The record-breaking high temperatures in June may have cost UK dairy farmers more than a litre a day in lost milk production per cow.
Data from Lallemand Animal Nutrition's heat stress project suggests average milk yield losses in June could have been 31.1 litres per housed cow, and 36.6 litres per grazing cow.
This equates to a monthly loss of 6,216 litres for a housed 200-cow herd, and 7,318 litres for a 200-cow grazing herd.
The project is now in its fourth year and is run by Dr Tom Chamberlain, founder of Chalcombe Ltd.
He says the estimated milk production losses for housed herds taking part in the project ranged from 14.4 to 39.8 litres per cow for the month.
The losses for grazing herds were estimated to be between 19.1 and 65.6 litres per cow.
"Any reduction in milk production is a total loss; there's no way to recoup anything as the input costs will have remained the same," says Dr Chamberlain.
"American work suggests that milk yield losses are only about 50% of the total production losses a herd experiences after a heat wave."
The temperatures in June, which the Met Office has confirmed as the hottest on record in the UK, are also likely to have impacted future lameness problems and herd fertility.
Excessive standing from cows trying to keep cool during the hot weather may lead to increased lameness, due to solar bruising and ulcers, that will manifest in late summer and early autumn.
Dr Chamberlain says: "The heat may also have caused bulling signs and conception rates to fall, resulting in a drop in numbers of cows calving next March; we have previously seen a 50% fall in calving numbers from heat waves in 2022."
He is encouraging farmers to take steps to mitigate the impact of heat stress as periods of hot weather are likely to reoccur this summer.
"It was an early heat wave in June and there will probably be more to come, so farmers need to work out a heat stress management plan," he explains.
"Cows will start to suffer when the temperature is about 19-20°C in the UK, and once they've been suffering for too long, they'll start to experience problems with milk yield and fertility, as well as a deterioration in rumen health."