Heat stress signs and mitigation strategies for dairy operations

07 June 20228 min reading

Rumen-stable fats relief the metabolism of the cow as they produce less metabolic warmth, compared to carbohydrates. Trials have shown that performance can be stabilized if components rich in starch are replaced by rumen-stable fats during summer season.

Nicolaas Vreeburg, DVM
Technical Services Manager
Lallemand Animal Nutrition

Since you are based in the Netherlands, it may be a surprise to readers that you are concerned with heat stress. Is heat stress a problem in the Netherlands, too?

Well, in fact, there is heat stress in most countries in the summertime. You know, unlike human beings, the optimal temperature for a cow is 10°C. Even in cooler climates, in the summertime, it can easily reach 25-30 C° these days. The other thing is that dairy cows are mostly kept inside, even in the Netherlands where grazing is still the standard. The climate inside a barn can be different from outside, with sometimes even a higher temperature.

Finally, let’s not forget that humidity plays a big role in heat stress (i.e., at the same temperature, cows will suffer more from heat if the humidity is high. The indicator to monitor is called the Temperature and Humidity Index, or THI). Under our climate, it is becoming a bigger problem every year.

So yes, farmers and farm advisers have to be aware of heat stress and the associated losses and consequences for dairy production. It is a growing concern in almost all countries now and farmers notice it more and more. Not only in the countries under warmer climates, but also the UK, the Netherlands, Germany, Denmark, Sweden, Finland…everywhere! 

For this reason, our ruminant team has developed a web tool to predict the negative impact that heat stress has on milk yield and milk quality for dairy cows, but also milk yield for dairy sheep and goats and growth for beef cattle in various parts of the world. This new prediction tool is based on:

• Recordings of temperature and humidity in several strategic regions for ruminant production. Temperature and humidity are keys to measure the environmental impact on animal performance, which is reported as the temperature and humidity index (THI). It is important to remember that the impact of stress is linked to the THI level and to the duration of exposure, both in terms of the number of hours per day and the number of consecutive days exposed to heat stress.

• Consolidation of multiple published analyses (7 publications in dairy cows), with experiences in different continents to better predict specific regional losses, including dairy ewes and goats.

This web tool is available as an interactive world map on

As an expert in cow behavior, or cow signals, can you tell us what are the first visible signs of heat stress?

There are some common signs of heat stress that can be easily observed in all farms.

1. First, the biggest change we observe is an increase in respiratory rate. You have to look very carefully because it is not something we typically monitor, and a lot of people don't have a clue that a normal rate for a cow is only 10 to 30 breaths per minute. A cow is considered panting if 60 breaths or higher occur.

2. Another thing that is easy to see is that cows are standing more often. During heat stress, cows are more apt to stand in the barn with fewer cows lying down in the stalls. These are the two, most visible signs that you can immediately observe in a barn.

3. Then, taking a closer look, you will see that cows are drinking more, spending longer periods at the water trough. A water meter for each pen is a good idea to help monitor heat stress exposure by the upward fluctuations in water intake per day.

4. Another, very often reported, sign is that cows are gathering and crowing together at cooler places in the barn, such as places with more ventilation and fresh air. Now, if you take a longer and better look, you can also see differences in eating behavior.

5. Cows are eating fewer, but bigger meals. Consequently, the percentage of cows with a rumen score of 2.5 or lower goes up. Again, to see the signs of stress, you need to have a good reference of what is normal.

What are the consequences of heat stress? Can you give us some figures?

The first and most visible consequence that farmers see, of course, is the loss of milk production. That's the easiest thing to see, and it can go up to 4 to 6 liters a day. With severe heat stress, I have seen cows losing 10 liters a day on average.

Longer term impacts for the herd can be lowered fertility, fewer cows get pregnant. That impacts the calving pattern or interval of the herd. A drastic reduction in pregnancy rates can have lasting effects and can take upward of 2 years to recover from.

Another long-term issue is for overall health. During and after heat stress, you can see an increase in poor foot health, lameness and even laminitis. There are several reasons for that. First, cows are standing more to try to escape the heat and uncomfortable feeling, and that can add more pressure on the foot and hooves. Even if everything is done correctly on a nutritional level poor rumen health can occur. The erratic eating and meal patterns of the cows leave the cow more prone to ruminal acidosis during or after a period of heat stress. Cows prefer to eat when the day is coolest, so they often overcompensate by eating bigger meals in shorter periods of time. This disrupted feeding behavior will affect rumen health, digestion and often increases the risk of sub-acute acidosis or bouts of clinical acidosis. For both these reasons, there is a higher risk of poor hoof health and locomotion in the herd with more hoof ulcers, white line lesions and even laminitis.

Finally, there is data that now shows an even longer-term impact of heat stress for a dairy operation. Offspring born to heat-stressed dams have lowered immune function at birth, more vulnerable to illness and are less productive during the first lactation.

There are huge consequences of heat stress in dairy cows, and farmers report it more and more. Farmers are concerned and are willing to invest in preventive solutions to ease the stress on the herd.

Talking about solutions, which one would you recommend in practice?

Make a customized plan with your team of advisors to help avoid the negative consequences of heat stress. You cannot affect the weather, but you can prepare your herd for it. There are three types of solutions: environmental, nutritional and management solutions. While environmental solutions are the most discussed, one aspect which is often overlooked, is the feed.

One solution to avoid the negative consequences of heat stress is to bring stability to the rumen, since changing feeding behavior due to heat stress impairs the rumen function and decreases the ruminal pH. With the live yeast feed additive Saccharomyces cerevisiae CNCM I-1077 (LEVUCELL® SC), the rumen pH can be stabilized at a higher level. Feeding live yeast will also support normal feeding behavior, helping cows to have more frequent and smaller meals even if they are under heat stress (Perdomo et al., 2020), as a result dairy production can be maintained (Figure 1).

Figure 1: Effect of live yeast supplementation on dairy cow feed efficiency (P<0.05) (adapted from Perdomo et al., 2020).

The other lever is to improve the digestibility of the ration. Feeding live yeast can also help improve the digestibility of the ration. Along with adding a rumen modifier, it’s good to check in with your nutritionist to ensure the diet includes more easily fermentable feedstuffs, for example beet pulp or cotton seeds, etc., so the cow is receiving the most nutrient dense bite she can during heat stress.

If we talk about feeding, let’s remember that the starting point is to have good quality and cool silage, which starts much earlier during the previous harvest season. So again, it is all about planning and using a high quality and proven silage inoculant at harvest! It is important to make sure to have cool and clean forages, especially during in the summertime. Moldy or poorly fermented silage, have a negative influence on feed intake, metabolic energy and nutrient availability. Here, selected silage inoculant (e.g. Magniva range) can really help.

Burgos Zimbelman R. and. Collier R.J. 2011. Tri-State Dairy Nutrition Conference, April 19 and 20, 2011.

St-Pierre N.R., Cobanov B. and Schnitkey G. 2003. Economic Losses from Heat Stress by US Livestock Industries. J. Dairy Sci.86. E52–E7.

Perdomo, M.C. et al. 2020. Effects of feeding live yeast at 2 dosages on performance and feeding behavior of dairy cows under heat stress. J. Dairy Sci., Vol. 103 (1) 325 – 339

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