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  • Jin Gan

Heat Stress In Dairy Cows

Heat Stress In Dairy Cows


The heat and humidity of a typical summer combine to make it a very uncomfortable environment for lactating cows. Heat stress has several serious negative effects on lactating cows. Among them are reduced feed intake, milk production, butterfat percentage, and lower reproductive performance. The cows will need extra energy in order to dissipate heat and regulate its body temperature. It is therefore very important to recognise and manage the heat stress in two ways:

  • Reduce the amount of heat cows are exposed to

  • Changing the feeding and management in order to help cows keep cool

Signs of Heat Stress

The obvious signs of heat stress experience by lactating cows are reduced milk production and lethargic behaviour of the cows. Moderate signs of heat stress occur when the temperature is around 25°C to 32°C with the humidity ranging around 50 - 90%. Signs of moderate heat stress include rapid shallow breathing, profuse sweating and an approximate 10% decrease in milk production and feed intake by cows. As temperature rises to about 32°C to 37°C and humidity remains 50 - 90%, cows will start showing severe depression in milk yield, up to 25% and in feed intake as well. Other signs will include open mouth breathing with panting and her tongue hanging out.

When the temperature/humidity index is greater than 90, it will usually result in severe heat stress in the high producing cow and moderate signs in the lower producing cow. In more severe cases, the cows may die from extreme heat especially when complicated with other stresses such as illness or calving. The reason why high producing cows exhibit more signs of heat stress compared to lower producing cows is because higher producing cows generate more heat as they eat more feed for higher milk production. They must get rid of the extra heat generated as a result of metabolising greater nutrients in the feed. As a general rule of thumb, the decrease in milk production normally results from less feed intake by the cow. Generally, 1kg of milk production is loss of every 0.5kg of decreased dry matter intake when temperature and humidity levels are high.

Temperature Humidity Index

Main Sources of Heat Stress

There are several heat factors that will affect the cows' performance and contribute to heat stress.

  • The main factor comes from the heat accumulated by direct radiation from the sun. Generally, dark coat cows accumulate more radiation heat than light or white coat cows.

  • Heat digestion plays a big role in the heat accumulation in cows. Heat of digestion of forages is higher than that of grains. Hence, cows on a higher forage ration are more incline to heat stress compared to cows on higher grain rations.

  • Conduction can also contribute to heat accumulation in cows. This usually occur when cows are crowded together. For example, when cows try to get into a limited shade of a tree, heat is "trapped" and passed by direct contact from cows to cows.

During hot weather, especially followed by hot nights, cows will accumulate heat and they are not able to dissipate the heat adequately. This will cause severe heat stress and production losses. In extreme cases, death from heat exhaustion may occur.

Effects of Heat Stress

Here are some of the important effects of heat stress on dairy cows:

  • Decrease in dry matter feed intake - The feed intake is reduced by approximately 10 - 15% during periods of hot weather. Early lactation and high producing cows are affected more severely compare to cows in the later lactation stage.

  • Decrease in milk production - As feed intake decreases, naturally, the milk production for cows will decrease as well.

  • Reduced fertility - Studies showed that there is a correlation between decreasing conception rates and increasing temperature of the environment.

  • Reduced rumen efficiency - Recent research shows that heat stressed cows have reduced rate of passage, lower rumen pH, higher rumen ammonia and change in volatile fatty acids. These effects seemed to be independent of changes in dry matter intake.

  • Reduced butterfat production (%) - Butterfat declines up to 0.3% units during summer period.

Important Steps In Controlling Heat Accumulation In Cows

The important measures in controlling heat accumulation are reducing heat load as well as increment in cows. Here are the 3 main sources of heat accumulation in cows:

  • Direct solar radiation from the sun - Keeping the cows out of sun in the morning around 8 - 9 AM to at least 4 - 5 PM will help to reduce heat accumulation. This is especially important for dark coloured cows. Keep the cows in barn. If there are shades outdoor, make sure there is enough space for all cows.

  • Heat of digestion - Feed the cows higher quality and higher digestible forages in the summer. It is also important to include fat in the diet in order to help maintain the energy intake when feed intake declines.

  • Heat conduction - It is essential to ensure that cows do not crowd during hot days. This usually occurs when there is inadequate bunk or shade space.

The steps above are important measures to help reduce heat accumulation in cows. With that being said, cows will still suffer from some distress from heat stress during the summer.

Measures To Help Reduce Heat Stress In Cows

  • Increase the water availability to the cows - During the summer, water supply needs to be increased. Place extra water points close to where the cows' spent most of their time. Ensure the water is fresh and clean.

  • Change the feeding routine - Try to feed a greater proportion of feed at night, usually 60 - 70% of feed. Watch out for feed heating in the bunk.

  • Increase the airflow and ventilation - Open up the sides of the barn to maximise the natural ventilation. It is also important to install fans in the barn. Important areas to ensure good ventilation are holding area, along the inside of the feed bunk and over the stalls.

  • Ration specification changes - Increase the ration concentration but do not compromise on fibre levels. It is important to also include fat to the ration. Increase the potassium to 1.5 - 1.7%, increase sodium to 0.45 - 5%, increase magnesium to 0.3 - 0.4%.

  • Ration changes - Feed a TMR. When feeding TMR, it is advisable to add water to increase water intake and to prevent the cows from sorting.

Alleviating Severe Heat Stress

Here are some indicators of heat stress that suggest the cows would need to dissipate heat:

  • THI increases to 78 or higher

  • 10 cows have respiration rate over 80 breaths/minute

  • Dry matter intake of cows is down by 10% or more

Any signs above would indicate that special attention is needed by the affected cows or else some of these animals would die.

  • Select the most comfortable place for cows - If the cows are outdoors, it is important to check the temperature and decide where is more comfortable for the cows - whether it is outside in the shade or in the barn. It is vital to ensure that the cows are not out in the sun.

  • Ventilation in the barn - Ensure that there is maximum natural ventilation and that the fans are running in order to increase airflow.

  • Indirect cooling - It is encouraged to use fine mist in the barn to cool the air. This will reduce heat load on the cows. Breathing cooler air will help cows cool faster.

  • Direct cooling - This is direct wetting of cows. The most effective is direct wetting of the skin in order to enhance the evaporative cooling from the body surface.

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