Ministry of Agriculture, Food and Rural Affairs

Heat Stress in Dairy Cows

Stress Threshold

You may be comfortable when the air temperature hovers around 22 degrees Celsius and the relative humidity sits at just 50 per cent. Your dairy herd, however, is starting to feel the effects of heat stress.

That weather equals a temperature-humidity index (THI) of 68. Until recently, a 72 THI was considered the point when heat and humidity began to stress dairy cows. University of Arizona researchers have found a 68 THI is a better indicator.

Since THI accounts both temperature and humidity, a temperature of 20 degrees C at 100 per cent relative humidity can also add up to a 68 THI. The table opposite shows the THI for a range of temperatures and relative humidity levels and how they affect your cows.

When your cows experience heat stress, they have higher body temperatures and respiration rates, and increased sweating and water consumption. Their feed intake, milk production and reproduction declines. Rumen acidosis and laminitis increase.

The Arizona researchers found cows had a milk yield reduction of 2.2 kg per day after 17 hours of exposure to an average THI of 68, or when the minimum average THI was 65 or greater.

Some research points to a THI of 65 as the point when heat stress begins in high-production cows, defined as having production over 35 kilograms of milk per day. As milk production increases to 45 kg from 35 kg per day per day, the threshold temperature for heat stress drops by five degrees C.

A University British Columbia research project demonstrated how cows prefer cooler temperatures. When given access to both pasture and indoor housing, they showed a strong preference for staying in the barn during the day, when temperature and humidity increased, and for pasture at night. Daily temperatures averaged 16.5 degrees C, with a range between 10 and 28 degrees C. The THI averaged 60.5, with a range of 50 to 75.

The key to reducing heat stress is a well-designed ventilation system. You can use one or more options of natural ventilation, tunnel ventilation, basket fans and large-diameter, low-speed fans. Fans should provide a minimum capacity of 500 cubic feet per minute (cfm) per cow and up to 1,000 cfm. Air velocity should be 220 to 500 feet per minute.

To take the next step for cooling cows, fogging and sprinkler systems provide effective options.

Fogging cools the air around cows-it does not wet animals or surroundings directly. The amount of cooling achieved depends on air temperature, relative humidity and the amount of moisture evaporated.

These systems use high-pressure nozzles at 200 psi to inject moisture directly into the air. Nozzles can be installed in a ring in front of fans, or used with tunnel ventilation systems by injecting water vapour close to the inlets.

In 2009, members of the Progressive Dairy Operators organization toured several Wisconsin farms that used misting to achieve significant barn cooling. Barn interiors were much more comfortable than outdoor sun and heat.

Sprinkler systems wet a cow's hair coat on the back to the skin with water droplets. Fans blow air over the cow's body causing evaporative cooling on her skin and hair coat, and heat from her body causes moisture to evaporate.

Droplets should be large enough to wet skin surface and have to be applied intermittently to allow time for the moisture to evaporate from the skin. Fans should run continuously when the sprinklers are on. If fans blow water on the feed or stalls, the droplet size is too small or the fans need to be repositioned. Udders should remain dry.

Kansas State University researchers looked at the effectiveness of cooling with sprinklers and fans. They measured cow respiration rates and core body temperature for eight different cooling strategies.

Average air temperature during the research was 31 degrees C at 57 per cent relative humidity. The THI during the study was 80, similar to what is typically experienced during Ontario summers. The London area, for example, averages 58 to 60 per cent relative humidity at 4 p.m. during July and August, while the Ottawa area averages 53 to 55 per cent.

Cows in the Kansas study were producing an average of 41 litres of milk per day. At the start of each trial, they had respiration rates of almost 100 breaths per minute (bpm) and core body temperatures around 39.5 degrees C. Researchers used these treatments:

• no fan cooling or soaking;
• fan cooling only;
• soaking one minute every 15 minutes;
• soaking one minute every 10 minutes;
• soaking one minute every five minutes;
• a combination of fan cooling and soaking.

The sprinklers delivered 3.4 litres of water per minute or 0.0015 litres per square foot. Results are shown in the figures on pages 38 and 39.

With no soaking or fan, the cow respiration rate stayed around 100 bpm, and core body temperature rose slightly to 40 degrees C. Using fans without sprinklers reduced respiration to 95 bpm after 90 minutes, and body temperature declined slightly.

As researchers decreased the interval between sprinklers use, they saw greater reductions in core body temperatures and respiration rates. Using fans with sprinklers improved results further. Sprinklers running one minute out of every five with the fans running reduced the cows' core body temperature to 38.7 degrees C and respiration rate to 50 bpm.

It doesn't take much of a heat wave to start stressing your herd, impacting high-production cows first. Having a well-ventilated barn provides the foundation for relieving the stress, and sprinkler or misting systems provide additional cooling relief.

This article first appeared in the Ruminations column of The Milk Producer Magazine in July 2011.