As UK springs and summers grow hotter and more humid, dairy farmers are increasingly confronted with heat stress challenges. Recognising the signs early and understanding the environmental thresholds that trigger this physiological strain is essential to be able to respond in a way that protects cow health and welfare, as well as milk production and farm business resilience. In a recent Climate Farm Demo webinar, PhD candidate Brigitte de Bruijn from Wageningen University shed light on the science behind heat stress and the signals that should prompt immediate attention.

 

What is heat stress?

Every cow has a thermoneutral zone—ranging from around -5°C to 15°C—within which it can maintain body temperature without expending excess energy. When temperatures exceed this zone, especially in combination with high humidity, cows begin to accumulate more heat than they can release. This is the onset of heat stress.

Above 20°C, risk begins to rise significantly. But it is not temperature alone that causes problems. Humidity plays a key role, as moist air hampers a cow’s ability to cool down effectively.

 

Temperature Humidity Index

The Temperature Humidity Index (THI) combines air temperature and relative humidity into a single value (0–100) to assess heat stress risk. Though helpful, THI does not account for solar radiation or airflow—factors that influence how heat is perceived by cows.

According to Brigitte, a THI above 68 sustained for four days or more typically qualifies as a heat stress event. Observable behavioural changes begin at a THI of around 58, while visible signs such as sweating or panting often occur at THI levels in the 70s and 80s.

 

Visible signs of heat stress in cows

Visible signs that indicate cows are experiencing heat stress include:

  • Increased sweating and drooling
  • Increased water consumption
  • Increased respiration
  • Decreased feed intake
  • Decreased rumination
  • Reduced activity
  • Increased standing

These signs reflect the cow’s attempt to regulate its body temperature. The physiological cost is high—less energy is available for milk production, immunity, or reproduction.

 

Impacts of heat stress

Heat stress in cows can impact milk production, feed intake, water intake, fertility and – in pregnant cows – the productivity and fertility of offspring.

Milk production

Speaking in the webinar, Brigitte cited a study which showed reduced milk production as THI increased above 68. Interestingly, this study showed that cows in early lactation reduced milk production by about 14% when experiencing heat stress, but cows in mid lactation reduced milk production by 35%.  

Feed intake

With feed intake, as THI increases dry matter intake decreases. Brigitte flagged research which shows early lactation cows have the most reduction in dry matter intake, despite milk production reducing less in this group. This makes fresh cows particularly vulnerable to heat stress, as they keep producing milk but don’t have sufficient feed intake.

Water intake

The impact of heat stress on water intake depends on both cow body weight and milk production, with larger breeds and high yielding animals having a greater increase in water intake as THI increases. With no heat stress, a 650 kg cow with a milk yield of 40kg/day will consume approximately 11 litres water per day. During heat stress this can increase to 200 litres per day to stimulate heat loss. This means there needs to be enough water available, and enough water points at different locations where they can drink, so all cows, not just those that are higher in the social hierarchy, have access to water.

Impact on fertility and during gestation

Speaking in the webinar, Brigitte referred to a study which showed that if the average THI was more than 72 during the 21 days before conception, the cows had a 61% greater risk of failed conception compared to those experiencing a lower THI.

This is because egg cells are sensitive to small changes to body temperature, and develop less during hot periods. Reduced fertility due to heat stress can continue into the autumn, as it takes time for cow to recover.  

With gestation, another study shows that if dams had heat stress during the last 8 weeks of gestation, the offspring had a lower life production and lower conception percentage as a heifer.

 

Cow signals of heat stress

Cows experiencing heat stress spend up to 30% less time lying down. This makes sense, as by standing up more they are exposing more of their body surface to air, making it easier to lose heat from the body via convection. However, this can result in an increased risk of leg problems as cows spend more time standing.

 Cows will also seek relief from heat by standing or lying in shade. Panting and rapid breathing at a rate of more than 60 breathes per minute is a sign that an animal is trying really hard to cool down. In Holstein cows, when temperatures increase above 28°C 70 – 80% of heat is released from the body through sweating, so seeing wet cows among the herd signals they are trying to lose heat as fast as they can.

While there are common signs and signals to watch out for, it is important to note that no dairy cow is the same, and the impact of heat stress differs between:

  • Milk production level - requires higher feed intake, which releases heat to digest
  • Milk production status - dry cows may suffer less from heat stress as not producing milk, but heat stress during pregnancy can have long term impact on offspring productivity, so this group should still be taken care of during hot periods
  • Breed – Holsteins are large animals, they require a lot of feed and produce a lot of milk, so don’t cope that well with heat. Smaller breeds that produce less milk and eat less can be more resilient to heat in their environment
  • Age of cow
  • Housing – whether cows are housed indoors or outdoors impacts how they are exposed to heat in their environment and how they cope with it.

Heat stress is a present and persistent risk. Understanding THI, recognising the signs, and acknowledging the variability among animals is critical to managing welfare and productivity.

To explore how to respond on-farm, including practical mitigation strategies, watch the full Climate Farm Demo webinar.