The primary reason cows decrease milk production during hot weather is that the cows eat less. Since cows will be consuming less as temperatures increase (Table 1), increasing the energy density of the diet can in part compensate for the decrease in dry matter intake.

Table 1. Relative changes in expected dry matter intake (DMI) and milk yield and water intake with increasing environmental temperature.


Expected intakes and milk yields



Milk yield

Water intake

























Sources: National Research Council. 1981. Effect of Environment on Nutrient Requirements of Domestic Animals. National Academy Press, Washington, D.C. Dr. Joe West, Extension Dairy Specialist, University of Georgia.


The data in table one is old data, but one can extrapolate it into today's numbers. 77 degrees F to 86 degrees F is a 5% drop in DMI, resulting in about an 8% drop in milk. If your herd is averaging 85 lbs. of milk, eating 57 Lbs. of D.M. You could expect to see milk drop to 78 lbs. and DMI drop to 54 lbs. Also, look at the volume of water that increases! Do you have enough drinkers? Is the drinker clean? Are any of the drinkers running out of water?

We at Nobis Agri Science (NAS) have a strategy to have "HEAT STRESS" rations ready to implement when the HIGH HEAT INDEXES strike your dairy. Ask your nutritionist to prepare you for the heat with the proper ration.

INCREASE ENERGY DENSITY OF THE RATION. High-quality forages should be available to the animal if possible. These forages may include summer annuals or a high-quality perennial. Silage, pasture, and hay are acceptable. The primary reason cows decrease milk production during hot weather is that the cows eat less. Since cows will be consuming less as temperatures increase (Table 1), increasing the energy density of the diet can in part compensate for the decrease in dry matter intake.

To increase the energy density of the diet, a fat or feed with a low heat increment (heat of digestion) may be included in the ration. The high-fat feed ingredients include whole cottonseed, either tallow or bypass fat, and roasted soybeans, which are available in some areas of the state. Lower-fiber feedstuffs usually result in fewer heat increments than high-fiber feeds such as grass hays. Diets high in grain and low in fiber cause less heat stress for lactating cows because of the lower heat of digestion. However, it is critical to balance the ration properly since milk fat may be depressed, and digestive orders may result when a high-grain ration is fed. Feeding buffers such as sodium bicarbonate and magnesium oxide allow higher concentrate rations to be fed and help alleviate the low-fat milk syndrome.

Also, the ration fed to cows in hot weather should be adequately balanced for minerals. Some mineral companies will have a "summer buffer." Increasing potassium to 1.3 to 1.5 percent, sodium to 0.5 to 0.6 percent, and magnesium to 0.3 to 0.4 percent may result in less heat stress by allowing the animals to dissipate heat. Chlorine usually is at least 0.25 percent of the diet, which is the recommendation throughout the year.

Do not overfeed highly degradable protein, i.e., 65 percent or greater degradable crude protein in the rumen. This also increases the heat increment and requires more heat to be dissipated from the animal. Proper supplementation of more undegradable protein appears to reduce the heat of digestion.

Cows that are under-conditioned will use body fat to drive milk volume, and the milk butterfat will be reduced, not to mention the numerous health implications.

Typically, a temperature-humidty index (THI) over 72 [75ºF (23.9ºC) with 65% relative humidity (R.H.) to 90ºF (32.2ºC) with 0% R.H.] was established as the lower threshold of heat stress. But with increased milk production per cow since initial development of the THI, a 22-pound-per-day increase in milk production will decrease the threshold for heat stress by 9ºF (5ºC). A recent re-evaluation of the THI has been modified due to improved milk production. The THI heat stress threshold was lowered to 68 [72ºF (22.2ºC) with 45% R.H. to 80ºF (26.7ºC) with 0% R.H.]. 

When cows experience heat stress, DMI decreases. At the same time, maintenance requirements are increased due to the activation of the thermoregulatory system. This can increase maintenance requirements by 7% to 25%. This decreased DMI can account for about 36% of reduced milk production due to shifts in post-absorptive metabolism and nutrient partitioning. Under heat stress, cows also have lower non-esterified fatty acids (NEFA) concentrations and a higher rate of peripheral glucose utilization. This reduced DMI precedes by several days of reduced milk production.

Heat abatement involves several actions: providing shade, air movement, misters, and fans, feeding earlier and later in the day, use of high-quality forages to minimize the heat of rumen fermentation and avoiding feeding fat sources that can reduce DMI or contribute to high levels of fatty acids such as linoleic or palmitic. When DMI is reduced as in heat stress, using a primarily saturated free fatty acid supplement with a combination of stearic and palmitic can increase energy intake by increasing energy density as long as DMI is not compromised.

Water is the primary nutrient needed to make milk, accounting for over 85 percent of the content of milk. Also, the cow's water requirement increases significantly as the environmental temperature rises. Cows drink up to 50 percent more water when the THI is above 80 percent. Table 1 shows that water intake goes from 21 to 32 gallons as the temperature goes from 86° to 95°F. Cows must be provided with cool water during periods of high temperature. It is also essential that cows have water near shade since they will not travel great distances for water in a hot environment. Water should be placed away from the milking parlor but in an exit lane from the barn as well as near the feeding location of the cows.

The holding pen is the most stressful location for milking cows during periods of heat stress if it is not shaded and cooled. Data indicate that proper cooling of the holding pen can pay for itself in two hot summers. Another publication, FSA4019, Cooling Dairy Cattle in the Holding Pen, provides additional details on cooling in this location.

Cool the holding pen area with a combination of shade, air movement, and water. When combined with air movement, water can increase the cooling ability of the cow. However, adding water in humid or poorly ventilated holding pens or barns can worsen the situation. The water can hold the heat in the cow if it does not evaporate from the cow. If cooling is done effectively with fans and water in the holding pen, less cooling is required between milking’s.

To provide cooling for the holding pen or loafing area, sprinkling enough water to soak cows to the skin and then running fans constantly at 5 to 7 miles per hour is recommended. These fans increase the evaporation of water which helps cool the cows.

One system that works very effectively is to sprinkle the cows for a short period, e.g., 0.5 to 3 minutes, and apply 0.05 inches of water per cycle, just enough time to soak the cows to the skin. Avoid allowing water to run onto the udder. If water does reach the udder, it is possible that bacteria can be transferred into the mammary gland and result in more mastitis. If possible, blow air onto the cows continuously. However, in some cases, you may want the fans off for the period when the sprinklers are running if the water droplets blow out of the holding pen.

A 1/2-horsepower, 36-inch fan rated at 11,000 to 12,000 cubic feet per minute (cfm) will blow 20 to 30 feet, while a 48-inch fan with a 1-horsepower motor rated at 21,000 cfm will blow 30 to 40 feet. Usually, it is necessary to have at least two widths of fans in the holding pen. For many producers, four fans are needed, two in the front of the holding pen and two approximately halfway down the length of the holding pen. The fans should direct a high-velocity breeze over the cows to allow them to dry faster so that the heat can be dissipated from their bodies (see FSA4019, Cooling Dairy Cattle in the Holding Pen).

An ample supply of water, usually 25 gallons per cow per day, is required to cool the cow throughout the day. This type of cooling system may be used in the holding pen and in the free stalls and areas of feeding. The floor of the holding pen should be grooved or rough-surfaced concrete or some other suitable footing so that cows do not slip in the wet environment. As a general rule, water should not stand in the holding pen, and the feet of cattle should be exposed to limited water.

If no covering is over a holding area, a shade cloth roof will be helpful, although a permanent structure is recommended. Additionally, shade cloth or a poultry curtain may hang down over the side of the holding pen where the afternoon sun shines in. The eave height of the holding pen should be at least 12 feet if possible.

For a few producers, a sprinkler in the exit lane of the milking parlor serves to soak the cows to the skin as they leave the milking area. The cows will be cooled as the water evaporates. Such cooling devices need to be used with care when mastitis or somatic cell count problems occur. The object is to wet the cow's back without water running onto the udder. A manual or electronic trigger should turn the water on the cow only for a few seconds.

Freestalls and feed bunk fans. One system that works very well uses 36-inch fans every 20 to 30 feet along the feed manger. This keeps the air moving continuously.


Keeping lactating cows cool can provide a good return on your investment as it makes cows more comfortable, thereby making them more productive. Shade and cool water should be available to cows and heifers at all times. If possible, cooling devices should also be installed in the holding pen and feeding area. The ration should be balanced appropriately, and generally, the energy density should be increased in the summer to help compensate for decreased dry matter intake of the cow.

Some of this information was adapted from:

Jodie A. Pennington
Professor-Extension Dairy Specialist

Karl VanDevender
Professor-Extension Engineer

University of Arkansas
Division of Agriculture
Cooperative Extension Service
Little Rock

Tags: Management Tips; Heat Stress