Heat stress significantly reduces feed intake in dairy cows, sometimes by 10 to 25 percent compared to normal conditions. This has direct consequences for milk production, energy balance, and the overall health of the animal. Read on to find out exactly how this works, which nutrients are the first to fall out of balance, and what you can do about it.
What happens inside a cow’s body during heat stress?
At high temperatures, a cow’s body automatically switches to a survival strategy: it reduces heat production by eating less, increases breathing rate to dissipate heat, and redirects blood to the skin for cooling. This disrupts virtually every physiological process, from digestion to hormonal balance.
Cows do not have a sweating system that works as efficiently as that of humans. As a result, they are far more vulnerable to heat accumulation. When ambient temperatures rise above 25 degrees Celsius and humidity is high, a critical threshold is reached: the so-called Temperature Humidity Index (THI). Above a THI of 68, most dairy cows begin to show signs of heat stress.
The physiological response is broad and far-reaching. Blood flow to the intestines decreases, weakening the integrity of the intestinal wall. At the same time, concentrations of stress hormones such as cortisol rise, suppressing immune function and disrupting metabolism. The result is an animal that eats less, digests less efficiently, and uses more energy for thermoregulation.
How much does feed intake drop during heat stress?
During heat stress, feed intake in cows drops by an average of 10 to 25 percent, depending on the severity and duration of the heat. Under extreme conditions — such as prolonged heat waves with high humidity — this can increase to 30 percent or more. This is one of the most direct and measurable consequences of heat stress in dairy cattle.
This decline is not coincidental. Cows deliberately reduce their feed intake because the digestion of feed generates heat, known as the heat of fermentation. By eating less, the body produces less internal heat. It is a survival mechanism, but it comes at a high cost: reduced energy intake leads directly to lower milk production, weight loss, and a negative energy balance.
What further complicates the situation is that cows also eat differently during warm periods: they consume larger portions in shorter intervals, particularly at night when it is cooler. This changes the fermentation pattern in the rumen and increases the risk of ruminal acidosis. Feed intake therefore declines not only in quantity, but also in terms of the quality of its distribution throughout the day.
Which nutrients fall out of balance first due to heat stress?
During heat stress, potassium, sodium, magnesium, and proteins are the first nutrients to fall out of balance. Through excessive panting, a cow loses large amounts of bicarbonate through respiration, disrupting the acid-base balance. At the same time, the demand for electrolytes increases while their intake decreases due to reduced feed consumption.
The electrolyte balance is particularly vulnerable. Cows lose large quantities of potassium and sodium through saliva and sweat. This affects not only fluid balance, but also muscle function, rumen activity, and milk composition. A potassium deficiency also leads to reduced feed intake, further reinforcing the vicious cycle.
Protein utilization also comes under pressure. During heat stress, the breakdown of muscle tissue increases as the body seeks alternative energy sources. At the same time, the efficiency with which proteins are absorbed in the intestine declines, partly due to reduced blood flow to the intestinal epithelium. The result is a deteriorating nitrogen balance and loss of body condition, even when the ration composition looks correct on paper.
How does heat stress affect rumen fermentation?
Heat stress disrupts rumen fermentation in several ways: it alters the cow’s eating pattern, reduces saliva production, and shifts the microbial composition of the rumen. This leads to a less stable fermentation process, a lower pH, and a higher risk of subclinical ruminal acidosis.
Saliva plays a crucial role as a buffer in the rumen. A cow that ruminates calmly produces large quantities of saliva that stabilize rumen pH. But during heat stress, cows ruminate less and eat more irregularly. Saliva production drops, buffering capacity decreases, and rumen pH falls. This promotes the growth of acid-producing bacteria at the expense of cellulose-digesting microbes.
The result is a shift in the ratio of volatile fatty acids: more propionic acid and butyric acid, and less acetic acid. This has direct consequences for milk fat production, as acetic acid is the primary building block for milk fat. Dairy farmers often notice this as a drop in the fat content of milk during warm periods — a signal that rumen fermentation is not running optimally.
What can farmers do to maintain feed intake?
Farmers can maintain feed intake during heat stress through a combination of management practices, nutritional strategies, and targeted supplementation. The most effective approach focuses on reducing heat load, adjusting the feeding schedule, and supporting electrolyte and energy balance.
Practical management measures with immediate effect:
- Provide feed in the early morning and late evening, when temperatures are at their lowest
- Ensure adequate shade and ventilation in the barn
- Optimize water supply: cows drink considerably more during heat stress
- Keep feed fresh by offering smaller, more frequent portions
- Protect TMR mixtures from heating and spoilage
At the nutritional level, it is advisable to adjust the ration to account for lower feed intake. This means a higher energy density per kilogram of dry matter, with extra attention to rumen fermentation balance. Products that keep the TMR fresh and palatable can make a practical difference by stimulating feed intake and preserving nutritional value.
Nutritional strategy, step by step:
- Increase the energy density of the ration by adding more bypass fat or energy-rich by-products
- Adjust potassium and sodium levels to compensate for losses through respiration and sweat
- Add an effective buffer to the ration to stabilize rumen pH
- Support liver and metabolic function with targeted supplementation
- Monitor milk composition regularly as an early indicator of rumen disturbances
Betaine plays a special role in this context. As an osmolyte and methyl donor, betaine protects cells against the effects of osmotic stress, such as that which occurs during heat stress. It helps cells maintain their fluid balance, supports liver function, and contributes to better nutrient utilization — even when feed intake temporarily declines.
How Jodoco helps with heat stress in dairy cattle
Heat stress is one of the most underestimated productivity risks in dairy farming. At Jodoco, we draw on more than 25 years of in-house research to develop targeted solutions that support the cow when it matters most.
Our approach to heat stress operates on multiple levels simultaneously:
- Cellular protection: Natural betaine as an osmolyte helps cells maintain their water balance and reduces the physiological stress of heat at the cellular level
- Energy balance: Acibet G combines betaine with glycerol and organic acids to support energy balance and stimulate liver function
- TMR quality: Acibet TMR prevents feed from heating up, keeps the TMR palatable for longer, and thereby directly improves feed intake
- Drinking water administration: Through our drinking water additives, targeted substances can be administered at the moment feed intake declines — because water intake remains more reliable than feed intake under stress
- Scientific foundation: Our formulations are developed in our own laboratories and validated in collaboration with national and international research institutions
Would you like to know which approach best suits your farm situation or that of your customers? Contact us and we will be happy to work with you on a practical strategy for the upcoming summer season.
