Heat Stress

Dairy cows in all areas of New Zealand are affected by heat stress during summer. The comfort zone for a cow is 4-20°C, about 10-15° lower than the comfort zone of a human.


  • Heat stress occurs when an animal's heat load is greater than its capacity to lose heat. Cows feel hot 10-15°C sooner than we do.
  • High air temperature, humidity, solar radiation and low air movement contribute to increased risk.
  • High relative humidity decreases evaporation and reduces the cow's ability to lose heat by sweating and breathing.
  • When air temperature is greater than about 21ºC and relative humidity is greater than 70%, cows begin to reduce their feed intake, and milk production is reduced. Jerseys are more tolerant of heat, with production losses insignificant until 25ºC
  • Cows radiate heat during the night to the cooler surroundings. Warm cloudy nights can reduce cooling, increasing the risk of heat stress.


As in humans, cows likely experience headaches, irritability and lethargy when they are too hot and have insufficient water. To cope with heat, cows use a variety of strategies including:

  • increased breathing rate and sweating
  • increased water intake
  • decreased feed intake and
  • decrease milk production
  • change in milk composition, e.g. fat % and protein % declines
  • change in blood hormone concentration, e.g. increased prolactin
  • changed behaviour:
    • seek shade
    • crowd together to shade each other
    • refusal to lie down
    • change orientation to sun
    • stand in water or next to water troughs.

Minimising heat stress

To minimise impacts on productivity and protect cow comfort, consider the following:

Lactating cows will typically require more than 100 litres/cow/day and will drink between two to six times per day. Ensure flow rates to troughs are high enough that the trough never runs dry. Most cows drink soon after milking, so install water troughs in races to meet that need.

Ensure summer pasture is of high quality. Feed with a high fibre content can increase the heat of fermentation in the rumen, increasing the heat load on the cow (e.g. non-irrigated summer pasture). Provide supplementary feed at night when it is cooler.

Use paddocks with shade trees during periods of heat stress – ideally 5m2 of shade per cow, to minimise competition (see Trees for Shade). Provide shade at the shed if possible. Install shade cloth in off paddock facilities.

Reduce the walking distance and speed to the dairy. Reduce the time spent in unshaded yards. Minimise handling stress. Isolate cows most severely affected by heat stress and provide shade and cooling. Milk earlier in the morning and later in the afternoon, or consider once-a-day milking.

Sprinklers can be used over the dairy yard to wet the cows coat and aid evaporative cooling for between two to six hours after milking. However, sprinkling can increase the humidity around the cows, especially when they are held close together. The effectiveness of sprinkling depends on the removal of water vapour by air movement, ideally by using a fan.

Temperature Humidity Index (THI) calculator

The calculator below can be used to help manage heat stress in cows. It can also be downloaded in xls format.

Calculate Humidity

Temperature (°C)
Relative Humidity (%)

Temperature Humidity Index:

Note: This calculator uses the following formula:
THI = 0.8T+[RH x (T-14.4)]+46.4 (where T is daily maximum temperature (°C) and RH is mean daily percent relative humidity divided by 100.

Production Impacts

When THI reaches 68 for Friesians and 75 for Jerseys (equivalent to 21 and 25.5°C respectively at 75% relative humidity) New Zealand cows begin to experience the effects of heat stress. This is seen by a reduction in feed intake and a drop of around 10g milksolids per day per unit increase in THI (Bryant et al. 2007).

For example, an increase in THI from 68-78 would equate to a 100g drop in milksolids/cow/day. Note that a drop in fat/protein percentage occurs before a drop in yield or milksolids is visible (Bryant et al. 2007).

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