Heat stress defines a condition that involves many factors. It occurs when a cow’s heat dissipation is exceeded by heat load exposure from high environmental temperature and humidity. Her metabolic heat – generated from maintenance and production requirements – adds to the environmental heat index.

The recognized clinical signs of heat stress are:

  • Increased respiration rate (panting)
  • Sweating
  • Reduced dry matter intake
  • Reduced production
  • General inactivity

General inactivity can be attributed to imbalances in energy, fluid, and electrolyte metabolism. This imbalance develops from a primary shift in energy metabolism and secondary to circulatory adjustments for water and salt losses induced by evaporative losses from skin and respiratory surfaces.

There are four ways a cow can cool herself: Conduction, convection, radiation and evaporation. The first three require a thermal gradient and evaporation that works on a vapor pressure gradient. Conduction occurs when a cow lies on a wet surface. Convection is when air moves past the skin’s surface. Radiation losses occur at night when the air cools. The fourth, evaporation, is what a cow relies on to lose the most heat.

The result of heat stress to the dairy industry is lost pounds of milk, lower milk components, reduced reproduction, compromised immune system, increased fresh cow metabolic issues, and hoof/laminitis problems. How we manage heat stress determines the magnitude of both the short- and long-term negative effect.


Unrestricted water

Water is an important nutrient. A cow will drink up to 30 to 50 percent more water at 80°F than at 40°F. However, water consumption can decrease when the temperature is over 90°F due to inactivity and decreased DMI. A cow’s water intake will be used to manage its heat stress. What’s left after maintenance requirements and heat stress usage goes to milk production. Therefore, access to quality water without restriction is important.

Fluids, including water, comprise a minimum average of 60 percent of the total bodyweight of the dairy animal. Of this total body fluid, 66 percent is found in cells and the remaining third is found in the soft tissue compartment and vascular system.

The rumen serves as a reservoir that releases consumed water to the body fluid compartments through an osmotic pressure gradient. As cells require fluid for metabolism, electrolytes and other nutrients exert osmotic pressure to continuously supply the cells with fluid for normal functions. A body fluid deficit results in reduced perfusion of fluid-transferred nutrients to the cells for metabolism. Management of the water content of the rumen should be employed to maintain both normal rumen fluid volume as well as other body fluid reserves to help cattle combat the dehydrating effects from heat stress.

To maintain normal cellular functions, the average water maintenance intake rate is 20 to 30 milliliters of fluid per pound of bodyweight daily. 1,000 pounds of bodyweight require 12 gallons (96 pounds) of water on a daily basis. If cattle consume 10 to 30 percent of their water needs from their feed ration, it would then be essential that cattle would require a daily water volume intake of 8 to 9 percent of their total bodyweight. For a lactating cow, each 10 pounds of milk produced requires an additional one gallon of water over maintenance requirements.

Severely challenged cattle can dehydrate in excess of 10 percent of their bodyweight. Clinical signs include eyes sunken into orbits, skin remains tented indefinitely, mucus membranes are dry, and a depressed attitude is evident. This degree of dehydration is potentially life-threatening.

Cattle with dehydration of 5 to 10 percent of their bodyweight will exhibit partial sunken eyes into the orbit, skin tenting that is 4 to 8 seconds in duration, tacky mucus membranes and reduced dry matter intake and a corresponding decrease in productivity. University studies indicate cattle with 7 to 8 percent dehydration levels have impaired immune response.

Cattle with 2 to 4 percent dehydration or less will have minimal observable clinical signs but physiological and performance efficiency can be reduced.

Combating dehydration

Clinical experience is the best way to assess the degree of dehydration encountered, the etiology causing the compromised fluid volume, and the method and amount of fluid required to return to normal hydration and vascular pressure status.

Rehydration and hydration maintenance for ruminates can be accomplished in three ways: 1) parental through IV or subcutaneous administration; 2) oral rumen drenching/pumping with large volumes; or 3) a combination of parental and oral volume of fluid or supplementation of patented pelleted electrolytes. These electrolytes can be added to the ration on an as-needed basis to improve fluid efficiency of hydration. It’s important for replacement fluid calculations to consider maintenance requirements, production needs and compensatory fluid losses.

Not all situations of dehydration in the bovine are easy to recognize. The rumen acts as a fluid reservoir by which body fluid balance can be maintained for a short period of time. This causes “shrink” of the animal’s normal bodyweight and, if not corrected, will lead into the stress observed as clinical dehydration.

Being aware of situations that will cause hydration issues gives an opportunity to be proactive and use preventative measures to insure body fluid homeostasis.

Providing assessable high-quality water, anticipating increased maintenance needs from environmental heat, or pyrexia associated with disease and fluid loss from diarrhea, are all examples of situations that can be addressed to maintain hydration with the use of water- or feed-added electrolytes on a pen or herd basis.

Reduced milk production is also associated with decreased dry matter intake (DMI), reduced blood flow to the mammary gland, and other factors. DMI can decrease up to 55 percent during severe heat stress compared to cows in a thermal neutral zone of 40 to 70°F. As DMI goes down, the cow’s nutrient requirements actually increase due to the physical efforts of cooling. It has been estimated that heat-stressed cows can be in a negative energy balance of 4 to 6 Mcal per day.

Adding supplements

Studies at Florida State as far back as 1987 found that adding electrolyte ions to the diet improved feed intake and water consumption. Current trials have expanded upon these studies and support the need for supplementation. When a cow sweats, potassium is lost and sodium is excreted to compensate for the potassium reduction. Therefore, potassium should be increased in the diet to at least 1.5 to 1.8 percent, sodium increased to 0.4 to 0.6 percent, magnesium (because of the higher potassium levels) needs to be between 0.35 to 0.40 percent, and chloride should be 0.3 percent. It is recommended this be used as a routine summertime diet providing the cow a reserve capacity for mild heat episodes.

Cows affected by moderate to high heat index will present as open-mouth panting and drooling. In this state she has used up her cooling capacity reserve. Respiratory rates are over 80 per minute, respiratory alkalosis is setting in, and metabolic acidosis will alter body chemistry. It is at this point that heat stress management makes the difference between disaster and a manageable situation.

There are times when a cow can experience heat stress for which she cannot cope, even with routine summertime diet and cooling techniques already in place. During these times of stress the use of yeast/fungal cultures and water or feed hydration electrolyte products can be very beneficial in reducing the effects of heat and humidity. Current trials with patented electrolyte hydration products have indicated less milk loss and the observation of better cow comfort. PD

Dr. Robert Ovrebo is a staff veterinarian at the Form-A-Feed and TechMix companies, headquartered in Stewart, Minnesota. For more information, send an e-mail to articleinfo@formafeed.com or call Ovrebo at (800) 422-3649.