HEAT STRESS hits earlier than you think

Owen Rees
Dec 21, 2025
4 min read

By Owen Rees, Senior Ruminant Productivity Consultant

With one of the coldest starts to summer on record, it feels almost ironic to be writing about heat stress—but warmer weather will arrive, and when it does, our cows will feel it quickly.

Nutrition 101 tells us that a cow begins to experience heat stress when the heat generated by the cow is greater than the cow can lose through the environment. This often begins around 25°C but can be brought on at lower temperatures with high humidity. For example, at 22°C with 45% humidity the stress threshold begins, respiration rate increases, and milk decline and reproduction losses can be detectable. Often, clear physical signs of cows experiencing heat stress begin at a Temperature-Humidity Index (THI) of around 72, which is classified as mild/moderate heat stress. This could look like 26°C with 45% humidity where cows will exhibit increased respiration, preference to be near water, bunching or even panting. Severe heat stress begins at 29°C.

Figure 1. Revised temperature humidity index for lactating dairy cows. | (Adapted from Zimbelman and Collier, 2009)

What does this mean for our cows?

As cows get hot their brain tells them to eat a little less. This will reduce the heat of fermentation and digestion, which will then lower heat output of their digestion system. Sadly, less energy intake means less milk/meat production. At the same time, the feed which is consumed is digested and metabolised less efficiently, fibre digestibility specifically and organic matter digestibility more generally declines, so the animal yields less energy from the rumen. This means the animals get a double whammy of decreased energy availability.

At the same time animals select against fibre, and for starchy feeds e.g. grain. If the cows are able to sort, they will end up with an unstable rumen, low rumen raft and lower rumen contractions. This will lead to cows that are prone to acidosis, due to the way they are eating, trying to minimise heat stress for themselves.

If our cows aren’t already in enough trouble her demands will ramp up rapidly trying to stay cool. Staying cool on hot days for a cow relies on evaporative cooling...sweating and panting.

For a cow panting takes a lot of energy, and cows can breathe at 80-100 breaths/min in real stress events. It takes massive effort and energy to keep themselves cool.

Maintenance energy requirements can easily raise 20-25% in heat stress events, right at a time when energy supply is dropping off. Energy partitioning always goes to maintenance first and then what is left goes to milk and meat production. So, no wonder why milk production drops off during heat stress events, and the rumen instability also means that often cows produce less milk fat.

What can we do on farm to help mitigate heat stress?

Sprinklers at the dairy in both AM and PM milkings. It takes cows up to 48 hours to cool down after being exposed to high heat and/or humidity, therefore turning sprinkler systems on in the morning plays a key role in cooling cows down.
Cleaning all water troughs regularly to ensure fresh water is presented to the cows with no limitation to intake. Regular cleaning should be at least 3 times per week as a minimum in dairy yards, feed pads and loafing areas, or before going to a new paddock.
In some cases, altered rations to support energy density with high quality forage that cows want to eat can help maintain DMI
Putting cows in shady paddocks on hot days, or providing shade cloths in loafing areas, will decrease direct sunlight heat

In addition to physical changes on farm, we can use certain additives to support cow metabolism and water retention, to keep her cool and productive. Betain is a supplement that has been used on farms now for over 15 years with strong research behind it, and great results in the field. Research has found that supplementing 15g of Betain per cow per day improves dry matter intake, internal body temperature, heat mitigation and milk production.

Betain is an osmolyte and methyl donor, which means it reduces microbe vulnerability to heat stress and saves energy that can be used for improved milk production and reproductive performance through periods of heat stress. During heat stress, particularly in dairy farms where the majority or all of the grain is fed through the dairy, cows are more susceptible to Sub Acute Ruminal Acidosis (SARA), particularly when dry matter intake has decreased. Rations will need to be balanced to assist further in maintaining a stable rumen environment.

Betain is suitable for use in all production systems and all classes of cows, and can be easily included in a mineral mix, grain mix, or TMR. Feeding it daily ensures consistent support and protection through heat stress. For tailored advice or to review your heat-stress ration this summer, please reach out to your TRAC Consultant. We’re here to help fine-tune rations and management strategies to keep cows cool, healthy, and productive.

If you have any questions, please do not hesitate to talk with one of our TRAC Ruminant Experts on 08 8733 1888 or email us at info@totalresult.com.au.