A study has determined that heat stress in dairy cows may be causing significant milk yield loss.

As a result, the study determined this may be leading to significant financial loss on dairy farms in both housed and grazing systems.

Although many farmers believe it may not be an issue on their farms, as it is usually associated with high temperatures – this study determined that this may not be the case.

Heat stress

The study, which was conducted by Lallemand Animal Nutrition and Chalcombe Ltd., found that throughout the summer of 2021, dairy cattle were impacted by heat stress on more than a third of the days during a six-month study.

The study was run across eight farms outdoors and six farms indoors, none of which had fans or misting systems in their buildings.

Dr. Tom Chamberlain, founder of Chalcombe Ltd., said: “Milk yield losses averaged 128L/cow over the summer period in housed situations, ranging from 84L to 191L.

“Potential losses were even higher in grazing situations at an average of 284L/cow.

“They ranged from 171L to 445L, but actual losses would depend on the amount of shade available,” he added.

The study determined that financial losses can amount to more than £20,000 (€23,569), on average, per 200 grazing cows and more than £10,000 (€11,785) on average per 200 housed cows.

These losses equate to the full sales value of the milk as no costs, such as concentrates fed, can be recovered.


Outling some measures farmers can can take to mitigate against heat stress in cows, Dr. Chamberlain said: “Generally, cows start to suffer when the temperature goes over 20°C.

“Farmers can monitor cows panting, to check if they reach over 60 breaths/minute which suggests that they’re suffering from heat stress.

“Cow behaviour is also a good indicator, such as when cows are standing more or clustering in the shade.

“To help mitigate heat stress, farmers should provide cows with as much access to shade as possible and ample cool water, as water intakes rise markedly during heat stress,” he explained.