Heat stress

Pensverzuring melkvee

Heat stress in dairy cattle

Heat stress in dairy cattle is a common problem when temperatures rise and humidity is high. This can lead to:

  • decrease in dry matter intake;
  • watery manure and scouring;
  • lower milk production;
  • mortality in the most extreme case.

One of the most important measures is to ensure good ventilation in the barn and clean and sufficient drinking water in a cool place.

How do you maintain your dairy cattle’s health and productivity, even during the hot summer? One of the most important measures is to ensure good ventilation in the barn and clean and sufficient drinking water in a cool place. Adjusting the dairy cattle ration with your feed advisor is also advisable. In this article:


The most commonly used indicator to determine heat stress in dairy cattle is Temperature. In 2018, Wageningen University (WUR) published a report on heat stress in dairy cattle during grazing in moderate climate regions. The report states that negative effects generally start at a THI between 60 to 70.

From a THI of around 55 to 60, dry matter intake in dairy cows decreases. And from a THI of 76, utilization shifts from fat to carbohydrates in the ration. Water intake increases linearly from a THI of 35.

The higher the THI, the more significant the drop in milk production and solids. The decline is most significant in dairy cattle with the highest daily milk production. Heifers are even more sensitive than cows from the second or third lactation onwards. In short, heat stress costs money!

Hittestress Koeien THI Index

Figure 1: Temperature Humidity Index

green = no heat stress

yellow = mild heat stress

orange = heat stress

dark orange = severe heat stress

red = lethal heat stress


The cost of heat stress in dairy cattle varies from farm to farm. The Netherlands was an example in the WUR report for the cost calculations.

For a dairy farm with 100 dairy cows and a milk production of 8,500 kg milk per cow, the costs consist of the following: 

  • less milk yields;
  • more costs;
  • long-term effects are not included in the calculations.

On average, heat stress in dairy cattle costs around $3,000 when the number of hot days is limited and rises to as much as $5,600 when challenging many hot days.

Lower milk yields are the highest cost in this regard. Heat stress causes a drop in milk production by up to -12%. Fat and protein content in milk can fall to -16% and -17%, respectively. 



During hot days, silage intake decreases because digestion generates heat. As a result, concentrate intake increases relatively. This increases the risk of rumen acidosis and hoof problems.

The WUR report states that dry matter intake decreases linearly by 3 oz per 1 unit increase in THI between 55 and 65. An average cow absorbs almost -6% less dry matter at a THI of 65 than at a THI of 39. At a THI of 74, it is almost -13% less dry matter intake.

Therefore, it is essential to support digestion by stabilizing the rumen pH throughout the day, thereby preventing the risk of rumen acidosis. This enables the cow to utilize more protein and energy from forage and pasture.

The rumen is the engine in the cow, and the microflora is like fuel. Therefore, the best lubricant is needed to keep the rumen running long-lasting and efficiently.


More than 70% of a cow’s energy is supplied from the rumen via fatty acids. pHom Norway is like a lubricant for the engine. It reduces friction and prevents the engine from clogging up.

pHom Norway is based on plant-based and arctic minerals and preserves a long-lasting balance in the rumen, even under heat stress. At the same time, the minerals are absorbed through the rumen wall.

That’s the Energy of Mother Nature!

The result with 3 to 3.5 oz of pHom Norway from the first day of lactation:

  • supports the fermentation of energy and fiber;
  • long-term and stable rumen pH;
  • reduces the risk of rumen acidosis;
  • for a healthy body condition score

100% Norwegian, 100% natural

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More Energy from your silage!