Dairy farmers are being advised to take action early summer to reduce heat stress in their herds after results from a UK study revealed significant fertility and production losses with temperatures as low as 14˚C.

Research by Cargill UK, which monitored temperature and humidity in 47 buildings housing milking cows across the UK from May to September 2021 and 2022, found heat stress was triggered most days during this period in all herds.

Temperature and humidity combine to cause heat stress. Based on 60% humidity and a temperature of 14˚C, which are common conditions in early summer in the UK, the Temperature and Humidity Index (THI) is 57. When the THI averages 57 for 24 hours or more, research has shown that oestrus behaviour in cows can be affected.

Conception rate is affected when the THI averages 65 or more – which will be around 20°C in the typical UK humidity. And studies have found that milk yield and protein yield will be affected if temperatures average at or above 22°C (triggering a THI of 68) for at least three consecutive days.

The results from the Cargill study found:

• A regional daily average THI of 65 or more was logged on at least 42 of the 153 days from May to September 2022, with the south recording this level on 38% of days.

• A regional daily average THI of 57 or more was recorded on 152 of the 153 days.

• On a Worcestershire farm, milk yields dropped from more than 40l a cow a day to 32l a cow a day when the THI exceeded 68 for several days.

Results from THI monitors in Scotland, from May, 2022, show that a daily average THI between 57 and 65 was recorded on 62% of units, and in June 77% of units.

While this dropped to 50% in July and August, as temperatures got higher, a daily average THI between 65 and 67 was recorded on 29% and 30% of units respectively.

This means that oestrus activity will have declined on most units from early summer, and in mid summer conception rates will decline.

"Many farmers recognise that fertility suffers during summer, but perhaps they don't realise just how vulnerable their cows are," says Cargill's technical sales adviser Donald Macleod.

"The consequences of depressed conception rates are not seen immediately, until perhaps conditions are cooler. By this point, it's too late to do anything except pick up the additional costs of extra fertility work and an extended calving interval."

He added that cows are resilient in maintaining milk yields during periods of heat stress with milk being the last thing they give up.

“High-production cows produce a lot of metabolic heat. Cows producing 46kg of milk a day produce 26% more metabolic heat than those producing 32kg of milk a day. It's a double-whammy – heat stress is a very real issue in dairy herds."

Managing heat stress

However, there are steps farmers can take ahead of the danger periods during the summer to help manage heat stress. They include:

• Improved ventilation

• Adapted management, such as cow flow

• Nutrition

The company looked at fertility performance on eight farms where THI monitors were installed, and a range of actions were taken to reduce the effects of heat stress on milking cows.

Some units took no action; others used fans, the specialised rumen buffer Equaliser CoolCow, or both.

The results showed that where fans and Equaliser CoolCow were used, the dip in fertility was minimised.

"If no action was taken, conception rates in the weeks where the highest THIs were recorded fell by 50%, from 40% at a THI 60 to only 20% when the THI exceeded 65,” added Mr Macleod.

The best improvements in fertility performance were seen when fans and the specialised rumen buffer were used. In these cases, conception rates remained more consistent and only dipped from 40% to 35% as the THI increased from 60 to 65 or above.

Equaliser CoolCow works by helping restore a cow's electrolyte balance and regulating her core body temperature. It costs around £20 a cow to feed in a concentrate or feed buffer for 150 days from May to September.

Mr Macleod added: "By keeping cows more comfortable, they remain active and maintain feed intakes, which supports more consistent fertility and milk production."

He said these actions should continue into September, when temperatures are typically still high enough to trigger setbacks in fertility.

“These conditions are becoming more normal on UK daily farms, so I’d encourage farmers to have plans in place to keep their cows – and themselves – cool through summer.”

“Because CoolCow plays an important part in countering the negative effects of heat stress, it's a cost that can be easily justified.”

Cargill UK will continue to monitor temperature and humidity through its cloud-based data loggers installed on 47 UK dairy units and publish live THI data by region on the web page www.provimi.eu/uk-coolcow