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Summer is here and even though temperatures have been lower than usual we need to be aware of the effect heat can have on cows. During past summers, we have seen cows being unable to cope with a number of consecutive days with temperatures in the high 30’s.
Like most mammals, cows need to maintain a core body temperature between 38.6oC and 39.3oC to remain healthy. Cows generate heat internally (metabolic heat) from eating, digesting food and moving. They also take on heat from their environment. The ‘heat load’ of a cow is determined by the amount of metabolic heat, plus environmental heat, minus the amount of heat lost to the environment.
The comfort zone or thermoneutral zone for a cow is when the amount of heat generated and absorbed equals the amount of heat lost. When the heat load is above or below this zone it is considered to be at a critical point and physiological changes occur that can impact on feed intake, production, fertility, health and welfare.
Cows continuously gain and lose heat using the processes of radiation, convection, conduction and evaporation. The first three occur when there is a difference in temperature between the cow and its external environment. The bigger the difference the greater and quicker the heat exchange. When the air temperature is higher than the cow’s body temperature the only way to lose heat is by evaporation. High humidity and hot nights make it difficult for cows to lose their heat load.
As the weather warms up it is important to be alert by monitoring weather forecasts and looking for signs of excessive heat load in your cows. The first outward sign of heat stress is an increased breathing rate. In summer, monitor the breathing rate of cows regularly and on hot days or after extended hot periods twice daily.

Check breathing rate

  1. Select a cow and observe the flank movements
  2. Use a watch and count how many breaths they take in 20 seconds
  3. Multiply this number by 3 to calculate the number of breaths per minute
  4. Monitor the breathing rate of your highest producing cows first (the demands of higher production increase metabolic heat)
  5. Monitor the breathing rate of at least 20 cows
Use the table below to assess heat stress

Strategies for hot weather

  • Provide shade
  • Spray cows with cool water
  • On hot days change milking times to cooler parts of the day
  • Give cows access to cool drinking water at all times
  • Have a summer nutrition program
  • Mating management
  • Monitor weather forcasts and heat stress risk

Cool Cows website

Dairy Australia’s Cool Cows program and website offer extensive information and tools to help you stay ahead of and deal with heat stress.  www.coolcows.com.au
Contact Farm Services 1300 838 700 for more information on summer nutrition, mating management or if you are concerned about heat stress in your cows.