It doesn’t take a heat wave, or a spike in summer temperatures, to cause heat stress in dairy cows.

In fact, rising early summer temperatures – even as low as 14°C can have an effect on fertility.

“Long before we see any symptoms,” says Cargill’s ruminant technical manager Philip Ingram.

Estimates show that a swing in temperatures from 14⁰C to 22⁰C at a relative humidity of 60 per cent - which is typical in the UK in summer months - can cause 20 per cent of eligible cows in a year-round calving herd to slip a cycle, according to the German Institute of Animal Breeding and Genetics.

“Trial data showing the effects of summer temperatures on cow fertility, that becomes obvious after the problem has occurred, should encourage dairy producers to take action early. Cows can stop cycling and fertility will suffer before the cow shows any signs of heat stress.”

“Although UK temperatures are not typically extreme, we do see significant swings in temperature and humidity, even within one week,” says Dr Ingram.

“They have a significant negative influence on fertility.”

Reviewing fresh air flow, easy access to pushed up feed and, for both housed and grazing cattle, plenty of water availability and shaded areas will help cows to regulate their core body temperature.

“Amending the diet and including a cooling rumen buffer, like Equaliser CoolCow, are also part of toolkit for keeping cows on track,” adds Dr Ingram.

“It helps to cool them down and to maintain rumen function, so intakes don’t falter.

"The aim is to keep cows comfortable and enable them to maintain their normal pattern of lying and eating so that production and well-being is maintained throughout the summer.”

This rumen buffer from Cargill helps regulate core body temperature by hydrating the cow at the cellular level, due to the osmolyte, and by restoring the electrolyte balance.

Equaliser® CoolCow is a powder additive that is added to the lactating cow ration - the TMR or compound feed - at a rate of 100 to 150 g/head/day from May to the end of September to help to mitigate the effects of heat stress and ensure cover throughout the summer.

For dairy cattle, the classic threshold at which heat stress affects performance is often quoted to be a temperature-humidity index (THI) of 68, which, in the UK, would be 22⁰C. “We’re now aware that fertility can be affected at a THI of 57 which would typically be about 14⁰C in the UK with its typical 60% relative humidity.

“If temperatures increase above this, the effect on fertility would be far greater. Estimates are that at temperatures of 27⁰C, which is possible in the UK, the proportion of cows slipping a cycle could increase to 43 per cent.”

“The difficulty is that production and fertility can be affected after a hot blip, when temperatures may have dropped to more normal levels. The delayed response means that that producers don’t then associate a drop in performance with the hot spell,” says Dr Ingram.

“The chances are that once the problem becomes obvious, the damage has been done, and getting cows back on track is a greater challenge.”

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