Heat stress – the hidden costs


Do you see that your cows change their behavior and become increasingly lazy and drowsy during the summer heat? Have you also experienced an increase in the somatic cell count as well as a more udder disorders? Or have you had problems keeping the insemination rate and conception rate at the same level as usual? Then read on and learn more about the financial impact of heat stress.

Heat stress effects can be seen at 20-22 ⁰C

The consequences of heat stress can be both large and long-lasting. Heat stress can be difficult to identify, as well as see which cows are badly affected. In Denmark, the effects of heat stress can already be seen at temperatures of 20-22 ⁰C, depending on the stable system. However, it most often affects high-yielding cows. Depending on the temperature during the summer, and the attention level of the individual farmer on heat stress, the severity varies.

The heat affects both yield, reproduction, and mortality

Based on field studies in the summer of 2018 and 2019, Peter Dahlgaard from Din Kvægpraksis ApS, has made a SimHerd calculation of the effect of mild and severe heat stress. At a mild degree of heat stress, he saw a minor decrease in peak yield of – 4 kg EKM, a reduction in the insemination rate of 7%-points, and a 5%-point lower conception rate. An increase of the somatic cell count of 50,000, was also detected. In the case of mild heat stress, it will typically last for up to 63 days.

A severe degree of heat stress has larger consequences. Peter found a drop in peak yield, of – 7 kg EKM. The insemination rate was reduced by 30%-points and the conception rate was reduced by 15%-points. The somatic cell count was increased by 100,000, as well as a 4%-point higher cow mortality. These consequences can last from 63 days to 91 days, depending on how long the heat persists.

Consequences of heat stress

Based on calculations in SimHerd, the economic impact of mild and a severe degree of heat stress was calculated. In table 1, the economic loss from heat stress can be seen, calculated in GM per annual cow per year. In an average herd with 200 annual cows, a mild degree of heat stress will result in a loss in GM per year of 11,692 €. In case of severe heat stress, GM per year is reduced by 50,662 €. In the following, we will investigate different measures that can reduce heat stress in cows.


Mild degree of heat stress

Severe degree og heat stress



GM per. annual cow


GM per. annual cow

Peak yield

– 4 kg EKM

– 48 €

– 7 kg EKM

-122 €

Insemination rate

– 7%-point

– 3 €

– 30%-point

– 54 €

Conception rate

– 5%-point

– 2 €

– 15%-point

– 37 €

Somatic cell count

      + 50.000

– 5 €

+ 100,000

– 27 €

Cow mortality



       + 4%-point

– 15 €

Total loss in GM per year cows


– 64 €


– 255 €

Table 1 – Economic consequences of heat stress

Investments to reduce heat stress

To minimize heat stress and reduce the loss in GM, investments can be made in various options. One solution is to focus on air circulation and ventilation of the barn. The first step is to check if existing solutions are placed correctly and they are sufficient, or whether more fans should be invested in, for an increased air exchange. You can also check whether there is enough cooling in the form of sprinklers or enough water troughs for the cows. Finally, the focus can be on minimizing the amount of time the cows have to spend standing or without sprinkling in connection with milking, where they stand close together. Based on SimHerd calculations, we can help you calculate how large a loss you have in your herd, and which amount is feasible to invest in reducing the risk of heat stress. Contact us for more information.


If you have access to the SimHerd software, it is also possible to make your own calculations on a specific herd. Read more in the scenario catalogue.