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Dairy farming special: Reducing the impact of heat stress on dairy cows

With high temperatures already recorded in April, farmers should be aware of the problems heat stress can cause.

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Dairy farming special: Reducing the impact of heat stress on dairy cows

Forward planning and the introduction of small but effective management changes can help reduce the impact of heat stress on dairy cows this summer.


Dr Laura Tennant, ruminant technical specialist with Trouw Nutrition, explains heat stress occurs when a cow’s heat load is greater than its capacity to lose heat.


She says: “Cows suffering from heat stress will produce less milk with poorer milk composition with declines in both fat and protein content.


“Heat stress can affect rumen function. Chronic heat stress and reduced rumination can lead to a depressed immune system, triggering a decrease in resistance to bacteria and an increase in somatic cell counts.”




She advises trying to keep cows on good quality forage, explaining that more mature grass and other feeds with a high fibre content increase the heat generated by rumen fermentation, increasing the internal heat load at a time when the cow is trying to cool down.


She recommends offering buffer and other supplementary feeds in the evening when it is cooler to promote better intakes.


When cows are at grass, she recommends using paddocks with shade during periods of heat stress and reducing the walking distance and speed between the grazing and parlour.


“Consider changing the rotation, so cows are in unshaded paddocks in the evening. Allow cows access back to shaded yards and reduce the time spent in unshaded yards,” she says.

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She also advises taking steps to minimize any handling stress and to isolate cows most severely affected by heat stress and providing greater shade and cooling.


“Water access is crucial. Cows require at least 60 litres/head/day and may need 120 litres depending on yield and temperature,” Dr Tennant adds.


Trough space


“At pasture, cows should not have to walk more than 250 metres to a drinking trough and all troughs should allow cows unrestricted access.


“Ensure trough space for 10 per cent of the herd to drink at the same time and allow 70cm of trough space per cow.


“Troughs need to replenish quickly so ensure flow rates are high enough that troughs never run dry. Water bowsers are an effective short-term solution to provide extra water when needed.”



WHILE the prolonged high temperatures last summer raised our awareness of heat-related production and fertility issues, heat stress will affect cows here in the UK, to varying degrees, in most summers.


Philip Ingram, ruminant technical manager at Cargill, says: “Conditions that can trigger heat stress in cows are not unusual here in the UK. Last summer’s prolonged high temperatures highlighted the effects of temperature, and humidity, on cow performance.


“Many producers saw daily yield fluctuations and changes in cow behaviour that affected intakes and signs of heat. This impacted on fertility and the effects were felt for many months afterwards.”


Heat stress in dairy cows arises from a combination of temperature and humidity.


“Temperatures do not need to be especially high, but if it is humid too, cows will feel uncomfortable and suffer heat stress,” he adds.




“They will alter their normal feeding and lying routines. As a result, intakes drop, milk yield falls and fertility will suffer.”


A temperature humidity index (THI) is used to determine heat stress conditions for dairy cows. Cows start experiencing heat stress when the THI is above 68. When the combination of humidity and temperature falls in the orange area, cows will be experiencing heat stress.


An ambient temperature of 22degC with relative humidity of 60 per cent – conditions not unusual in the UK – equates to a THI of 68. Cargill has compiled data by region for the past three summers to show producers, by month from 2010 and by day through June to August in 2018, when temperature and humidity triggered heat stress conditions.


“Even in northern England and Scotland, the THI has been above 68 in July and August for two of the past three years,” Mr Ingram adds.


“Last year, in 2018, the THI was above 68 right across the UK for three months.”


Conservative estimates put the damages of heat stress, through lost milk, decreased fertility and less efficient use of feed, at £40- £85 a cow in a typical UK year.




“This will depend on conditions, type of cow and stage of lactation – among other things – but it is a cost that can be reduced with forward planning,” he adds.


“Fine-tuning management and adding an additive can be easily justified if yields and fertility are kept on track.”

Tips for managing heat stress


Philip Ingram, Cargill, offers these tips to manage heat stress in the herd.


■ Measure temperature and relative humidity and monitor THI. THI meters monitor
temperature and humidity and can be placed in the shed or in a convenient corner of the yard.


■ Identify when cows are experiencing heat stress.


■ Use nutritional solutions. Specific feed additives can mitigate the effects of heat stress on milk production and fertility. Use these to reduce mineral losses associated with heat stress, support rumen function and help maintain feed and water intake. And start early – two weeks ahead of the likely ‘risk’ period.


■ Take practical steps in housing and grazing management. Ensure sufficient ventilation, fresh feed and clean water is available 24/7. Provide shelter for grazing cows, or access to buildings.

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