How to protect dairy yields by tackling heat stress in cows

Heat stress in dairy cows can cut milk yields, reduce fertility and cause health issues such as acidosis and mastitis.

Heat stress occurs when animals experience high air temperatures and are unable to cool down enough to maintain a healthy body temperature.

Signs of heat stress


1. Changes in feeding behaviour

Cows eat less to reduce the amount of metabolic heat produced. They may eat smaller amounts more often and tend to consume most overnight.

Food sorting may occur as they choose feeds that produce less heat during digestion, such as grains and protein over forages.

2. Changes in body position

Hot cows spend more time standing and less time lying to increase body surface area exposed to wind.

3. Panting

The normal respiration rate is 26-50 breaths/min. Anything over this indicates heat stress – flank movements are easy to count.

4. Other behaviour

Reduced activity levels or crowding around troughs. In extreme heat, cows may salivate or drool more.

Effects of heat stress

1. Poor fertility

Shorter oestrus and a weaker expression causes lower conception rates. Embryo mortality increases. Foetus growth is slower, lowering calf birthweights.

2. Decreased milk yield

Caused mainly by reduced feed intake, nutrient deficiency and reduced energy. If affected in early lactation, it can reduce milk yield for the whole cycle.

Milk quality also declines, reducing fat and protein content and raising somatic cell counts.

3. Reduced feed efficiency

Energy is used to keep the cow cool, leaving less available for milk production.

Minerals are lost through increased sweating and urination. Feed sorting can lead to acidosis.

4. Lower immunity

Increases susceptibility to infection, such as mastitis.

Minimising heat stress

Farmers can minimise the effects on production and fertility by taking some simple steps.

The ration

  • Ensure it contains enough energy – cows use more for maintenance during heat stress.
  • Fat is a good energy source as it is highly concentrated, but doesn’t increase heat load much during digestion.
  • Avoid high protein in hot conditions as it requires a lot of energy to excrete.
  • Increase palatability and digestibility to encourage cows to eat.
  • Some forage is needed to prevent acidosis, but digesting concentrates produces less heat.
  • Adding yeast can help prevent acidosis.
  • Sodium and potassium minerals can be added to replace those lost through respiration.

Feeding time

  • Provide fresh feed at cooler times of day to stimulate intakes.
  • About 60% of the ration should be fed between 8pm and 8am.


  • Cattle will drink more to support digestion and make up for losses to evaporative cooling.
  • Water should be clean and fresh, with adequate pressure to refill tanks and troughs.
  • Ensure there is enough trough space per animal (15cm) to minimise competition

Stocking density

  • Temperature in housed areas or holding pens is influenced by stocking density as animals generate body heat.
  • Minimise the time in poorly ventilated areas, for example the holding pen, on hot and humid days.
  • Painting metal roofs white and adding insulation directly beneath will reflect and insulate against the sun, reducing thermal radiation.
  • Install windows and gates to improve ventilation.
  • Increase direct airflow over the animals, for example by fans to improve cooling when temperatures are very high.
  • Provide additional shaded areas outdoors to help avoid solar radiation during grazing.

Research on heat stress

Cows in the UK could be more prone to heat stress – and as a result production losses at lower temperatures – than initially thought according to a 21-year study at Scotland’s Rural College (SRUC) Langhill.

The study on a herd of Holstein Friesians, revealed that both milk yield and quality are affected by UK weather, particularly during the summer months.

Researchers Davina Hill and Eileen Wall found milk production peaked at 24.03kg milk a grazing cow, at a temperature of just 12C and 70% humidity.

As these parameters increased, both fat and protein content decreased along with yield.

As a result of the study, SRUC discovered a much lower optimum temperature humidity index (THI) for dairy cows – a formula that is used to measure when cows are becoming heat stressed.

Traditionally the accepted international THI threshold was 72, but the study has discovered it to be much lower at just 55 (see production differences table).

“These results show that milk yield and quality can be impacted by high temperatures and humidities under conditions experienced in the UK,” says Dr Hall.

In fact the study showed THI 55 was exceeded on nearly 40% of the days cows studied.

Graph showing THI effect on yield

How to calculate your own THI

Instead of just looking at cows alone to see if they are heat stressed, a new app could help farmers calculate individual THI, which is a more sophisticated measurement of the environment.

The Cool Cow app from Purina calculates the THI.

All farmers need to do is measure the temperature and humidity using a thermometer and hygrometer and the app does the maths.

Kevin Herrick, senior manager of dairy and livestock technical services at Purina says some producers may not feel that a THI of 55 is warm enough to be problematic; however, it could be for the cow.

“What may feel comfortable to you is not necessarily what is most comfortable to the cow.

“I worked with a vet who always said that if I was walking through a barn and started questioning if I needed to turn on the fans, it was already too late.”

“This is called evaporative cooling, and includes processes such as sweating and panting. When the atmosphere already contains a lot of water vapour [for example, relative humidity is high], evaporation is less effective,” she says.

The UK has relatively high humidity, averaging 70-90% or higher still in poorly ventilated buildings. As the relative humidity at any temperature increases, it becomes more difficult for the animal to cool itself.

This means cows can experience heat stress at relatively mild temperatures when humidity is high, she says.

“Cattle are not good at cooling down. Their sweating mechanism is not very effective and they produce a lot of heat during digestion,” adds Dr Hill.

Other factors that can influence heat stress include cow size – larger animals suffer more – age – juveniles or the very old are most affected – and pregnancy as the developing foetus produces more heat.


Temperature humidity index (THI)




Milk protein content (%)



Milk fat content (%)