It has been a slow start to summer but farmers should start preparing to make cows more comfortable as the temperature increases. These days, dairy cows work a lot harder than they used to, and as farmers strive to increase milk production the heat load on cows becomes tougher. Australian cows are exposed to one of the hottest continents, and whether or not greenhouse is increasing, production benefits result when efforts are made to cool cows down over the summer.

How do we measure the heat load on a dairy cow?

Heat load in dairy cows is a combination of temperature and humidity, and is termed the temperature humidity index (THI). The higher the humidity, the lower the temperature required to obtain a given THI. Cows start to get into trouble when the THI increases above 75. For example, at 50% relative humidity, the temperature required to give a THI of 75, 78 and 82 is 28, 30 and 33 respectively. At an 80% relative humidity, the temperatures required are 25, 27 and 29. Figure 1 shows the relationship between temperature, humidity and THI. A THI of 75 will start to reduce conception rates in dairy cows; a THI of 78 will result in reduced milk production, while a THI of 82 results in big drops in milk production.

Table 1: The chances of a THI of 78, or a THI of 82 ( ) or higher being exceeded for 2 days or more at various locations in Victoria.


What factors influence the effect of heat load in a cow?

Not only are environmental factors important for cows, there are differences between cows which influence the effect of heat stress. These include:

  • Breed: Channel Island breeds are more heat tolerant than Holstein Friesians;
  • Coat type and colour: Woolier hair coats increase heat flow to cows; a black hair coat will absorb more heat than a white hair coat;
  • Younger animals: Have greater heat tolerance than older animals because of the higher body surface area to weight ratio (this is also why they are more affected by cold exposure);
  • Higher producing cows produce more metabolic heat and have less heat tolerance;
  • Cows fed more grain produce more heat than cows fed less grain- this effect is more pronounced above 5kg per day of grain feeding.

What happens as cows warm up?

Generally, cows are able to cope with heat until the THI hits a critical value after which productivity reduces. Excessive heat loads usually build up over a number of hot days. Symptoms include seeking shade, refusal to lie down, reduced feed intake and crowding around water troughs. More severe signs include restlessness, open mouthed breathing and excessive salivation. The most important effect on the dairy farm is reduced feed intake, and therefore reduced milk production.

Costs of no control measures for heat stress in a 400 cow herd.

A simple economic analysis of a herd of cows located at Warrnambool without any management systems in place to control heat stress in the herd shows the costs of heat stress to be considerable. With a summer milk price of 32 cents per litre, and a range of 0.5 to 1.0 litres per THI unit above 72 in reduced milk production, the likely costs associated with heat stress in this herd is between $8960 and $17920. This cost does not include those associated with reduced component tests, reduced reproductive performance or any increase cell counts associated with stress. The same analysis showed that with good control of heat stress losses associated with heat stress were reduced to $2048 to $3968.

Methods used to cool cows down.

It is best to adopt an integrated approach rather than relying on a single technique when managing heat stress in dairy cows. Some techniques include:

Milk cows later in the day: cows milked two hours later in the afternoon will produce 1 to 2 litres more milk than cows milk mid afternoon;

Sprinkling cows before the afternoon milking will cool cows down as long as there is a breeze blowing. Use the sprinklers to cool the yard, then provide a 2 minutes on 5 minutes off for 30 to 60 minutes before milking. Providing shade at the dairy will aid cooling. There are advantages in fly control and yard cleaning when sprinklers are used. It is important to provide a water droplet size which wets cows to their skin: providing a mist that sits on the coat surface will increase heat load by acting as an insulating layer around the cow;

Feeding cows a mixed ration of conserved feeds during the day, and preferably under shade, and allowing access to good quality pasture at night will increase milk production. Cows have trouble maximising pasture intake during the heat of the day;

Provide shade: natural shade trees, shade cloth and galvanised iron roofs have all been examined. In trials, cows usually prefer the iron roof. Any roofing system should be vented and be at least 3.7m above the ground. Iron roofs with a sprinkler system provide luxury accommodation for most cows.

Addition of salt to the diet: cows sweat to cool down and therefore lose sodium and potassium. On irrigated pastures potassium requirements of cows will be satisfied. However trials in northern NSW showed that cows supplemented with 120g of salt per day had better milk production than unsupplemented cows. Cows grazing dryland ryegrasses are likely to be potassium as well as salt deficient in summer, and may need supplementing.

More information.

Much of the information in this article has been obtained from the Dairy Australia publication
“Cool Cows”. Additional information can be found on the following websites: