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How heat-stressed are Canada’s dairy cows?

Dean Wood for Progressive Dairyman Published on 31 May 2019

From coast to coast, the Canadian climate sees large variations in levels of heat and humidity, but even a few days of elevated temperatures and moisture in the air can cause a dairy cow to experience heat stress and the negative impacts on production, reproduction and health that follow.

That’s why ventilation systems play a critical role in helping cows relieve excess heat, while minimizing losses to the bottom line. 



What is heat stress?

In order to design systems to minimize and prevent heat stress, we need to define it and identify the costs associated with it. The term heat stress is commonly associated with hot temperatures and the direct impact those temperatures have on production. In general terms that is accurate, but there are numerous variables that both directly and indirectly affect the level of heat stress in a herd.

Quantifying the impact of heat stress during a short time frame is difficult. It can directly impact feed intake and nutrition, milk production, fertility and overall cow health. All of these are critical to the success of any operation. 

We understand cows absorb heat from their environment and generate metabolic heat from movement. Cows then radiate this heat to both the air and ground around them, while also losing heat from sweat evaporating and breathing. That’s where the heat stress and temperature-humidity index (THI) chart shown in Figure 1 comes in handy.

temperature-humidity index

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The inclusion of relative humidity greatly affects the level of heat stress induced on the herd. As those levels increase, so does the impact (as outlined in the chart’s key). It becomes clear the specific geographic location of the dairy directly affects the level of heat stress induced.

Heat stress levels across Canada

Once we understand the THI, we need to investigate the climate in our area. The following data has been compiled to provide knowledge of the average climatic conditions of four geographical areas (Table 1).

canadian heat stress numbers

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Both Quebec and Ontario share similar climates. The historical temperature and humidity data confirm this and identifies a period of 123 days as a concern for heat stress. Of note, the year 2005 holds the record for highest total amount of days above 30ºC, with 41 days of primary concern (approximately 11% of the year). Assuming the worst-case scenario, a dairy farmer in Ontario or Quebec needs to be more observant for heat stress indicators for approximately one-third of the year. 

Although Alberta shares a similar amount of humidity, the temperature conditions differ from that of Ontario and Quebec. The data indicates a period of only 92 days for primary concern. Again, assuming the worst-case scenario, a dairy farmer in Alberta needs to be more observant for one-quarter of the year.


British Columbia represents a completely different set of climatic conditions that neither Alberta nor the Eastern areas can match. With more days of primary concern and higher humidity during those days, a dairy farmer in British Columbia has their own unique challenges. However, it should be noted the spikes in high temperatures in July and August are not as severe (nor are the spikes in low temperatures in December, January and February). 

How to relieve heat stress

Now that we’ve grasped both the climate-related issues and the problems they can cause, how do we relieve the stress on dairy cows caused by elevated heat and humidity?

Cows get rid of body heat through four primary ways: conduction, radiation, evaporation and convection. The Ontario Ministry of Agriculture, Food and Rural Affairs (OMFRA) suggests a three-step approach:

1. Ensure the ventilation system is working effectively by reducing any barriers to airflow around buildings and checking wall design. Some screening used to keep birds out can greatly reduce airflow.

2. Provide adequate water space and volume. Water consumption increases as temperature increases. It’s important to have an unrestricted water supply when and where cows need it.

3. When ventilation cannot handle the heat load, supply supplemental cooling over the cows. This can increase their convective cooling rate significantly. If heat stress is still a problem, sprinkler or fogging systems can be used to increase the evaporative cooling rate. 

Selecting the proper ventilation system is the key to controlling the environment and preventing heat stress from occurring. But what ventilation system is right? Do you base the entire design of a barn around 11% of the year? These are all questions that should be discussed directly with your ventilation supplier because there are pros and cons to every option. The key is to understand your geographic variables. Discuss the alternatives openly and thoroughly with a knowledgeable and experienced ventilation company. 

Based on the selection of ventilation equipment, a corresponding discussion is necessary to determine proper design of the supplemental cooling equipment. Type, size, placement, performance, life span, capital cost, operational cost and rebates all play a significant role in the success of any cooling system.  end mark

References omitted but are available upon request. Click here to email an editor.

Dean Wood is a sales manager with Envira-North Systems Limited. Email Dean Wood.