Heat Stress


6 min read

Watch for these changes Checking breathing rate Minimising heat stress Is heat a problem in my area? North Island South Island Why do cows get hot so easily? Podcast

Heat stress in cows happens when they can't dissipate the heat they generate, leading to discomfort and lower milk yields. Unlike humans, cows can start feeling the stress at temperatures below 20°C. Providing shade and ample drinking water is the first line of defence against heat stress. You might notice that your cows breathe faster, graze less, drink more, and move slower during heat stress. You can also make strategic changes to their management and feeding routine. Checking the impact of heat in your area can guide your approach. The podcast included provides in-depth strategies and scientific explanations on managing heat stress.

Heat stress occurs when cows have more heat than they can get rid of and leads to discomfort and lower milk production.

Cows begin to experience heat stress at much lower temperatures than humans and prefer temperatures below 20°C. All areas of New Zealand get hot enough to cause heat stress during summer. Access to shade and plenty of drinking water are the best line of defence but cooling with water and changes to milking and feeding routine can help when shade isn’t enough.

Watch for these changes

  • Cows breathing faster – check their breathing rate
  • Cows standing more but grazing less
  • Increased water intake and cows hanging around troughs
  • Cows slower walking to and from the shed
  • Less milk in the vat

Checking breathing rate

The earliest indicator of heat stress is increased breathing rate. Ideally, observe 10 cows on a warm summer afternoon, but you could start with just one – a high producing black cow will be most at risk.

Minimising heat stress

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


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 over off paddock facilities.

Cooling with water

Sprinklers can improve evaporative cooling for 2-6 hours after wetting. However, water must RUN OFF the cows; simply wetting them only increases humidity, making things worse, especially when they are held close together. Fans can be used to increase the effectiveness of sprinklers by moving the water laden air away.

Drinking water

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 low. Most cows drink soon after milking, so install water troughs in races to meet that need. Cows should not have to walk more than 250m to get a drink. Providing larger troughs or more than one trough will reduce the impact of ‘guarding’ by dominant cows. Cows will drink more water if it is clean and palatable.


Reduce the walking distance and speed to the dairy. Reduce the time spent in unshaded yards. Allow more yard space per cow at milking times. Milk earlier in the morning and later in the afternoon or consider once-a-day milking.


Consider offering more of the daily feed during cooler hours of the day, especially high fibre feeds which generate more heat in the rumen. Feed more of the pasture allocation in the night break and any energy dense supplements in the hotter part of the day, when cows are too hot and bothered to eat bulk feed.

Is heat a problem in my area?

Check the table to see how many hours you and your cows need to manage in summer.

These tables show the average risk and impact of heat for December, January, and February, between 2017 to 2020. Estimates are for Friesian and crossbreed herds, which start to reduce production when temperature is greater than about 21ºC. Jerseys are more tolerant of heat, with production losses insignificant until 25ºC.

North Island

Average hours per
day too warm
for comfort
Days warm enough
to reduce milk
Estimated milksolids
impact per cow
each summer
Whangarei 16 83 6.9
Kerikeri 16 86 6.5
Dargaville 15 68 4.9
Hamilton 14 77 6.8
Karapiro 15 77 6.0
Matamata 14 79 6.8
Te Kuiti 14 80 7.0
Bay of Plenty
Te Puke 15 77 6.2
Whakatane 16 82 5.5
Rotorua 12 65 4.6
Stratford 9 46 3.0
Lower North Island
Waipawa 11 66 6.2
Palmerston North 12 58 4.1
Dannevirke 10 53 3.6

South Island

Average hours per
day too warm
for comfort
Days warm enough
to reduce milk
Estimated milksolids
impact per cow
each summer
Top of the South Island
Richmond 14 63 4.0
Reefton 9 56 4.5
Arapito 10 49 2.8
Canterbury/North Otago
Balmoral East 9 48 3.5
West Eyreton 7 39 2.5
Lincoln 8 43 3.1
Winchmore 7 44 3.4
Orari 7 42 2.6
Timaru 5 25 1.2
Waimate 5 29 1.6
Middlemarch 7 45 4.0
Southland/South Otago
Gore 5 33 1.9
Invercargill 4 21 1.1

Why do cows get hot so easily?

We all have an ideal temperature range, within which we feel comfortable and our immune system and organs function properly. The comfortable temperature range for a cow is 4-20°C, lower than for a human.

That’s because cows generate enormous amounts of energy digesting food and producing milk. This is handy during winter but a challenge during summer, when cows absorb more heat from the environment and it’s harder for them to maintain an ideal body temperature.

One way cows get rid of excess heat is by evaporation of their breath and sweat.  To increase evaporation, they breath faster and sweat more, though their ability to sweat is limited. When this isn’t enough, they eat less to reduce the production of heat in their rumen, so their milk yield declines.

High humidity and low air movement increase the risk of heat stress. Evaporation is less effective, making it hard for the cow to lose heat by sweating and breathing.


How does heat stress affect cows, what are some warning signs, and what can you do on farm to make life more comfortable for your cows? In this episode, Tom Buckley, farm manager for Owl Farm in Cambridge, goes into detail about the strategies they’re using to combat heat stress. Meanwhile, DairyNZ’s Jac McGowan talks about the science behind heat stress: how it affects cows, the warning signs, and what current research is underway

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Last updated: Sep 2023
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