Heat stress and feed intake in cattle: what happens and what can be done
Published 14 May 2026 · Updated 2 August 2026
When ambient temperature rises above the point where a cow can shed her metabolic heat comfortably, the first thing she does is eat less. That drop in dry-matter intake has been measured and described many times, and it is large: in high-producing Holsteins a sustained heat wave can reduce intake by ten to twenty-five per cent within days, which means the energy and protein supply to the udder falls by roughly the same fraction. What follows is a cascade that runs through milk yield, body reserves, fertility and health, and it costs the global dairy industry more money each year than most diseases.
The temperature-humidity threshold
Cattle lose heat by radiation, convection and evaporation, and when the air is both hot and humid the evaporative channel narrows. The temperature-humidity index, which combines the two into a single number, is the standard indicator. Holsteins begin to show measurable stress at a THI of about 68, which in practice means an air temperature of around 22 degrees Celsius at 50 per cent humidity. At a THI above 72 the intake depression is usually obvious, and above 80 the animal is in serious trouble.
Beef breeds, Bos indicus cattle, and crosses with tropical blood tolerate higher thresholds, but none of them is immune. The difference is in the slope of the decline, not the direction. A Brahman-cross steer in a Queensland feedlot eats less in January than in July; he just starts later and falls less steeply than a British-breed steer would.
What happens beyond the intake drop
Reduced intake is the visible part; the metabolic changes underneath it are less obvious but just as consequential. A heat-stressed cow shifts her maintenance energy expenditure upward, because panting, sweating and increased peripheral blood flow all cost energy. At the same time her net energy intake is falling, so the gap between supply and demand widens from both ends. The deficit is covered from body reserves, primarily fat, and in severe or prolonged heat stress the cow enters negative energy balance in much the same way she does in early lactation—except that in early lactation the deficit is temporary and the system is adapted to it, whereas in a heat wave the cow is spending reserves she did not budget for.
Protein metabolism is also disrupted. There is evidence that heat-stressed cattle increase the oxidation of amino acids, which means that the amino acid supply to productive tissues falls by more than the intake drop alone would predict. Muscle protein turnover appears to shift toward catabolism, and the nitrogen balance deteriorates. The practical consequence is that a ration formulated for thermoneutral conditions undersupplies both energy and amino acids during heat, and the shortfall in amino acids may be proportionally larger.
This was one of the subjects taken up in the plenary programme at the Granada meeting in 2022, where a lecture examined the physiological impact of amino acid supply specifically during heat stress. The argument was that adjusting the amino acid profile of the ration, not just its crude protein content, could mitigate some of the production loss—a fine-grained approach that requires knowing which amino acids are being burned and in what proportion.
Nutritional strategies
The nutritional response to heat stress has three layers. The first is to increase the energy density of the ration so that the cow gets more energy per kilogram of dry matter eaten. In practice this means more fat, more concentrates and less forage, within the limits that rumen health will tolerate. The second is to shift the timing of feeding toward the cooler hours, because cattle eat more at night when daytime temperatures are high. The third is to adjust the mineral and electrolyte supply: sweating and panting increase the loss of sodium, potassium and bicarbonate, and replacing them keeps rumen buffering capacity closer to normal.
None of these fully compensates for a severe heat event. A cow that has dropped her intake by twenty per cent cannot be fed back to her previous yield by reformulating the ration alone. The ceiling is set by what she is willing to eat, and that ceiling is set by her thermal state, not by the composition of what is in front of her.
Management and facility design
The largest returns come from keeping the cow cooler, not from feeding her differently once she is hot. Shade, fans, sprinklers and combinations of all three are the tools. Evaporative cooling—wetting the coat and blowing air over it—is the most effective, but it requires water that is not always available and infrastructure that takes investment. In housed systems the building design itself determines the upper limit of what can be achieved: roof insulation, ridge ventilation, inlet area and eave height all matter, and retrofitting a barn built for a temperate climate is expensive.
The genetics side is slower. There is genetic variation in heat tolerance within Holstein populations, and genomic selection for it is now technically feasible, but the trait is antagonistic to production: the most heat-tolerant animals within a breed tend to be the lower producers, because producing less generates less metabolic heat. Breeding for both simultaneously is possible but the progress is slow, and most commercial herds cannot afford to wait for it.
The pragmatic answer for most dairy operations in warm climates is a combination of all of these: cooling infrastructure that prevents the worst of the intake depression, ration adjustments that make the best of whatever intake remains, and a willingness to accept that summer production will be lower than winter production and to plan the cash flow around it. The research conversation, including the work discussed at Granada, is about narrowing that seasonal gap, not eliminating it.