Heat stress

Heat stress increases maintenance energy requirements and it lowers dry matter intake, making it difficult to meet energy needs. Therefore, feeding management and forage quality become critical during heat stress.

Rumen acidosis must also be controlled. Raising DCAD to 30-45 meq/100 g DM can help control metabolic acidosis. Heat stress also affects reproduction negatively. The goal should be to keep body temperatures below 102.5oF (39.2oC) and respiration rates below 80 per minute by providing ventilation, shade, and misters.

Energy Needs Associated with Heat Stress:

When we develop rations, the computerized programs that we normally use assume that a cow only lives in a barn with a temperature of 68oF (20oC), the cow is never subjected to wind, rain, high humidity, or snow, and the cow is not standing in mud halfway up her body. As we all know, no matter how hard we try, this is not the environment of all cows all of the time.

Dairy cows need energy to make milk, grow, and develop a calf. They also need energy to maintain themselves. Maintenance energy is the energy needed for walking, breathing, eating etc. A cow’s maintenance energy requirement increases if she is heat stressed. Environmental heat will increase body temperature, respiration rate, and loss of electrolytes. The critical temperature for a cow ranges from 80o to 85oF (26.7o to 29.4oC). This is the temperature at which the cow may exceed her capacity to get rid of extra body heat and rectal temperature may rise. Cows must meet their energy needs for maintenance before they can use energy to make milk. So, cows under heat stress must be supplemented with extra energy or they will have less energy to make milk. If a cow cannot get rid of her extra heat, it is estimated that milk production will decrease by 1.22 pounds (0.55 kg) for every 1oF (~ 0.6oC) increase in rectal temperature. Relative humidity, air movement, and solar radiation all contribute to heat stress. High humidity will intensify the effects of high temperatures.

The maintenance requirement of lactating dairy cows can increase by as much as 30% if temperature rises from 80o to 104oF (26.7 to 40oC) for 6 hrs/day due to the cost of panting and changes in tissue metabolism. To put this into perspective, according the NRC (1989), a 1300-pound (591 kg) milking cow requires 9.57 Mcal of NEl to maintain herself. In the above heat stress situation, she would require 12.44 Mcal of NEl to maintain herself. If she were producing 90 pounds of milk per day (41 kg), she would require 28 Mcal for milk production. The heat stress needs would take her total ration energy requirement from 37.5 Mcal to 40.5 Mcal/lb. This level of energy may not be practical or healthy to achieve. As a consequence, many farmers in severe heat stress situations will simply sacrifice milk production.

Animals in open lots are more often subjected to direct sunlight. This increases heat stress. In one study, where cows were either exposed to direct sunlight or were shaded, it was found that those animals in the direct sun had 15% less dry matter intake and 22.7% less milk yield. It is important to recognize that on many farms the greatest heat stress arises with replacement heifers and dry cows because of limited housing. They too will suffer from heat stress and incur added energy costs.

Formulating Rations for Heat-Stressed Cows:

As described above, cows require more energy to combat heat stress. It is therefore normally recommended that dietary energy intake increase during times of heat stress. But, the total energy consumed by the cow is difficult to increase due to poor dry matter intake in heat-stressed cows. Acidosis concerns and dietary fat limitations usually also limit increases in dietary energy density to 2-3 Mcal/lb (4.5-6.5 Mcal/kg).

Heat stress lowers dry matter intake, especially forage intake. Since fiber undergoes more fermentation in the rumen than grain, it increases body temperature more. Cows will often decrease forage intake during hot weather. Extra effort should therefore be made to feed forages with higher NDF digestibility during times of heat stress in order to reduce the heat of fermentation in the rumen.

Feeding management becomes even more critical during heat stress times. One may need to have more evening feedings and better overall bunk management. Feeds will spoil more quickly in warm weather. It may be especially helpful to feed more often. Definitely, do not let silage sit for hours in a wagon prior to feeding. Of course, sweeping out the feed bunk on a daily basis becomes even more critical in the summer months.

Relative changes in maintenance requirements and dry matter intake for 1300 lb. cows producing 60 lb milk of 3.7% fat at various ambient temperatures

Temperature (oF) Maintenance Req’t(% of 68oF Req’t) Expected DM Intake (lbs/day)
68 100 40.1
77 104 39.0
86 111 37.3
95 120 36.8
104 132 22.5

Relative changes in maintenance requirements and dry matter intake for 591 kg cows producing 27.27 kg milk of 3.7% fat at various ambient temperatures

Temperature (oF) Maintenance Req’t(% of 68oF Req’t) Expected DM Intake (lbs/day)
20 100 18.23
25 104 17.73
30 111 16.95
35 120 16.73
40 132 10.23

(adapted from McDowell et al., 1976)

We rely on forage intake to stimulate cud-chewing which produces saliva to buffer the rumen. When cows eat less forage during heat stress, rumen acidosis becomes a concern. Fluctuating eating patterns caused by heat stress can create slug-feeding problems that can further increase the risk of acidosis. It is imperative to pay attention not only to total forage fiber but also fiber length. It may be especially helpful to add 2-3 pounds (0.9-1.4 kg) of long hay to diets of cows undergoing heat stress. Addition of rumen buffers (one ounce per 10 pounds of milk) (28 g /4.5 kg of milk) should be helpful in maintaining rumen pH. Yeast culture (2-4 oz./cow) (57-114 g/cow) may also help to maintain dry matter intake.

Balance heat-stress rations for lower expected dry matter intake. Make sure adequate amounts (pounds, kg, or Mcal) of total protein, energy, minerals, and vitamins are consumed when dry matter intake is depressed. For example, actual percent protein in the ration may need to increase. Unfortunately, slight increases in the concentrate:forage ratio may be necessary to accommodate for the decrease in dry matter intake. A balancing act must therefore be done between providing enough energy to the cow but not causing acidosis. Watch total NFC levels, especially fast-digesting NFC’s, because high levels can cause acidosis. Slight increases in dietary fat (without exceeding 5% rumen available fat and 7% total fat in the ration) should be helpful. Fat provides 2.25 times more energy than a pound of carbohydrate and it is not fermented to acid in the rumen.

Mineral Needs of the Heat-Stressed Cow:

Recent research has shown that we need to be concerned not only with rumen acidosis but also with metabolic acidosis. We know that the rumen microbes suffer under acidic conditions. The cells of the cow’s body also have trouble when they encounter too much acid. Acids change enzyme activities and affect the structure of molecules. The cow’s regulation of blood pH is almost as important as her need for oxygen.

Although cows do not have many sweat glands, they will sweat a certain amount during hot weather and lose electrolytes. Heat-stressed cows lose a lot of potassium and they can become potassium deficient. The loss of potassium increases blood acidity. Researchers have found that they can increase blood pH by increasing the dietary cation-anion difference (DCAD). This is the difference between the amounts of positively charged cations (especially sodium and potassium) in the diet and the negatively charged anions (especially chloride and sulfur) in the diet. Raising DCAD increases the ability of the cow’s blood to buffer acids and this raises blood pH (decreasing acidity).

Researchers have found positive milk production responses when they have raised DCAD to 35-45 meq/100 g DM. The author has been able to achieve 30-35 meq/100 g DM on commercial dairies in northeastern U.S. and has seen positive results. With the current mineral sources available, it is difficult and costly to achieve 35-45 meq/100 g DM. A study was conducted on a commercial dairy in Florida. They only raised DCAD from 19 to 25 and raised milk production by 3 pounds (89.5 vs. 86.5 pounds) (40.7 vs. 39.3 kg).

Generally, it will require 1.6-1.8% dietary potassium, 0.75-1 pound (0.34-0.45 kg) of added buffer, and 0.40% sodium to significantly increase DCAD. Chloride levels also need to be controlled (0.40%). Therefore, potassium carbonate will need to be used rather than potassium chloride (a more popular source of potassium in the feed industry).


Cows that are heat stressed tend not to show heats, and if they are bred, they have low conception rates. Insufficient energy consumption, reduced blood flow to the reproductive tract due to diversion of blood flow to peripheral tissues for heat dissipation, and high temperatures within the reproductive tract have all been suggested as possible causes of poor reproductive performance during heat stress. However, there is still much speculation among researchers. Arizona researchers calculated that for every 10oF rise in environmental temperature over 50oF at the time of artificial insemination, conception rate declined 5%. For this reason, one may choose not to breed a cow and just short cycle her with prostaglandins (and hope for cooler weather the next time) when the weather is very hot.

Florida researchers found that calf embryos were more resistant to heat stress as they aged. Embryos developed substantial resistance to heat stress by day 3 after conception.


Since water is the most essential nutrient and milk contains 87% water, the importance of an adequate supply of clean, fresh water cannot be overemphasized. Cows drink more water right after milking or after eating. Therefore, attention to water availability is especially important at those times of the day. One may also want to consider adding some water to the silage or TMR offered. Adding water to a TMR can increase water intake by 5-10%. Cows that are hot need more water. The table below shows the differences in water intake that may be seen with changes in environmental temperature. (See Water)

Drinking Water Intake at 40oF (gal/day)
Drinking Water Intake at 60oF (gal/day)
Drinking Water Intake at 80oF (gal/day)
Heifer, 200 lbs 2.1 2.4 3.2
Heifer, 400 lbs 3.2 4.5 6.1
Heifer, 600 lbs 6.3 7.9 10.5
Heifer, 1200 lbs 8.7 10.8 14.5
Dry Cow, 1400 lbs 9.7 11.8 16.1
Milking Cow, 1400 Ibs, 60 lbs milk 21.8 23.9 24.7
Milking Cow, 1400 lbs, 80 lbs milk 26.8 31.8 38.7
Milking Cow, 1400 Ibs, 100 lbs milk 31.8 37.1 45.5

Estimated Drinking Water Intake of Dairy Cattle at 4.4, 15.6, 26.7 oC

  Drinking Water Intake at 4.4oC (l/day)  Drinking Water Intake at 15.6oC (l/day)
Drinking Water Intake at 26.7oC (l/day)
Heifer, 90.9 kg 7.95 9.08 12.11
Heifer, 181.8 kg 12.11 17.03 23.09
Heifer, 272.7 kg 23.85 29.90 39.75
Heifer, 545.5 kg 32.93 40.88 54.89
Dry Cow, 636.4 kg 36.72 44.67 60.94
Milking Cow, 636.4 kg, 27.27 kg milk 85.52 90.47 93.50
Milking Cow, 636.4 kg, 36.36 kg milk


120.37 146.49
Milking Cow, 636.4 kg, 45.45 kg milk 120.37 140.44 172.23

(Adapted from Eastridge and Watson, 1990)

(The charts shows average drinking water consumption of cows fed a ration with average moisture (45-55%)

Note: 1 gallon = 8.34 pounds of water

In general preference studies, cows like to drink warmer water (86oF) (30oC). But, there is one study where researchers looked into the effects of offering cool water (50oF) (10oC) versus warm water (82oF) (27.8oC) when daily environmental temperature was over 95oF (35oC). Intake of cool water was 42% higher than that of the warm water. Cows drinking the cool water also had higher dry matter intakes and milk production. But, in two other heat stress studies, there was shown to be no benefit to cooling water.

Heat Stress and bST:

The feeding and management recommendations for a cow treated with bST are not any different from that for any high-producing cow. Nutrient intake needs to be increased with special attention paid to energy and water needs. Keeping the cow as cool as possible is important. There are farmers who feel that with the extra stress of heat, cows won’t benefit from bST. Israeli research indicated that the response to bST under high environmental temperatures may have been slightly reduced. Work conducted at the University of Missouri, showed that heat-stressed injected cows responded to bST with increased milk production and dry matter intake. Hot temperatures did not reduce the effects of bST.

Reducing Heat Stress on the Cow:

By far the best solution to heat stress is not to have it in the first place. The dairy producer’s goal should be to keep body temperatures below 102.5oF (39.2oC) and respiration rates below 80 per minute. Heat stress should be evaluated at the cows’ nose level both lying down and standing at the bunk and in the holding pen. Often, considerable heat stress occurs in the holding area while cows are waiting to be milked.

Ventilation is critical. Natural ventilation using side-wall curtains works very well. Many farmers have been creative in modifying existing facilities for greater natural ventilation without a lot of cost. Often times the extra milk produced during a hot summer will significantly help to pay to put curtains on in the fall if walls are just ripped off in the summer. Fans are usually needed, especially over the feed bunk and in the holding pen. Florida experts suggest that the air speed over the cow should be 400 to 600 feet (122 to 183 meters) per minute. Usually this requires at least one 36-inch (91 cm) fan (with airflow of 11,000 cfm) for every 30 feet (9.1 meters). Fans need to be angled downward. Shade needs to be provided. Sometimes even inside freestall barns, cows avoid the afternoon sun shining in on one side of the barn. Shade cloth can be very effective. Misters are also helpful for enhancing evaporative cooling.


Ax, R.L. 1994. Summer heat stress reduces conception. Hoard's Dairyman. May 25, 1994. p. 412.

Bray, D.R. and R.A. Bucklin. 1994. What you can do to keep cows cooler? Hoard's Dairyman. May 25, 1994. p. 14.

Coppock, C.E. 1986. J. Dairy Sci. 69:1004. 69:1013.

Ealy, A.D., M. Drost, and P.J. Hansen. 1993. Developmental changes in embryonic resistance to adverse effects of maternal heat stress in cows. J. Dairy Sci. 76:2899.

Eastridge, M.L. and M.E. Watson. 1990. Water for dairy cattle. In: Proceedings of the Ohio Dairy Day, July 13, 1990, p. 68.

Harris, Jr., B. 1994. Water must be plentiful, good and handy. Hoard's Dairyman. May 10, 1994. p. 370.

Huber, J.T. and G.E. Higginbotham. 1986. Feeding systems for heat stress in lactating cows. Anim. Health and Nutrition. May-June 1986.

Johnson, H.D., R. Li, W. Manalu, K.J. Spencer-Johnson, B.A. Becker, R.J. Collier, and C.A. Baile. 1991. Effects of somatotropin on milk yield and physiological responses during summer farm and hot laboratory conditions. J. Dairy Sci. 74:1250.

Lotan, E., H. Sturman, J.I. Weller, and E. Ezra. 1993. Effects of recombinant bovine somatotropin under conditions of high production and heat stress. J. Dairy Sci. 76:1394.

McDowell, R.E., N.W. Hooven, and J.K. Camoens. 1976. Effect of climate on Holsteins in first lactation. J. Dairy Sci. 59:956.

West, J.W., B.G. Mullinix, J.C. Johnson, Jr., K.A. Ash, and V.N. Taylor. 1990. Effects of bovine somatotropin on dry matter intake, milk yield, and body temperature in Holstein and Jersey cows during heat stress. J. Dairy Sci. 73:2896.

Related Links:

Feeding and Managing Cows in Warm Weather
B. Harris, Jr., Ph.D., University of Florida
Describes the different strategies cattle use to lose body heat and how to take advantage of these in a feeding management plan

How to Reduce Heat Stress in Dairy Cattle
J. Keown & R. Grant, University of Nebraska - Lincoln
Discussion of general issues important to consider to relieve heat stress in cattle.

Air Velocity Improves Summer Cow Comfort in Freestall Barns
Brian Holmes, University of Wisconsin
Details various ventilation and air movement systems that optimize body cooling in freestalls.

Engineering Options for Reducing Dairy Cow Heat Stress
Kevin Janni, University of Minnesota
Discusses a variety of facility options aimed at reducing heat stress in dairy cows. Most are based on freestall operations.

Heat Stress in Dairy Cattle
J. Pennington and K. VanDevender, University of Arkansas
Article offers a variety of ways to detect and reduce heat stress. Includes recommendations for portable shades, holding areas and cooling systems in the barn.

Economics and energy considerations when managing the dairy enterprise
D.B. Fischer
Brief discussion of issues such as shade and cooling systems, with an emphasis on the costs involved with implementing such systems.

Circulation Fans vs. Wind Tunnel for Freestall Barns
B. Holmes, University of Wisconsin
A comparison of two ventilation systems commonly used in freestalls. Provides general recommendation for each type of system.


Mary Beth de Ondarza

Mary Beth de Ondarza
45 articles

Nutritional consultant for the dairy feed industry at Paradox Nutrition, LLC.

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Dr. de Ondarza received her Ph. D. from Michigan State University and her Masters Degree from Cornell University, both in the field of Dairy Nutrition.

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Paradox Nutrition

Paradox Nutrition

Paradox Nutrition, LLC is a nutritional consultation business for the dairy feed industry. Mary Beth de Ondarza, Ph.D. is the sole proprietor.

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