Heat Stress

Lactating cows prefer temperatures below about 20-21°C.  To cope with warm weather, they use a variety of strategies to maintain normal body temperature, including standing in shade, drinking more water and spending more time around water (especially if there is no shade), increasing their sweating and breathing rate, and reducing feed intake.  When this is insufficient to keep their body temperature below 39.3°C, they exhibit symptoms of clinical heat stress, such as panting and drooling, and their welfare, health, and milk production can be severely affected.

Cows with high production, Friesian genetics or dark coats are more susceptible to heat stress – lower-producing cows and Jerseys may be comfortable until about 25°C.  High air temperature, humidity, solar radiation, and low air movement increase the risk. 

A distinct decline in milk yield means that the cows have not been able to stay cool; this may not be observed until two days after the hot weather.  Night-time cooling can help reduce the negative effects of heat load on production, but it does not reduce the welfare impact on the cows during the day. 

In the summer of 2021 the Waikato experienced more than 105 days when humidity and temperature created conditions that were too hot for cows. There are, on average, 6-8 hours a day in summer that are hot enough to impact milk production, and another 4-5 hours a day which are too warm for cow comfort.  The estimated impact on milk production is 6 kg MS per cow over summer, or $12,000 for a 300-cow herd at $6.80/kg MS.

When thinking about how to reduce the impact of heat, there are three categories to consider:

1.               Reduce the heat from the sun, e.g. provide shade and also cooling reflective surfaces such as yards and cowshed roofs.

2.               Reduce the internal heat load – that’s a whole lot of smaller things, like making sure they have plenty of water, allowing extra space on the yard, shifting ‘hot’ feed to cooler hours of the day, and limiting walking during the heat.

3.             Finally, remove the heat.  Options here are sprinklers, misters and fans, that pull the heat off the cow through evaporation.

Ideally, farms should be set up so that cows can access shade and water whenever they want it.  But that is easier said than done. Over the next ten years, we will be expected to have comprehensive mitigation plans and probably a lot more shade available. But, as a starting point for the next 1-2 years, we will be expected to know how many hours our cows might be too hot and make the most of the resources and management options already available to us. 

Strategies at Owl Farm

Check and manage water flow and quality 

Lactating cows can drink more than 100 litres/cow/day and will drink between two to six times per day. Most cows drink soon after being milked, which is why we often see troughs running low or dry straight after milking. If cows cannot drink when they want to, their daily water intake will be reduced, even if they could have drunk it all later.  Water intake is directly related to milk production, partly because water intake regulates feed intake. Ensure flow rates to troughs are fast enough that the troughs never run dry.  Work on 14 litres/hour/cow the trough needs to serve.  Herds larger than 400 cows need two troughs in the paddock, but any herd will benefit from extra troughs.

Tom’s tactics:

Check water use overnight via Halo to respond to higher than expected volumes which would initiate a water leak check.

Check water toughs in each paddock every time you are in there.

Turn off water lines when cows are not grazing a block to increase flow to other areas.

Monitor the impact of Mg, Zn etc. on palatability and add flavourings to the water if needed.

Install extra water troughs in the races

Rule of thumb: if troughs cause cow flow issues in the paddock or in the race, too many cows are thirsty, and more troughs are required. Install troughs where cows are already spaced out, rather than where they already congregate.

Tom’s tactics:

Last season two more troughs were provided on the races – one new 1100L trough,  and a change in fence line to provide access to another.  There are now three troughs on the main trunk line, these are all 100-400m from the shed.

Use sprinklers on the yard

Cows cannot sweat much so they have limited capacity for evaporative cooling off their skin.  Spraying them with water works like artificial sweat – the water droplets absorb heat from the cow’s skin, then either evaporate or run off, taking the heat with them.  To be efficient, the droplets need to be big enough to wet the cow to her skin.  Fans or wind can be used to shift the humid air away on humid days when evaporation is slower.  Higher flow rates may also be used provided the water drips off the sides of the cow rather than running onto the udder.

In New Zealand studies, cooling with sprinklers for 10-90 minutes before afternoon milking reduced respiration rate by 60% and reduced body temperature for at least four hours after milking.  Because sprinklers are so effective at cooling, cows don’t need them for long periods when the Temperature Humidity Index (THI) is lower than 69, which is typically about 24°C.  Cows that were under sprinklers for 90 minutes when THI was less than 69 showed signs of hypothermia (Kendall et al. 2007).

Tom’s tactics:

Set-up includes one sprinkler on the shed roof and one at the back of the yard. Flow rates were measured this year at 45L/min on each sprinkler @ $1.49/m3 water.
Triggers for using the sprinkler are usually temperatures around 25°C or lower if it’s humid or the cows have had limited access to shade during the day. 

When cows are on once a day milking (OAD) they can be brought into the yard for a cool down before moving to another paddock for their night feed. Sprinklers sometimes get used in the mornings when night time temperatures don’t fall below a comfortable 18°C.

It’s a loud pump to run so although it cools the shed down it can reduce milker comfort and it costs approx. 13c/min in water to run.

Use data for decision making. We collect temperature and humidity data from the farm and use the DairyNZ website to calculate THI.  Monitor breathing rate (normal 24-40 breaths per minute) to check cows’ response to the environment.

Alter milking times 

Changing milking times can achieve multiple outcomes, depending on your system. Reducing the number of milkings in the hot afternoons can reduce heat-generating exercise for the cows and heat generated in the yard (if not using effective cooling systems).   Milker comfort can be increased by milking before lunch or later in the evening.

Tom’s tactics:

Last season we used 3 n 2 milking interval after mating finished.  Milking times were 5am then 6pm then 11am the next day.  This worked well with crop allocation and removed the afternoon milking time during the heat of the summer.

Plan for shade when allocating feed 

Cows who have access to shade for 90 minutes before afternoon milking in New Zealand reduced respiration rate by 30% and lowered body temperature by 0.3°C compared to cows without shade (Kendall et al. 2007). The body temperature remained lower for 2-4 h after milking.

Pasture grazing cattle will spend 1-3 h per day in shade, depending on weather conditions and the amount of shade available per cow. Cows use shade to reduce the heat they’re absorbing from the environment.  Shade trees reduce solar radiation by >50% and the ambient temperature can be up to 10°C cooler under trees in summer. In New Zealand studies, cows start using shade from 20°C, and cows with access to shade had lower body temperatures, lower breathing rates and higher milk production. 

Contrary to opinion, providing shade does not result in animals spending less time grazing. Cows provided with shade actually spend the same or more time grazing per day than those without shade, and can have increased dry matter intake.  Providing shade for as many animals as possible will help prevent large groups gathering under too few trees, which leads to high nutrient loading, soil compaction and pasture loss.  For a 300 cow herd, that would be about seven mature oaks, or 60 mature poplars, or a 36m x 36m shade structure. If there is less than 4-5 m2 of shade per cow, there will be competition for the space and some cows will miss out. 

Tom’s tactics:

Plan summer crops with trees for shade as cows will be on these during the day feeds. If a week of hot weather is predicted the grazing plan will be adjusted to allow more shade access.  In summer cows are only getting 50% of their diet in pasture so there is more flexibility when choosing which paddocks to graze.

In 2021 we invested in an extensive planting plan which also includes shade and shelter outcomes.


In New Zealand, where solar radiation levels are high, shade is likely a more important resource to cows than water cooling. However, cooling with water is efficient if cows are showing signs of being too hot (e.g., panting, drooling) on warm days, and in particular after a long walk to milking. Compared to shade alone, cooling with water spray reduces body temperature, respiration rate and air temperature more efficiently.  So bringing the herd in for cooling is a good option when there isn’t enough shade available.

To practice counting breathing rates have a go watching this video