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See the signals to combat heat stress

Amelie Mainville Nadon and Danielle Kiezebrink for Progressive Dairyman Published on 30 April 2018
Fans in the barn

It may still be spring, but summer heat is just around the corner. It’s no secret warm temperatures cause stress on dairy cows.

Some effects are noticed immediately, such as reduced dry matter intake and drop in milk production and components. However, other performance indicators, like reproductive performance and lameness, will not show their negative impacts until weeks or even months later.



The physiological mechanism of heat stress is briefly explained in Figure 1.

as the temperature-humidity index exceeds 68, cows begin to pant to dissipate heat

As the temperature-humidity index (THI) exceeds 68, cows begin to pant to dissipate heat. Panting increases the cow’s respiration rate, leading to increased amounts of carbon dioxide being exhaled, subsequently increasing blood pH levels. This condition is known as respiratory alkalosis. In response, the cow secretes extra bicarbonate into the urine to maintain homeostasis.

Respiratory alkalosis reduces the amount of bicarbonate that would normally recycle through saliva to buffer rumen acids created from feed fermentation. Lower buffering capacity decreases rumen pH.

As the rumen becomes more acidic, receptors within the rumen recognize the change and slow down rumination. The cow will chew less and produce less saliva, further reducing the rumen buffering capacity.


The behaviour of the cow will also be affected by heat stress as stress hormones like cortisol and epinephrine are released, negatively impacting milk letdown. Cows will stand more in order to increase the amount of surface area to help dissipate heat through conductive cooling.

This attempt to cool down by circulating more blood to the skin and ears reduces the amount of blood going towards the feet and the reproductive and mammary system, thus affecting hoof health, milk production (-4.5 kilograms per cow of milk at 80 THI) and reproduction (8 percent decrease in gestation rate with 65 to 69 percent humidex).

It is also important to remember it takes about 80 to 100 days for a follicle to develop into an oocyte that can be fertilized, so summer heat stress will impact fall breedings.

Contrary to what many may think, the situation worsens at night with cooler temperatures because cows will begin to pant less, increasing carbonic acid that has accumulated and needs to be excreted.

This inconsistent level of carbonic acid, blood pH and rumen pH leads to metabolic acidosis and impairs production and components.

As time goes on, sub-acute rumen acidosis may also further degrade hoof integrity. This impact will not show up until two to three months after the initial start of heat stress.


The above signals and metabolic challenges make it critical to have a heat abatement plan. There are three sections of the CowSignals “diamond” (Figure 2) that can be maximized to help reduce the impact of heat stress.

there are three sections of the cowsignals diamond that can be maximized to help reduce the impact of heat stress


Cows need fresh, dry air to breathe. Cows cool down efficiently by breathing in dry air and exhaling wet air (sweat through their lungs). Replacing ammonia and bacteria-contaminated air, and maintaining dry floors and beds, will reduce the incidence of mastitis and lameness.

Where should fans be placed? Close-up animals are the highest-risk group under the most stress (nutritionally, physiologically and metabolically).

Second, place fans pointing in the opposite direction of cow flow in the milking holding area. Cows experience the most heat stress in this area. Compounding effects of close confinement, standing, jostling and milking will cause the cows’ body temperatures to increase very quickly in a short period of time.

For every 1ºC increase in body temperature, it takes twice as long for a cow to cool down. So pay extra attention to the holding area fans, air flow and water cooling systems as the cows leave the parlour.

Cows should spend 50 percent of their daily time budget resting. To maximize comfort and lying time, fans should be placed over the stalls to encourage heat abatement as well as resting.

Fans should not be placed over the feed alley. Cows will spend roughly 25 percent of their time at the bunk eating. If fans are placed at the feed fence versus the stalls, this will encourage cows to stand where the air flow is versus lie down and rest.

As an important reminder, proper fan maintenance is a low-cost way to improve the fans’ efficiency. Wind speed should be maximized at 8 kilometres per hour. Dirty fan blades are less efficient. By keeping fan blades clean, fan efficiency will improve by 25 percent or more.


On average, milk is 87 percent water. The more the cow drinks, the more milk the cow will produce. Under normal conditions, a ratio of 5-to-1 for water and dry matter intake is a good starting point.

For example, a cow consuming 20 kilograms of dry matter intake will consume 100 litres of water. Another rule of thumb suggests for every litre of milk produced, a cow will drink at least 3 litres of water.

In heat-stressed conditions, the volume of water consumed will double. The cow will consume most of its water intake after milking. Ensure there is easy water access exiting the parlour. It is important to clean the water troughs (including around the floats) three times per week to improve water quality and stimulate intake.

Ensure there are multiple water sites evenly spaced throughout the barn, with a target of 3 to 4 inches per cow of linear space, and water flow and pressure are maximized at 16 to 20 litres per minute and 60 pounds per square inch, respectively.

If your facilities permit, cool cows by evaporative cooling. Water cooling should be used when temperature exceeds 28ºC. Look into soakers or misters to find out what system works best for your management style.


Heat stress will impact dry matter intake negatively. Formulating for higher-density nutrients to meet production demands is critical. To help the cow’s metabolic system, adjust minerals and buffers, and consider increasing the potassium, sodium and magnesium levels within the diet.

Evaluate forage-to-concentrate ratio, as reducing the forage levels may temporarily help maintain milk production, but it can set up the herd to be acidotic.

Aim to distribute fresh feed during the coolest parts of the day (early morning and in the evening). If feeding only once per day, feed at night. This will provide feed at the coolest time of the day, as well as ensure cows have feed available throughout the night.

Ensure the moisture of the TMR is adequate to prevent sorting and potential subsequent rumen acidosis. Look into proven additives that will help the cow improve its ability to buffer the rumen and improve rumen fermentation, such as sodium bicarbonate and yeast.

Consider the cost-benefit of proven specialty cooling additives to improve heat abatement. Cooling additives can work intracellularly to reduce animal body temperature as well as hydrate the body in order to encourage intakes and maintain milk production and components.

There are many management and nutritional factors that can improve the cow’s ability to combat heat stress. In combination, the listed heat abatement strategies will enable your herd to overcome heat stress more effectively, reducing negative monetary losses through upheld herd performance.

Working alongside your nutritionist and veterinarian and paying attention to the signals will enable you to effectively battle the upcoming summer heat.  end mark

Danielle Kiezebrink is a formulator and ruminant technical support for Cargill Animal Nutrition Canada/Purina. Email Danielle Kiezebrink.

PHOTO: Air, water and feed are three areas to focus heat abatement strategies. Courtesy Photo.

Amelie Mainville Nadon
  • Amelie Mainville Nadon

  • Canadian Dairy Technical Services Manager
  • Cargill Animal Nutrition Canada/Purina
  • Email Amelie Mainville Nadon

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