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:

Shade

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.

Growing Trees For Shade

PDF Guide

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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.

Management

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.

Feed

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 production	Estimated milksolids impact per cow each summer
Northland
Whangarei	16	83	6.9
Kerikeri	16	86	6.5
Dargaville	15	68	4.9
Waikato
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
Taranaki
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 production	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.

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’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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