Heat Stress in Cows

Cows take on heat from the environment and generate metabolic heat from eating and digesting feed. Problems start to occur if temperature and humidity increase and cows don't have opportunities to balance their metabolic and environmental heat gains.

What causes cows to get hot?

Like most mammals, the dairy cow needs to maintain its core body temperature between 38.6oC and 39.3oC. The core temperature changes slightly throughout the day, reaching a peak in the early evening and a low early morning.

In hot environmental conditions, cows off-load heat with a range of physiological and behavioural strategies.

Metabolic heat is being produced all the time. During the day this heat is not as easily dispersed. If night time conditions are sufficient to allow adequate dispersal of heat the cow will not suffer ill effects. If this diurnal cycle of heat accumulation during the day and loss during the night is disrupted by high night time temperatures the effects become more noticeable.

Factors that determine the level of environmental heat a cow is exposed to over time are:

  • air temperature and relative humidity
  • amount of solar radiation
  • degree of night cooling that occurs
  • ventilation and air flow
  • length of the hot conditions.

How do cows keep themselves cool?

In hot environmental conditions, cows off-load heat with a range of behavioural and physiological strategies. 

Dairy cattle may change their behaviours by:

  • looking for areas with greater air movement or standing to increase or exposure to air
  • seeking water and shade
  • changing their orientation to the sun
  • panting or sweating, or
  • stopping or reducing feed intake which decreases rumen heat production.

Heat gain loss

As heat load builds the cow's body struggles to cope.

Awareness of the signs will inform your decision-making around the management and response to heat stressed cows. 

Looking for subtle changes in behaviour will give you plenty of time to act. If the period of excessive heat load lasts more than a few hours signs of heat stress become more marked.

Open mouth breathing, group seeking of shade and excessive drooling are all signs of prolonged heat stress and call for urgent attention.

 

Recognise the early signs of excessive heat load and allow for early intervention with effective management and mitigation strategies, such as access to shade and cooling infrastructure, water access and feed pad usage.

 

Physiological responses to excessive heat load

Apart from observable changes in behaviour, there are also a number of unseen physiological changes that occur within the cow:

  • feed intake decreases by 10-20% when the air temperature is more than 26°C;
  • core body temperature rises;
  • blood hormone concentrations are changed; and
  • blood flow distribution is altered, blood flow to gut, uterus and other internal organs is decreased, blood flow to skin is increased.

The unseen changes can have far-reaching consequences on the productivity, health and welfare of cows. 

 

Increasing air temperature and humidity reduce the cow's ability to cool itself.

Heat exchange between the cow and the environment occurs through radiation, conduction, convection and evaporation processes. 

The direction of heat exchange depends on the temperature difference between the cow and the surrounding environment.

When the air temperature is higher than the cow's temperature, heat flows into the cow - heat is absorbed. When the air temperature is lower than the cow's temperature, heat flows from the cow - the cow off-loads heat and cools down.

The greater the temperature difference, the faster the flow of heat.

 

Excessive heat load

A dairy cow manages the body heat load that it carries within itself all the time. If the sum of metabolic heat produced by the cow and the heat gained from the external environment begins to exceed that lost, the cow's heat load starts to build.

The cow must ensure it stays within the optimal range through thermo-regulation. This means balancing the metabolic and the absorbed environmental heat using a a range of strategies, such as increased breathing rate and sweating.

It is important for dairy farmers to know the signs of excessive heat load so practical strategies can be implemented to help the cows cope.

Once heat load reaches a critical point changes start to occur in metabolism, hormonal regulation and feed intake. This in turn affects milk production, milk quality, fertility and health.

Heat scale 

 

  • Heat transfer and behaviour

    Cows try to off-load heat by changing their behaviours, some changes are less obvious than others.

    Read more
  • Dry Cows and Heat Stress

    Cows respond to heat stress in many ways. Identify the subtle signs and intervene early.

    Read more
  • Heat Stress and Productivity

    Heat stress reduces fertility and calving rates, encourages infections, and surpresses feed intake & nutrition. Deveop responsive strategies & get better production rates.

    Read more