Heat stress reduces milk production in dairy cows by an average of 10 to 25 percent, depending on the intensity and duration of the heat. This loss is no coincidence: when a cow’s body temperature rises, her energy balance shifts dramatically. In this article, we answer the most frequently asked questions about heat stress in dairy cattle, from physiology to practical feeding management.
What physiological processes does heat stress disrupt in dairy cows?
Heat stress disrupts a range of interconnected physiological processes in dairy cows: energy balance, hormonal regulation, gut integrity, and cellular metabolism all become dysregulated when body temperature remains structurally elevated. The cow invests energy in cooling down rather than in milk production, with direct consequences for her performance and health.
As ambient temperature rises, the cow increases her respiratory rate and boosts blood flow to the skin to dissipate heat. This requires energy. At the same time, feed intake drops, reducing the amount of energy coming in. The result is a negative energy balance comparable to that seen in early lactation.
At the cellular level, osmotic stress occurs: cells lose moisture and fall out of balance. The production of heat shock proteins increases as a protective mechanism, but this comes at the expense of other protein synthesis processes. At the same time, oxidative stress rises, weakening the intestinal barrier and increasing the risk of infection. The gut wall becomes more permeable, leading to a heightened risk of endotoxemia — a condition in which bacterial toxins from the gut enter the bloodstream.
How much milk production is lost due to heat stress?
Heat stress in dairy cows typically leads to a production loss of 10 to 25 percent, but in extreme cases this can rise to 40 percent. The loss results from both reduced feed intake and direct physiological disruptions that impair milk synthesis, regardless of the nutrients available.
Interestingly, research shows that part of the production loss cannot be explained by reduced feed intake alone. Even when cows eat sufficiently, they produce less milk during heat stress. This points to direct hormonal and metabolic disruptions: insulin levels rise, glucose availability to the udder declines, and the synthesis of milk fat and milk protein decreases.
The economic impact is significant. Dairy farmers in warmer climates and during hot summers in Belgium and the Netherlands see not only daily production fall, but also a slower return to normal production levels after the heat period. Cows that have experienced severe heat stress recover more slowly and sometimes perform structurally worse in the following lactation.
Why does milk quality decline at high temperatures?
At high temperatures, milk quality declines because heat stress directly affects milk composition: fat, protein, and lactose content decrease, while the somatic cell count rises. This makes the milk less valuable for processing and increases the risk of udder health problems.
A rising somatic cell count is a direct indicator of udder inflammation. Heat stress weakens the cow’s immune system, making her more susceptible to mastitis-causing pathogens. At the same time, the elevated body temperature accelerates bacterial growth in the cow’s environment, further increasing the risk of infection.
The decline in fat content is linked to changes in rumen fermentation. Heat stress disrupts the ratio of acetate to propionate in the rumen: relatively more propionate is produced, which comes at the expense of fatty acid synthesis for milk fat. Meanwhile, protein content falls because fewer amino acids are available to the mammary gland, partly because the cow redirects them toward recovery processes.
Which cows are most vulnerable to heat stress?
High-producing dairy cows are most vulnerable to heat stress, because their intensive metabolism already generates more body heat than low-producing animals. In addition, cows in the dry period, in early lactation, and in late pregnancy are particularly susceptible to the effects of high temperatures.
The following groups deserve extra attention:
- High-producing cows: They generate more metabolic heat and have less capacity to dissipate additional heat.
- Cows in early lactation: They are already in a negative energy balance and have fewer reserves to absorb the additional burden of heat stress.
- Dry and late-pregnant cows: Heat stress at this stage has consequences for calf development, colostrum quality, and production in the subsequent lactation.
- Older cows: Their thermoregulatory capacity is less efficient than that of younger animals.
- Cows with a high body weight: A larger body volume generates more heat and is harder to cool down.
Breed-related factors also play a role. Holstein-Friesian cows, the dominant dairy breed in Belgium and the Netherlands, are inherently more susceptible to heat stress than dual-purpose or beef breeds, due to their high milk production.
How can feeding management reduce heat stress in dairy cattle?
Feeding management can reduce heat stress in dairy cattle by increasing the energy density of the ration, stimulating feed intake through timing and palatability, and deploying targeted supplements that support osmotic balance and metabolism. A well-considered feeding strategy significantly reduces production losses.
Practical adjustments to feeding management during warm periods include:
- Feeding during cooler hours: Cows prefer to eat at night or early in the morning when temperatures are lower. Adjust the feeding schedule to match the daily heat pattern.
- Increasing energy density: Add more fat-rich ingredients to partially compensate for lower feed intake without overloading rumen volume.
- Adding buffers: Sodium bicarbonate and other rumen buffers help stabilize pH when rumen fermentation is altered by heat stress.
- Maximizing water intake: Ensure an adequate number of drinking points with fresh, cool water. A dairy cow can drink up to 150 liters per day during heat stress.
- Providing osmotic support: Betaine is a proven osmolyte that protects cells against dehydration and stress. Through drinking water and feed additives, betaine can be applied in a targeted manner during periods of heat stress.
In addition to the ration, TMR stability is also important. At high temperatures, the total mixed ration ferments more quickly, which reduces palatability and puts further pressure on feed intake. Products that keep the TMR fresh for longer, such as Acibet TMR, help to limit this problem.
When is heat stress a problem in Belgian and Dutch livestock farming?
In Belgium and the Netherlands, heat stress in dairy cattle occurs when the temperature-humidity index (THI) exceeds 68. In practice, this means that at an outdoor temperature of just 25 degrees Celsius combined with normal humidity levels, cow production and health already come under pressure.
Traditionally, heat stress in our region was seen as an occasional summer problem. That view is no longer accurate. Due to climate change, heat waves are becoming more frequent, longer, and more intense. In 2026, the likelihood of periods with multiple consecutive tropical days during the Belgian and Dutch summer is greater than ever before. Dairy farmers who fail to anticipate this risk recurring production losses every year.
Moreover, many farmers underestimate the impact of nighttime temperatures. Cows recover from heat stress at night, but when nighttime temperatures remain above 18 degrees Celsius, that recovery is incomplete. Chronic, low-grade heat stress lasting for weeks can therefore sometimes be more damaging than a short but intense heat wave.
Barns with insufficient ventilation compound the problem. Even at outdoor temperatures of 20 degrees Celsius, barn temperatures can rise above 30 degrees — particularly in older buildings without mechanical ventilation. Early monitoring using THI calculations and adjusting management before summer begins makes a significant difference.
How betaine helps with heat stress in dairy cattle
Betaine is one of the most scientifically supported natural ingredients for addressing heat stress in dairy cattle. As an osmolyte, betaine actively protects cells against moisture loss and osmotic stress — precisely the mechanisms that heat stress triggers at the cellular level. More information about how this ingredient works can be found on our page about betaine.
In ruminants, betaine works through multiple pathways:
- Osmotic support of cells during heat stress
- Stimulation of microbial protein synthesis in the rumen
- Positive effect on fiber digestion and short-chain fatty acid production
- Support of liver function and reduction of negative energy balance
- Contribution to higher milk production and improved milk quality
At Jodoco, we have incorporated betaine as a core ingredient in products specifically developed for the challenges of the summer period. Acibet G combines natural betaine with glycerol and organic acids to provide targeted support for energy balance and liver function in dairy cattle. Trial results show that cows receiving Acibet G better maintain their milk production, even with less concentrate feed per 100 kg of milk.
Would you like to know which approach best suits your farm situation? Contact us to speak with our team for personalized advice on heat stress management and the right feeding strategy for your dairy operation.
