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Heat Stress: A reflection

By Mikaela Baker, (Ba Ag. Sci., M Ag Sci) TRAC Ruminant Productivity Consultant

Reflection is equally an important practice in agriculture, business and personal development as is planning for the day and mapping out the future. Reviewing the past and reflecting on what happened, what was learnt, what went well and what went poorly creates a strong base for future resilience in a business enterprise.

Heat stress and heat abatement is a topic that is covered so regularly, potentially to the point that not enough focus is given now, it is ‘assumed knowledge’ and part of regular practice. However, with the recent four day heat wave that swept across our southern states, the range in outcomes on farm shows that heat abatement practices may not be practiced to the extent they should be. With temperatures exceeding 37 degrees daily, putting the cows thermal humidity index (THI) in the moderate - severe heat stress range. This was a time to employ every strategy on farm, to help keep cows cool throughout the day and maximise cooling during the night and early morning.

Heat stress affects cows on many different levels, such as decreasing feed intake, increasing metabolic costs and as such, decreasing fertility success and milk production. Interestingly, only 50% of the decrease in milk production is attributed to a decrease in dry matter intake, the other 50% is due to impaired metabolism and feed conversion, highlighting the importance of trying to keep cows as cool as possible.

When cows are heat stressed, they can decrease food intake from temperatures of 25 degrees and above, significantly dropping intake from 40 degrees onwards by 20 to 40% compared to normal. Cows may try to avoid roughage when they are feeling heat stressed, as roughage can cause an increase in body temperature through digestion. The decrease in roughage can be problematic for dairy farms that feed high amounts of grain through the dairy, as their forage to concentrate ratio will go from a safe range to an inverted ratio, putting cows at risk of acidosis. This can still be challenging for TMR farms, where cows are able to sort through the ration or where high concentrate rations may be fed. As cows begin to feel hot, their endogenous bicarb production (buffering from rumination) decreases, which further puts them at risk of sub-acute ruminal acidosis (SARA) or clinical acidosis or grain poisoning. It is important to ensure hay, silage and TMR are presented in a way where there is easy access and no competition, so we do not further hinder their chances of eating forage. In addition, keeping cows cool through the use of sprinklers where possible and having food fresh and available in the coolest parts of the day will help keep intake up.

When decreased forage intake is known through heat events, the first step is to manage heat stress through physical means such as shade and sprinklers, however tools such as feed additives can add an extra layer of support. Products such as Acid Buf, help to maintain a more stable and lower rumen pH, which can reduce the risk of acidosis and SARA.

From a farm management perspective, there are key activities that should be included in all heat abatement summer plans. There should always be sprinklers used for both AM and PM milkings, set with timers. When the THI chart indicates there is some heat stress, sprinklers are beneficial. It is essential to use the sprinklers in the morning, even if we feel cool, because the cow is still suffering from stored heat internally and could take days to cool down depending on daily maximum temperatures. Cows feel the heat from 24 degrees onwards, and even more severely in humidity. The added challenge of humidity means that sprinklers need to be set to large droplets, on timers which allows the water to dry off the cows back allowing her to cool. Misters are lovely for us in the dairy but increase the humidity and heat stress of the cows.

In addition to sprinklers AM and PM, putting cows in shady paddocks will help keep them cool. TMR or feed can be brought to them, there is potential there will be extra wastage, however by ensuring the cows are cooler, they will eat more food and maintain more milk. Milking times can also be adjusted. They can be moved either way in the afternoon. Either brought earlier to the hottest part of the day where they can be actively cooled by sprinklers and fans, or pushed later in the evening when it is cooler.

From a physiological perspective, betaine is an additive that can be used through the dairy grain mix or added to a TMR when required. Betaine is a naturally occurring extract from sugar beet, that assists in maintaining cellular fluid balance (osmoregulation) to keep the cows feeling cooler and more hydrated, it also assists with essential amino acid cycles. The work done by the University of Melbourne showed feeding betaine in summer resulted in a 6% increase in milk yield.

During recent periods of heat stress, we noticed farms lose up to 5 litres per cow, and others who even increased milk production. We found that the cows that lost up to 1 litre or even increased milk production, were moved to areas with shade and sprinklers/irrigation, had sprinklers both AM and PM, fresh clean water and had additional and best quality forage supplied. Heat abatement almost seems to work in a logarithmic effect, the more you do, the better results that you will get.

Take stock of the management practices utilised this summer and in particular over the recent heat wave and see what worked and what didn’t. Our TRAC Consultants, your Experts In Ruminant Productivity would be more than happy to reflect with you, and see what plans we can have in place ready to go for next summer, because in 7 months we may have forgotten.

For more information or to discuss this article further,

please get in touch with your local TRAC Expert In Ruminant Productivity

on 08 8733 1888 or email us at


Our Consultants


Tom Thorn

0427 243 319

Owen Rees

0429 437 823

Mikaela Baker

0457 243 319


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