Heat Stress in Dairy Cow

By Peter Robson, Progressus Dairy Production and Management Agrischool

High heat loads on the modern-day dairy animal leads to –

1.     Depressed feed intake

2.     Decreased milk yield, milk fat %, protein %

3.     Elevated somatic cell counts and risk of clinical mastitis

4.     Increased weight loss of cows

5.     Elevated rectal temperature 

6.     Increase respiration rate

7.     Reduced reproduction

Unless the farm is well equipped to measure the above 7 points on the cows in the dairy farm, it is likely the farm just accepts the consequences of heat load / heat stress on this cow.

It is significantly important for the operating dairy farm in tropical and sub-tropical regions to be aware of heat stress and the associated THI (Temperature Humidity Index).

The THI for cattle can be compared to the HHI (Human Heat Index) as a guide for understanding THI for the dairy cow and how the THI affects many life aspects of the dairy cow.

The consequences of THI of the milk producing dairy cow –

1.     Increased maintenance energy required – decreased milk production, increased respiration rate

2.     Increased acidosis risk – reduced feed intake

3.     Impaired rumen balance & function - reduced rumination

4.     Impaired rumen function – reduced milk production

5.     Lower immunity - increased somatic cell count in milk – possible risk of mastitis

6.     Oxidative stress - reduced reproduction, conception rate

All the above will increase production costs & decrease returns to the farmer & reduce the overall health of the cow. The risk of ignoring THI stress for the dairy cow is significant.

There are a number of nutritional and management options that can help to relieve the dairy cow of some of the burden associated with Heat Stress and THI.

1.     Fans

2.     Water misting

3.     Nutrition & Diet options – feed additives

4.     Feeding times – in the cool of morning and evening. Feed available overnight

5.     Available clean water

6.     Adequate space for cows to lay down and be comfortable

7.     Shade

8.     Number of times feed is presented to cows over the course of 24 hours

THI relates to the combination of Temperature and Relative Humidity. Cows will start to feel the effects of THI when temperatures reach 22 degree C to 24 degree C and Relative Humidity reach 40% to 60%.

As the temperature and relative humidity increase so does the THI and the effect of THI on the cow moves from Mild to Moderate to Severe Heat Stress. Hence to cows will exhibit the signs list above and then the consequences of the THI will be displayed by the cow as the THI increases.

THI affects all categories of dairy cattle – including milking cows, dry cows, growing heifers, and calves – with implications that impact every stage of the animal’s life cycle. These effects and practical strategies to mitigate heat stress will be thoroughly explored at the Dairy Production and Management Training Course, taking place from August 5th to 8th, 2025, in Thailand, and held annually by Progressus. This course combines in-depth classroom theory with hands-on farm visits, offering participants a comprehensive understanding of heat stress management and broader best practices in tropical and sub-tropical dairy production. It is a valuable opportunity for farmers, veterinarians, nutritionists, and dairy professionals looking to improve performance and welfare on their farms.