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Cool Cows and Climate Change - Information for Dairy Farmers
How susceptible is my herd?

Dairy cattle herd susceptibility

How susceptible is my herd?

Before looking at your current farm set-up and what strategies you may decide to use to keep cows cool in the hot season, consider how inherently susceptible your herd is to heat stress.

Location

Where you farm within Australia is obviously a major factor. Regions and specific locations within regions vary significantly in terms of:

  • day and nightime air temperature
  • relative humidity levels;
  • amount of solar radiation; and
  • winds

These all determine the level of environmental heat that a cow gains or losses over time.

In addition to where you farm, there are three major animal factors which influence your herd's inherent susceptibility to heat stress. These animal factors affect the amount of metabolic heat the cow produces, and/or the ease with which heat is transferred to and from the cow's external environment.

Breed

  • Tropical cattle breeds such as the Brahman tend to be able to cope better than European breeds. They have better heat regulatory capacities than European breeds, due to differences in metabolic rate, food and water consumption, sweating rate, and coat characteristics and colour. As European breeds have a higher heat loading at the skin, they must evaporate substantially more sweat than tropical breeds to maintain normal body temperatures.
  • Of the European breeds, the Brown Swiss and Jersey are least vulnerable to heat stress, then the Ayrshire and the Guernseys. The Holstein-Freisian is the most vulnerable.

Age and liveweight

  • Younger animals are more heat tolerant due to a greater surface area to weight ratio than larger, older animals. This allows more heat per kilogram of liveweight to be unloaded through sweating. Younger animals, however, will absorb more heat from the environment due to this same greater surface area to weight ratio. Generally, younger animals with lower milk yields have lower metabolic heat loads.

Level of milk production

  • When feed is consumed and digested, metabolic heat is produced and excess amounts must be unloaded to maintain normal body temperature.
  • High producers eat more feed and generate more metabolic heat yet must still dissipate their heat load from a similar body surface area as lower producing cows. This makes high-producing herds (and higher producing cows within herds) more susceptible to high environmental heat loads.

Consider the following.

My Herd

Lower Susceptibility

Moderate Susceptibility

Higher Susceptibility

What breed are most cows in my herd?

Brown Swiss Jersey

Other European breed or cross-breed

Holstein-Friesian

What proportion of cows are in their first or second lactation?

More than 40%

 

Less than 40%

What is the herd's average milk production level?

Less than 5,500L or 400 kg MS/cow/year

5,500 - 8,000L or
400 - 600 kg MS/cow/year

More than 8,000L or 600 kg MS/cow/year

There are several other factors that affect the amount of metabolic heat a cow produces and how effectively she transfers heat to and from the external environment.

Coat colour & type

Black coated cows absorb more solar radiation than cows with lighter coloured coats during the day. At night, black cows will re-radiate heat more effectively.
Cows with dense, flat coats like Brahmins resist heat transfer to the skin better than cows with woolly coats (European breeds).  

Temperament

Temperament may also play a small role in heat tolerance. Animals that are calmer are more heat tolerant than animals that are more excitable

Diet 

Some feeds produce more metabolic heat than others.
Other dietary factors that affect the amount of metabolic heat produced include the amount of fibre versus grain / concentrates in the diet.
Any restriction in the availability of fresh, cool drinking water will of course increase
animals susceptibility to heat stress.

Previous exposure to hot conditions

Cow that have not been preconditioned to hot weather will have a greater stress response (higher breathing rate, higher body temperature). Cows need at least three weeks to acclimatise.

Activity level

Cows that must walk longer distances over hilly terrain each day to and from the dairy generate more metabolic heat.

Australian Government - Department of Agriculture, Fisheries and Forestry