The Effect of Heat on Dairy Cows Kicks In Quickly

Once cows begin to show physical signs of heat stress the damage is done. Research shows that fertility is affected at lower summer temperatures than previously thought, and that this can easily go unnoticed.

The Effect of Heat on Dairy Cows Kicks In Quickly

Fluctuating temperatures in spring and summer are typical in the UK and estimates show that a swing in temperatures, from 14⁰C to 22⁰C at a relative humidity of 60%, can cause 20% of cows in an all year round calving herd to slip a cycle.

This latest published data, from Thuringian State Institute of Agriculture and the Institute of Animal Breeding and Genetics in Germany (or simplify to ‘a German animal breeding and agriculture research group’) comes as no surprise to Cargill’s ruminant technical manager Philip Ingram.

“Although UK temperatures are not typically extreme, we do see significant swings in temperature and in humidity, even within one week. And it’s hard to manage this eb and flow. Air conditioning or sophisticated temperature-controlled fans are not commonplace on our units, and grazed herds are affected by spikes in warmer weather, just as much as housed herds,” adds Dr Ingram. 

He points out that during the summer, cow fertility is the first parameter to be affectedfollowed by milk yield and milk components. And all these performance drawbacks occur before any physical behavioural symptoms usually associated with heat stress are seen.

“If we see cows exhibiting heat stress – such as crowding in cooler corners of the building or field, standing rather than lying and panting – it’s likely that the fertility and yield damage is already done,” he says. “And they may not be picked up, as they occur after a hot blip rather than during it. We don’t relate the problems to higher or fluctuating temperatures.

“So we can’t rely on our perception of the cow’s behaviour to assess when and how temperature and humidity are affecting dairy cow performance.”


Fertility suffers first

The temperature humidity index (THI) is taken as an industry benchmark for accessing heat stress in livestock systems. For dairy cattle, the classic threshold at which heat stress affects performance is often quoted to be a THI of 68.

In the UK the humidity is almost always more than 60%, so this THI would be triggered at 22⁰C.

However recent research has highlighted that fertility can be affected at a THI of 57. In the UK this would typically be about 14⁰C.

The findings come from a study on 22,212 high production Holstein cows kept in 15 large-scale dairy herds in Germany for three years, from 2013 to 2015.

It showed that when temperatures exceed this modest threshold of THI 57 (14⁰C at 60% relative humidity) oestrus behaviour changes, fewer cows are seen bulling and the number of services falls. And conception rates start to fall from THI 65, equivalent to 20⁰C at 60% relative humidity.

So, by the time the temperature reaches 22⁰C in the UK fertility will have suffered significantly with 20% of cows slipping a breeding cycle.

And if a classic case of heat stress was to occur, the effect on fertility would be far greater. Estimates are that if temperatures rise to 27⁰C, which is possible in the UK, the proportion of cows slipping a cycle could increase to 43%.

“Rather than long spells of hot weather, we tend to see blips through summer,” says Dr Ingram. “Producers often think they’ve got away with any performance dips due to a few hot days.

“But what you see is that both production and fertility can be affected after the hot blip – a delayed response that they don’t then associate with the hot spell.”


Grazers not exempt

Long-term UK experimental datafrom Langhill Farm, Edinburgh University and Crichton Royal Farm, in Dumfries, also point to the negative effects of temperature at lower thresholds than often assumed, as highlighted in trials on grazing herds.

Where cows were outdoors, milk yield was shown to peak at a THI of 54.9, or 13⁰Cand average humidity, and then decreased as THI increased.

Butterfat in housed herds started to fallwhen the temperature increased above 10⁰C and the studies showed that milk fat gradually decreased as the number of hours of sunshine increased in grazed cattle.

“This all points to the need to prepare cows for the summer months,” adds Dr Ingram.

He encourages producers to look at diets and use them to prepare the cow for warmer conditions.

“We’ve seen success on farms where producers have included a rumen bufferthat can maintain rumen function and keep the cow hydrated. If we can do this, then intakes are likely to hold up and the negative effects on fertility and production caused by increased temperatures can be countered.”

He mentions UK trials where the rumen buffer Equaliser CoolCow– specifically designed to condition cows to deal with fluctuating and high summer temperatures, helped to maintain milk fat in diets when compared with a control group that suffered significantly reduced milk fat (of 0.34%) during the hottest weeks.

Fat-and-protein corrected milk yield was numerically 0.5kg higher in the Equaliser CoolCow group throughout the trial, and 1.5kg higher in the during the hottest weeks.

“We also saw that fertility results, for those cows that got pregnant during the trial, were numerically better with the Equaliser CoolCow group getting in calf 17 days earlier than the control group.”

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,” says Dr Ingram. “Acting early in the season, to ensure that cows are kept comfortable, with shade while grazing andgood ventilation plus the use of ‘cooling’ buffers will help to pre-empt problems through summer,” adds Dr Ingram.

 Key points:

  • Fertility and production suffer before visual signs of heat stress are seen
  • Fertility is affected from as low as THI 57 (14⁰C and 60% relative humidity for example)
  • Meteorological data from the UK during the past four years show THIs above 57 from May to August.
  • Therefore, in a typical year, cow fertility can suffer from May to August.
  • A dip in performance may be seen after the hot spell; producers can believe that they’ve got away with it when, in fact, the impact is delayed.
  • Producers are well advised to prepare cows for summer ahead of an increase in temperatures
  • Cargill’s cooling buffer, Equaliser CoolCow, is proven to keep cows performing throughout the temperature fluctuations in summer.

Article from Cargill, first published in Dairy Farmer May Edition.




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