With many parts of the UK set to see a heatwave in the coming days, Katie Jones takes a look at heat stress in dairy cattle, and looks at ways to minimise the problem.
Heat stress occurs when a cow’s heat load is greater than its capacity to lose heat.
Dr Laura Tennant, ruminant technical specialist with Trouw Nutrition, 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,” she says.
“Allow cows access back to shaded yards and reduce the time spent in unshaded yards.”
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,” she adds.
“At pasture, cows should not have to walk more than 250m 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.”
Heat stress in dairy cows arises from a combination of temperature and humidity.
Philip Ingram, ruminant technical manager at Cargill, says: “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.