadis.org.uk/disease-a-z/cattle/managing-heat-stress-in-dairy-cows/ 

May your summer be filled with just the right amount of sun... 

Us Brits are never happy when it comes to the weather, but in this case, for the sake of our cows, we hope you have a sunny and warm—but not too hot—summer where productivity is high and heat stress is low. 

And on the subject of heat – a lesser-known dairy farming fact to leave you with: 

Did you know: An average cow produces the same heat output as a 1.4kW electric heater.  A cow yielding 18 litres per day will generate 28% more body heat than a dry cow. A cow yielding 31 litres a day produces 48% more body heat than a dry cow.  

We hope this advice will help you and your herd enjoy a perfect summer! How do you manage the summer heat on your dairy farm? Share your thoughts and get some insight of your own with our farming communities on Facebook and Twitter.

You may also be interested in: 

>> Recognising and preventing heat stress in dairy cattle

>> Cow cooling: 7 ways to keep your dairy herd healthy this summer