Now is the time to think where changes can be made on the dairy to lessen the impacts of hot weather on dairy cows. Besides changing the cow’s environment to lessen the effects of heat stress, dairymen may also modify their feeding program in order to give their animals additional relief during hot weather. The main objective of feeding cows during heat stress should be to maximize feed intake.

Voluntary intake of dry matter during hot weather has been shown to decrease to about 55 percent of that eaten by cows which are in the comfort zone. The comfort zone is from about 40°F to 75°F. Dairy cows, as all animals do, continually produce heat. As an animal digests feed, heat is generated by the body. In heat stress conditions, cows will voluntarily reduce feed consumption in order to reduce the amount of heat generated. The roughage portion of the ration is the main source of generated heat. If given a choice, cows will generally consume more concentrate in relation to roughage during heat stress periods. This is due to less heat being generated in the digestion of concentrates than to alfalfa hay.

One mechanism dairymen can use to increase feed intake during hot weather is to provide palatable feedstuffs. Feeds which are somewhat wet in nature are relished by cows. Dry, dusty feeds are not as preferable during hot days. For this reason, wet feeds such as wet brewers or distillers grains, wet citrus pulp, corn silage, winter forage, alfalfa haylage or green chop should be utilized during hot weather.

It is important to remember wet feeds can mold easily; therefore, a load should be consumed within five to seven days. If one is unable to utilize wet feeds in their ration, simply adding 10 to 20 percent water to a mixed dry feed can do wonders for feed intake during hot weather. When feeding a wet ration, it is important to keep the total ration matter above 55 percent. Feeding too wet a ration can cause depressions in dry matter intake, so moisture content of a ration should not be ignored. One should also clean the feedbunk area frequently in order to prevent mold buildup.

The time you feed your cows during heat stress can also influence feed intake. In periods of hot weather it is recommended to feed 60 to 75 percent of the mixed ration after 6 p.m. The remaining 25 to 40 percent is to be fed before 6 a.m. Cows do very little eating during the day in heat stress conditions so more feed needs to be offered in the cooler hours of the day. Feed that is in the sun all day is not very palatable to the cow.


Possible feed additives


If a total mixed ration is not currently being fed, you are probably offering forages (hay and silage) separate from the concentrate portions of your ration. In this type of feeding program, cows in hot weather will generally consume more grain in relation to hay or silage. As mentioned previously, forages generate more body heat than concentrates, so cows naturally decrease forage consumption during hot weather.

Decreases in forage intake will lead to rations containing over 60 to 65 percent concentrate. This will lead to digestive upsets known as rumen acidosis. This is due to the pH of the rumen becoming too acidic which leads to the growth of undesirable rumen bugs. Generally, symptoms such as cows off feed, low butterfat test or loose manure may indicate an acidosis problem.

To help achieve optimum rumen fermentation during the summer, rumen buffers are generally recommended. Buffers, by definition, are substances that resist changes in acidity (pH) of a solution to which they are added. They are included in dairy rations to maintain a desired rumen pH of 6.2 to 6.5. Buffers are generally viewed as insurance against possible feeding problems.

Choice of concentrates can also influence rumen pH. Rapidly fermentable feeds such as corn, barley, wheat, hominy and bakery waste can rapidly drop the pH in the rumen with subsequent digestive upsets. Slower fermentable feeds such as wheat millrun, beet pulp, citrus pulp, soybean hulls and rice bran should be included to avoid any possible chances of rumen acidosis. If at all possible, try to maintain levels of 21 percent acid detergent fiber in total ration dry matter.


Feeding fat sources such as oilseeds (whole cottonseed), animal tallow, animal-vegetable blends and rumen-protected fats can be quite beneficial during heat stress. Feeding of fats increases the energy density of the ration enabling one to decrease slightly the amount of concentrate fed. Research has shown lactating cows can utilize energy from dietary fat much more efficiently than energy from body fat or from concentrates.

When feeding fat, be careful not to feed over approximately 6 to 7 percent total fat in the ration dry matter. Higher levels can affect palatability and fiber digestion by rumen microbes. Several ration adjustments should be considered when fat is added:

1. Feed adequate amounts of fiber to maintain rumen digestion.

2. Higher levels of calcium (0.8 to 1.0 percent) and magnesium (.25 to .30 percent) levels in the total ration dry matter.

3. Add one percent of a low rumen-degraded protein for each three percent additional fat in the ration dry matter. Since fat does not influence energy levels in the rumen, supplemental protein should also be available to the cow and not the rumen microbes.

Minerals: Sodium (Na) and Potassium (K)

A need for considerably higher Na and K levels for lactating cows during hot weather than previously recommended has been reported by Florida and Texas researchers. Chloride is a third mineral studied for its relationship to heat stress, but the general consensus is that most diets are sufficient to compensate for increased requirements due to hot weather. Raising dietary Na from 0.18 to the 0.4 to 0.5 percent range of DM resulted in up to a 10 percent increase in milk yields. The need for more Na in heat-stressed cows was attributed to increased urinary secretion of Na which was associated with lower K in serum and urine and depressed plasma aldosterone, which controls Na secretion.

Studies at Texas A&M University increasing K to 1.53 percent of dietary DM resulted in greater feed intake and higher milk yields. The increased dietary requirement of K in heat-stressed cows was attributed to greater excretion of K in sweat in hot compared to cool weather. Also, less forage is eaten in hot weather, which usually decreases K content of the ration. Positive responses in milk yields have been obtained in cows fed as high as 1.5 percent K (of DM).

Fungal cultures

Four studies at the University of Arizona have shown reduced rectal temperatures and respiratory rates as well as increased milk yields in cows fed an Aspergillus oryzae extract. Control cows were generally lower in feed intake than Amaferm-fed cows. We examined the addition of Aspergillus oryzae during hot weather on a commercial dairy in Fresno County. Cows fed Aspergillus oryzae had lower rectal temperatures compared to controls for nine of 12 weekly determinations. Treated cows also had significantly higher percentages of milk protein and SNF.

In another trial conducted under hot summer conditions in Arizona, cows fed yeast culture produced two pounds more milk per day than controls. Moreover, milk was of higher protein and lower somatic cell content. The higher milk yields in cows consuming fungal additives have been associated with better rumen utilization of fiber, perhaps due to increased numbers of cellulolytic organisms and more stable rumen conditions. However, the reason for rectal temperature and respiration rate reductions during heat stress periods needs clarification.

Level and type of protein

Feeding excessive levels of protein is not only expensive but can provide an additional energy drain to the animal. The nitrogen from excess protein not utilized by the animal is excreted in the urine with an associated energy cost. Studies that we conducted at the University of Arizona showed high protein diets of high rumen degradability are detrimental to cows subjected to hot summer temperatures.

Three trials involving 60 cows subjected to hot summer conditions (from May to September) in Tucson, Arizona resulted in lower milk yields and feed intakes when cows were fed 19 percent protein of high degradability compared to a 19 percent protein diet of lower degradability or the two 16 percent protein diets (of normal and high degradability). When these diets are compared at moderate temperatures, protein level and degradability affected cows much differently than in hot weather.

Results from this study show that one should avoid rations high in protein of high degradability. This is due to the energy expenditure by the animal in handling nitrogen from excess protein.


Milk production decreases during heat stress, primarily because of reduced feed intakes. Energy deprivation is magnified during heat stress because of increased maintenance requirements. Several cooling systems now available for relieving heat stress result in improved feed consumption, higher milk production and better reproductive performance.

Diets high in grain and low in forage reduce heat stress for high producing dairy cows because of lower heat of digestion. However, milk fat is depressed and digestive disorders increase during hot summer conditions when forage intake is severely limited, either voluntarily or through restriction. Feeding of buffers and supplemental fat often allow for feeding high concentrate rations without the undesirable effects. Several byproduct feeds (beet pulp, soy hulls, citrus pulp, etc.) might also aid in keeping milk and milk fat at acceptable levels during heat stress.

Work at the University of Arizona shows milk yields and feed intakes are decreased in heat-stressed cows fed diets high in protein of high degradability; whereas, cows in moderate temperatures reacted differently to protein alterations of their diets. Milk yields were higher in heat-stressed cows when Na and K in the diet were increased. Feeding of fungal cultures (both from Aspergillus oryzae and Saccharomyces cerevisiae) modified heat stress effects while increasing milk yields and feed intakes. PD

—From University of California 21st Century Dairying Newsletter, May 2007