Heat stress

Why do cows get hot so easily?

Cows generate lots of heat digesting food and producing milk. This is handy during winter but a challenge during summer. Their ideal temperature range, within which they feel comfortable and their immune system and organs function properly is just 4-20°C, well below our ideal temperature range.

Heat stress occurs when cows have more heat than they can get rid of. When it gets too hot, one of the ways cows try to cool down is using evaporation. To increase evaporation, they breathe faster and sweat more, although their ability to sweat is limited. High humidity, little cloud cover, and low air movement increase the risk of heat stress as evaporation is less effective. If that's not enough, they eat less to reduce the production of heat in their rumen, and milk production drops.

All areas of New Zealand get hot enough to cause heat stress during summer.

How to recognise heat stress in cows

Watch for these five signs:

Cows breathing faster – check their breathing rate
Eating less - cows standing more but grazing less
Drinking more and cows hanging around troughs
Cows walking more slowly to and from the shed
Less milk in the vat

Checking breathing rate

The earliest indicator of heat stress is increased breathing rate. Check breathing rate by observing a number of cows on a warm summer afternoon. More than ten breaths in ten seconds means a cow is not comfortable and is an indication that the herd needs cooling.

The three stages of heat stress

Stage 1: No visible signs of heat stress

No drool
Mouth closed
Difficult to see chest movements.

Stage 2: A Cow starting to show signs of heat stress

Increased panting
Some drool
Mouth open sometimes
Visible chest movements

Stage 3: A heat stressed cow

Fast and obvious panting
Whole body movement
Neck extended
Tongue out
Lots of drooling
Heavy panting

Do cows experience heat stress in my region?

Dairying regions in New Zealand often experience days when the temperature exceeds a cow’s optimal range for comfort and productivity. Environmental factors such as solar radiation and wind speed can further influence a cow’s heat load.

To help predict conditions that pose a risk of heat stress, New Zealand has developed the Grazing Heat Load Index. This index includes ambient temperature, solar radiation, and wind speed to assess the thermal load on outdoor cows. When the Grazing Heat Load Index exceeds 55-60, cows are at risk of heat stress unless they have access to shade or cooling. GHLI can reach 55 at just 18°C when there is no cloud and little breeze, whereas on a day that is overcast or with a moderate wind, the temperature could be 24°C before GHLI exceeds 55.

Average number of days a year that dairy cows are at risk of heat stress across New Zealand.

_{Data presented is based on the temperature, humidity, solar radiation and wind speed captured in the Grazing Heat Load Index (GHLI). This index records heat stress above GHLI value of 55. The information was collected during 2006 to 2024 (Woodward, et al., 2024).}_{Colour scale represents the numbers of days that cows are at risk of heat stress. Purple lines depict local regional councils.}

Modelling indicates that cows are at risk of heat stress annually for 40 to 85 days in New Zealand and this is expected to increase in the future. The number of heat stress risk days annually differs by region, for example, Waikato 69 days, Bay of Plenty 69 days and Canterbury 80 days. The regions with the lowest number of heat stress risk days are Taranaki 38 days, Manawatu 47 days and Otago-Southland 37 days.

By 2050, the number of heat stress risk days is predicted to rise even in regions previously less prone to such conditions.

Minimising heat stress

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

Providing shade

Cows are highly motivated to seek shade when it is sunny, hot, and with little wind, specifically when:

Air temperature is greater than 22 ℃.
Wind speed is less than 7 km/h.
Solar radiation is greater than 2 MJ/m2/h.

Solar radiation is a measure of how much sunlight is hitting a surface in a specific time and space. It is measured in megajoule per square metre, per day. 2 MJ/m2/h might feel like standing outside on a sunny day, around noon, with no shade; the sun would feel strong on your skin, and you would feel warm quickly.

Cows use shade when there is as little as 1 m²/cow available, it’s recommended to have up to 5 m²/cow available for access by the entire herd to minimise competition.

Cooling with water

Sprinklers can improve evaporative cooling for 2-6 hours after wetting.

Wet the dairy yard before cows arrive to remove heat from the concrete.
Ensure all cows are wet in the first 10 minutes of arriving to the dairy shed.
If you need to conserve water, run sprinklers on an on/off cycle, for example, around 3 minutes every 15 minutes.
The water should be large droplets to reach the skin and pour down the side of the cow.
Position sprinklers approximately 2m above cows to reduce risk of wetting udders and of water getting into cows’ ears.
Fans and/or extra space on the yard can increase airflow and cool cows faster by moving humid/moist air away.
After a warm night, sprinklers can be used in the morning milking to reduce heat load, especially if there is little shade available for cows during the day.

Note: Sprinklers are not effective unless they thoroughly wet the coat, as this only increases humidity, making heat stress conditions worse.

Using sprinklers can keep cows cooler for 2-6 hours.

Drinking water

Lactating cows typically require more than 100 litres/cow/day in summer and drink between two to six times per day. A cow can drink 20 litres per minute.

Inspect the troughs at peak drinking times (after milking) and ensure flow rates to troughs are high enough.
Water pipes should be 75mm in diameter with enough pressure to provide 20 litres per cow per hour. Large volume troughs will also help maintain supply during high demand.
Clean troughs regularly. Cows will drink more water if it is clean and palatable.
Consider installing extra troughs in raceways and at the dairy yard.

Check flow rates. Keep your troughs clean. Cows will drink more and feel cooler.

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Install an extra trough along the exit race of the cow shed.

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Milking

Cows walking to milking and standing on unshaded yards during the heat of the day increases risk of heat stress.

Reduce the walking distance and speed to the dairy. Plan to use paddocks closer to the shed for afternoon milkings.
Allow more yard space per cow at milking times.
Reduce the time spent in unshaded yards by milking earlier in the morning and later in the afternoon or consider once-a-day milking.

Feeding

Peak heat production in the rumen occurs about three hours after cows eat. The impact of modifying diet is small relative to the impact of providing shade and cooling. However, there are some changes that can help keep your cows cooler:

Increase metabolisable energy (ME) and nutrient densities of feed, especially during the day to reduce a drop in appetite due to heat stress.
Feed more of their daily allowance earlier in the morning or overnight so that heat is produced during cooler parts of the day.
Cows lose more salts in sweat and urine when they are hot so offer extra potassium, sodium and magnesium via salt licks or supplement.

Podcast

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 use 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 research is underway.

Listen on: