24th June 2024

Could heat stress be affecting your bottom line?

Weather patterns over the last few years have shown that farmers and their cattle can expect hotter weather, more frequently.  Research from 2023 found that the average production loss caused by heat stress annually is between £24,000 and £90,000 depending on herd size.


What is heat stress? 

Heat stress occurs when cows are generating more heat than they can lose to the environment.  

The thermal comfort zone of a cow is between -15°C to +25°C. However, when relative humidity in the environment is high, heat stress can be experienced at lower temperatures. 

Even a THI score of 57, which indicates mild to moderate heat stress, can impact productivity as the below graphic shows. 


Graph showing THI Levels

A cow’s rumen acts as a large fermentation vat during the feed digestion process. As she breaks down feed and converts energy into milk production, a vast amount of heat is produced as a byproduct. 

Environmental temperatures must be sufficiently cooler than the heat produced by the cow in order for her to lose this heat and remain within her thermal comfort zone. 

Higher producing cows create more body heat as their energy usage is greater. A cow with a yield of 18 litres per day will generate over a quarter more body heat than a dry cow, while a cow yielding 31 litres per day generates almost 1.5 times more heat than a dry cow. [NADIS]

Increased respiration and sweating are the cow’s natural mechanisms for cooling down. However, these are often insufficient for today’s ultra-high producing breeds in warm weather. 


How does heat stress impact milk production? 

Cows use more energy when working to dissipate heat, which has a knock on impact on productivity, as energy is redirected from milk production. 

This is further impacted by reduced appetites during hot weather, where dry matter intake (DMI) is reduced by 10-30%. 

When temperatures exceed 25°C (or THI of 68),  for more than 24 hours, milk yield can drop by 20%, which has a direct impact on farm profitability. Heat stress can also alter the composition of the milk, with protein and fat content dropping with increasing THI. [nih.gov] 


How does heat stress impact fertility? 

Reduced fertility rates in the summer are one of the most costly effects of heat stress. 

Even mild heat stress, at a THI of 57 (sustained for 24h+) can have an impact on fertility, with oestrus expression and detection being reduced. Once THI reaches 65, conception rates begin to decline, with more inseminations required per pregnancy. 

Heat stress can cause a decrease in conception rates by up to 30% due to reduced eggfollicle development. Calving intervals (CI) are also extended as cows struggle to get back into calf. Each day a cow is open is worth £5 in lost revenue, meaning the difference between a CI of 365 and 400 days is worth £175 per cow per year. 

Furthermore, an extended period of ‘late lactation’ means the cows have low milk yields for longer. With this reduced energy demand, cows can become overfat towards the end of the milking period, increasing the risk of transition disease.

Although dry cows generate less metabolic heat than milking cows, this group should not be overlooked during hot weather. Environmental heat stress during the dry period negatively affects performance in the following lactation and can have lasting effects on the unborn calf. 

A 2019 study showed that cooling cows for the entire dry period increased milk yield up to 30 weeks into the subsequent lactation, with production being increased from 4 to 7.5kg/day relative to cows which were not cooled.  

For more information on how to manage heat stress in your dairy cows, please click here. 


VentTube Cool


Galebreaker has 40 years of experience in providing the best solutions for keeping cattle cool and comfortable. 

Get in touch to find out how our range of innovative products can help mitigate heat stress in your herd this summer. 

Click here to contact our expert team. 

Could heat stress be affecting your bottom line? Cows lying under VentTube ST55GRE Mid Mount R7


Steps to reduce livestock heat stress as temperatures soar – Farmers Weekly (fwi.co.uk)

NADIS – National Animal Disease Information Service

Impact of mild heat stress on dry matter intake, milk yield and milk composition in mid-lactation Holstein dairy cows in a temperate climate – PubMed (nih.gov)

Heat stress and seasonal effects on reproduction in the dairy cow—a review – ScienceDirect

Reproductive physiology of the heat-stressed dairy cow: implications for fertility and assisted reproduction – PMC (nih.gov)

How to Improve Your Calving Interval – ABS Global UK (www.absglobal.com) 

Extending lactation length: consequences for cow, calf, and farmer – Journal of Animal Science (nih.gov) 

Effect of heat stress during early, late, and entire dry period on dairy cattle – Journal of Dairy Science 

Late-gestation heat stress impairs daughter and granddaughter lifetime performance – Journal of Dairy Science 

Maternal heat stress reduces body and organ growth in calves: Relationship to immune status – JDS Communications