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​Temperature Impacts on Dairy Cows

Heat stress is the main challenge dairy cows will face in a changed climate. Heat stress assessments take into account

  • solar radiation
  • wind speed
  • humidity.

The lowest daily temperatures are as important as the highest temperature because the ability to have some respite from the heat helps restore the cow’s metabolic equilibrium and minimise the decline in milk yield.

There is a temperature range where normal body functions occur, the actual range depends on many factors including the animal’s age, breed, body condition, milk yield and diet. The bigger they are, the lower the surface area to mass ratio, the harder it is to cool. Also the higher their milk production is, the less tolerant they are of heat stress. Cows can acclimatise to an increase heat load over a period of a few days to several weeks depending on species, breed and production levels.

Reduced feed intake is a complex response to heat stress but it is only part of the reason for reduced milk production. Changes to the blood circulation and panting in hot conditions effects the digestive system and compromises rumen health.

Heat induced behaviour changes in cows occur before changes to production become apparent.  In hot conditions cows:

  • seek shade
  • change grazing time
  • increase water consumption.

Research results suggest that dairy cattle are compromised in temperatures over 25C and particularly above 29C. 

Animal Disease

Climate change will introduce the risk of increased disease occurrence and severity from existing or new sources. New diseases may come from a foreign source, emerge or evolve from existing sources. The same climate conditions that favour disease development may also be stressful to animals and increase the impact of the disease on the farm system. For example , there is a link between hot conditions, the weakened immune system of heat stressed cows and incidence of mastitis.

In warm humid conditions, pastures are at risk of being infected with fungi-related (mycotoxic) diseases. Two of these diseases are facial eczema and ryegrass staggers. Animals are more exposed to the risk of ryegrass staggers in hot humid conditions. When pasture growth rates slow down, such as during a dry period, pastures are likely to be grazed to lower residual lengths, there-by exposing the animals to higher levels of the particular mycotoxin that causes ryegrass staggers because the toxin producing fungi are mainly found in the base of the plants.

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