9th June 2023

How heat stress impacts dairy cattle fertility

As British weather patterns continue to shift due to climate change, we can expect hotter, drier summers[1], which will increase the risk of heat stress. 

Although we associate heat stress with the intense summer heat, cows can become stressed at temperatures of 19°C, especially if there is high humidity.

What is heat stress?

Cows produce heat by digesting their feed and producing milk. But when the outside temperature and humidity are too high, they cannot cool down efficiently, leading to heat stress.

Even a low-output cow producing 18 litres of milk a day will generate 28% more body heat than a dry cow[2].

Heat-stressed cows will pant, slobber and may become lethargic. They may also reduce their dry matter intake (DMI) by 10-30%[3].

How heat stress impacts fertility

Traditionally, farmers have used milk yield as an indicator of their cows’ heat stress, as milk yield falls when the cow becomes heat stressed.

However, new research indicates that the threshold for fertility is lower than for lactation, meaning reproduction could be affected at much lower temperatures than first thought[4].

Heat stress can cause a decline in egg quality, decreased fertilisation rate and an increased risk of abortion[5], which can lower the herd’s conception rate by up to 30%[6].

This decline in fertility can last beyond the summer months and can carry on into the autumn. This suggests that heat stress affects follicles (undeveloped eggs) which become the eggs that are released for fertilisation 50 days later[7].

This increase in empty days can affect those on a block calving schedule as they will not be able to calve in time to receive the best prices in their contract.

Additionally, cows which endured heat stress while still in utero are also impacted – being older at first calving, having a higher culling rate, longer calving intervals and lower milk yield[8].

How to reduce the impact of heat stress on fertility

The main way to reduce the impact of heat stress is to either cool the cow down or to avoid her heating up.

Galebreaker’s VVS side curtains use temperature, wind speed, wind direction and rain sensors to continually optimise the temperature and humidity within housing by opening and closing to make full use of natural airflow. They are perfect for cooling, without the running costs of cooling equipment.

Rollerscreens provide variable ventilation and weather protection and are perfect for use above gates and feed barriers, although they can be adjusted to fit any opening.

A tube ventilation system, such as Galebreaker’s VentTube Cool, is an energy-efficient solution and provide an efficient cooling effect by wicking heat away from the animals’ skin. Tubes can keep the airflow consistent and they can be positioned to direct airflow where the cows need it, while also reducing the humidity and ammonia levels.

Other areas that can help reduce heat stress include looking at their diet. As cows will decrease their feed intake during hot weather, providing them with a more energy dense, easier to digest ration during the summer will maximise the cows’ nutritional intake without overheating them.

Cows also need an adequate water supply as they will drink up to 10% more during hot weather[9]. Providing 10cm of trough space will give enough space for 10% of cows to drink at one time, per AHDB guidelines[10].


[1] Climate change in the UK – Met Office

[2] NADIS – National Animal Disease Information Service

[3] NADIS – National Animal Disease Information Service

[4] Dairy cow reproduction under the influence of heat stress – PubMed (nih.gov)

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

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

[7] rep 179 Roth (psu.edu)

[8] Season of conception is associated with future survival, fertility, and milk yield of Holstein cows – ScienceDirect

[9] NADIS – National Animal Disease Information Service

[10] Guidance on improving water provision for dairy cows – Farmers Weekly (fwi.co.uk)