Dairy cattle have a narrow ‘thermoneutral zone’ of between -0.5°C to about +20°C with relative humidity of between 60 and 80 per cent, which is the range of temperature and humidity where the cow is comfortable and does not expend much energy either trying to stay warm or keep cool.
Many people think that it is only a high temperature that causes heat stress but it is actually the combined effect of temperature and humidity that contributes to heat stress. This is known as the Temperature Humidity Index or THI
Heat stress occurs when a cow cannot adequately dissipate heat, leading to an increase in her core body temperature.
Dairy cows are especially vulnerable due to their high metabolic rate. Metabolic heat is generated during digestion contributing to heat stress in high producing cows.
Australian summers can have long periods where the cow is exposed to extreme temperatures and THIs that can have significant impacts on their health and welfare as well as their production and fertility.
It is especially significant during heatwave conditions when there is no overnight reprieve.
Cows have this incredible ability to heat load during the day and offload the heat during the cooler conditions overnight, but if they are unable to dissipate all the excess heat overnight and carry it forward into the next day, heat stress will accumulate and cause quite significant losses.
This ability to heat load also gives us several great opportunities to manage heat stress in the milking herd by providing active cooling at milking time, even on farms with little other heat management infrastructure.
Cooling the concrete in the cow yard prior to bringing the cows in and running sprinklers off and on using a timer during milking time is an excellent way of cooling cows.
The aim is to cool, not saturate the cows, and because most heat is lost during evaporation adding fans to increase evaporation is ideal.
Bigger picture, mitigating heat stress requires a multi-pronged approach and is best tailored to the farm’s environment, infrastructure and resources.
Provide adequate shade
- Natural shade from trees or constructed shelters can significantly reduce heat exposure.
- If possible, provide shade in the dairy holding yard.
- Ensure shade structures are large enough to accommodate all cows without overcrowding.
Improve ventilation and airflow
- Increase airflow in sheds or shade structures and milking areas using fans or natural ventilation.
- Open-sided shelters and ridge vents can help maintain a cooler environment.
Active cooling systems
- Sprinklers or misters: Regularly spraying cows with water reduces body temperature effectively. Use alongside fans for maximum impact.
- Evaporative cooling: Systems combining water mist and air movement can lower temperatures in covered areas.
Optimise feeding and milking
- Adjust feeding times: Offer feed during cooler parts of the day, such as early morning or evening.
- High-quality forage: Provide easily digestible feeds to reduce metabolic heat production.
- Increase water availability: Ensure cows have constant access to clean, cool water. Position water troughs near shaded areas and ensure troughs have been cleaned of sediment and silt and water supply lines are adequate.
- Adjust milking time: Milk earlier in the morning or later in the evening to avoid the hottest time of day
Breeding and genetics
- Selecting heat-tolerant breeds or crossbreeds can improve herd resilience.
- Breeding programs can focus on traits that enhance heat tolerance.
Monitor and adapt
- Use technology like temperature monitors and weather forecasts to anticipate heat stress events. Subscribe to services that provide warnings if the Temperature and Humidity Index is expected to be high.
- Develop a heat stress management plan and train staff to respond promptly.
In recent years, the effects of heat stress on dry cows has become more widely understood.
Heat stress in pregnant dry animals will not only affect the health and wellbeing of the cow and her future production but also has a very significant effect on the unborn calf that can reduce the health outcomes of the calf and reduce their lifetime production for years to come.
For this reason, providing adequate shade and shelter and actively managing heat stress in dry cow groups is highly recommended.
Dairy Australia has some excellent resources on managing heat stress in dairy cows and I would strongly recommend all dairy farmers review their knowledge of heat stress management and their preparedness for heat stress events.
Lastly, I cannot emphasise enough my belief that having back-up plans for your back-up plans to provide drinking and cooling water for the herd in the face of extreme events like prolonged power failures, fires, etc is an essential part of your animal health plan.
Rob Bonanno is a ProDairy herd health veterinarian based in Gippsland.
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