Have you done your summer diet adjustment? With these hot, sultry summer days upon us, the idea of cozying up in the warm kitchen with a steaming bowl of soup or a hearty home-cooked meal does not appeal to many.

Instead, we adjust our diets to grilled foods, cold salads, fruits and lots of cool refreshments. Summer diet adjustments also change in the dairy cow as she, too, needs to accommodate the stress of summer.

As environmental temperatures increase, the amount of humidity in the air the animal can tolerate decreases. The cow’s normal rectal temperature is between 101.3°F and 102.8°F. To maintain this temperature, she becomes reliant on dissipating her body heat primarily through the thermoregulatory function of evaporative cooling.

The temperature-humidity index (THI) is a measure of the combined effect of ambient temperature and humidity. At 90°F and low humidity (<40 percent), the THI is 72 and the animal is in a mild heat stress situation. She adjusts by seeking shade and increasing her respiration rate to more than 80 breaths per minute to start dissipating the extra heat.

If the temperature remains 90°F and the humidity is 40 to 85 percent, the THI will be 80 to 90 and the animal is now in a moderate heat stress situation. Her saliva production and respiration rate will increase. Feed intake becomes depressed and water consumption will increase.


Severe heat stress occurs with humidity of 90 percent and over, as the THI is 90 or above. The animal responds with even more rapid respiration (>100 breaths per minute) and excessive saliva production.

The severity of heat stress is a combination of factors such as the THI, the duration of the heat, the cooling temperatures at night and the facilities, such as shade, fans and sprinkling systems, the animal has access to.

As these conditions persist, the body temperature increases. The heat increment of the cow also increases her body temperature. This is the heat produced by rumen fermentation in addition to the metabolic process of digestion.

When the cow cannot dissipate enough body heat to maintain her normal temperature, her response is to lessen her body temperature by lowering her heat increment.

This is first done by eating less. Less dry matter intake (DMI) results in less feed to ferment and metabolize; this will be physiologically helpful to her. However, for every one-pound decrease in DMI, we see a two-pound drop in milk production, which results in a financial loss to producers.

However, research by Rhoades and Baumgard found that the decrease in DMI explained about 45 percent of the production loss and that other heat stress-induced changes decrease milk production.

Energy and protein adjustments

Diet adjustments can help your cows through this stress. Different feedstuffs contribute different amounts of heat because of the heat of fermentation or the efficiency of use by the end products. Corn and other concentrates generally contribute less heat of fermentation than forages.

The increased corn comes at the result of decreasing fiber. If fiber is inadequate in the diet, the rumen mat becomes diminished and there is less rumination, resulting in a decreased amount of chewing activity. Less chewing results in less natural buffering by the saliva and rumen pH is lowered. resulting in acidotic conditions.

Forages can be lowered in the diet slightly, but rations still need to provide ADF levels of 19 to 20 percent and NDF levels of 28 to 30 percent to be adequate. Feeding a high-quality, digestible forage will enable the dietary ADF to be lowered, but still provide energy for the animal. Including sources of non-fibrous byproducts can help provide an effective source of fiber to the animal to maintain rumination.

To maintain energy in the diet with the decreased DMI, incorporate fat into the diet. Fat contains 2.25 times more energy than carbohydrates. Fat produces less heat during digestion and does not add to the heat increment.

Fat levels should not exceed about 5 percent of the diet dry matter. Continue to watch the sources of fatty acids as to not induce milk fat depression.

Consider the amount of protein and amino acids you are feeding your cows during the summer. It’s critical for the diet to supply adequate protein and a good balance of carbohydrates and soluble protein for microbial growth.

If too much soluble protein is fed, the animal will excrete this excess as urea through the urine. This is a biochemical pathway that expends a lot of energy.

Mineral balance

Increased fluid losses due to sweat, panting or urination will alter the balance of carbonic acid and bicarbonate. Increased bicarbonate is excreted in the urine to maintain this balance. Potassium is also lost through sweat.

These losses affect the cow’s cation-anion balance. The dietary cation-anion difference (DCAD) – the positive cations of potassium and sodium compared to the negative anions of sulfur and chlorine – should be about 25 to 30 mEq/100g DM during heat stress.

Depending on the other components of the diet, suggestions are to feed about a half-pound buffer as sodium bicarbonate or sodium sesquicarbonate. Sodium should be about 0.4 to 0.5 percent of the diet DM.

Potassium carbonate can also be fed to provide buffering and to raise the dietary potassium to about 1.6 to 1.8 percent. Maintain chlorine at about 0.4 percent of the diet DM.

Other dietary factors

Just as summer is the time we enjoy certain treats like ice cream or popsicles, it is a good time of the year to include some “extras” in the diet of the cow. Products containing Aspergillus oryzae or Saccharomyces cerevisiae can promote a microbial population suitable for fiber digestion.

Liquid feeds can be added to the diet to alleviate any sorting out of the forages the cow may be prone to do. Bunk stabilizers can also be added to diets to lessen the effects of feeds that have questionable bunk stability. Electrolyte hydration products have also shown to provide benefits in heat-stress situations.


Because milk is 85 percent water, it’s no surprise that when water intake decreases, milk production will also decrease. A temperature rise from 86º to 95°F increases the need for water from 20 to 32 gallons per cow.

Ample water should be offered near the feed bunk and in return lanes. Water also needs to be fresh and clean. Troughs need to be cleaned on a regular basis.

Feeding behavior

Adjusting the feeding times of the diet can result in more stable feeds in addition to more comfortable animals. Feeding early or late in the day will allow the animal to consume more feed during the cooler part of the day.

Since the animal produces the most heat of fermentation three to four hours post-feeding, this allows her to dissipate more heat before enduring the extreme heat of the day. In addition, less feed will sit in the feedbunk during midday when the animal does not eat, resulting in less spoiled feed.

As you enjoy your summer diet, think about the diet of the cow with her nutrients, forage quality or non-nutritive extras. As you migrate to cool conditions with a cool beverage, think about her water supply and feedbunk management. Keeping her comfortable and on feed are critical during the summer season. PD

Dr. Kuehn is a nutritionist at the Form-A-Feed and TechMix companies, headquartered in Stewart, MN. For more information, call Dr. Kuehn at 800-422-3649 or send an email to articleinfo@formafeed.com

References omitted due to space but are available upon request to editor@progressivedairy.com .

Photo by PD staff.

Carla Kuehn