Heat Stress. What is it? Do you think it is having an impact on the health and productivity of your cows each summer? And should we even worry about it?

To answer the first questions, Heat Stress is experienced by cows when they gain more heat than they can get rid of, occurring at any age and stage of lactation.

Heat Stress is measured and assessed using the THI (Temperature and Humidity Index) calculation, assessing the relationship between temperature (°C) and Relative Humidity (%), with a THI level of 68 being the standard benchmark – anything over 68 THI tells us that cows are experiencing heat stress and we can expect to see health and performance related issues, see Table 1.0.

One of the main misconceptions is that heat stress only occurs during periods of very warm weather, which we may only see for short periods of time in the North of England and Scotland each summer, so why should we be concerned?

The reality is that humidity plays a big role in heat stress, even at lower temperatures, a high humidity level can cause heat stress conditions. For example, a temperature as low as 21°C with 65% humidity is enough to reach the limit of 68 THI, conditions that are quite common during UK summer months.


Stress Level

Signs/Performance Losses


No Stress

62 – 67

Low Stress

  • milk loss of up to 1 litre

  • oestrus behaviour and conception rates affected

68 – 72

Moderate Stress

  • milk loss of up to 2 litres

  • lower DMI intakes

  • butterfat depression

  • rises in SCC

73 – 79

High Stress

  • milk loss of up to 4 litres

  • visible signs of stress, cows standing and herding together

  • butterfat and milk protein depression

  • SCC increases


Severe Stress

  • all of the above plus

  • >10% reduction in rumination

  • milk losses greater than 4 litres

  • visible metabolic health issues and risk to life >90 THI.

Table 1.0 THI and signs of Heat Stress

To assess the real on farm severity and impacts of heat stress in different housing environments, the Davidsons Dairy Tech team in conjunction with Lallemand Animal Nutrition have been gathering unique data on farms across Scotland and the North of England, during the summer of 2022, see Graph 1.0. The data gathered has been extensive, assessing over 35,000 data points looking at temperature and humidity over a 3 month period, 1st June 2022 – 31st August 2022.

The findings of the studies should be a real eye opener for dairy farmers, whether cows are housed or grazed, with some of the headline findings being:

  • Heat stress occurred on 50% of the days (46 days out of 91)

  • Up to 17.9% of the total time cows experienced heat stress

  • Highest THI recorded was 86 THI on 19th July 2022

  • Single longest continuous period above 68 THI was 20 hours, 40 minutes on 14th / 15th August 2022

  • Up to 64 minutes/cow/day reduction in rumination (11% reduction)

  • Losses of up to 1.5kgs milk/cow/day

  • Reduced conception rates and pregnancy rates

Visible signs of heat stress on the study farms varied dependant on the level of THI being experienced at any given time, but included observation such as, reduced dry matter intakes, cows crowding, panting, reduced signs of oestrus, increased standing time, reduced cudding rates and higher SCC levels.

The Dairy Tech Team at Davidsons actively encourage farmers to seriously consider the impacts of heat stress on their herd. Start to put plans in place for the summer ahead and also consider any longer term investment to reduce the impact of heat stress.

The approach taken should form two parts; environmental and nutritional.

The environmental aspects to consider should be, ventilation, especially for dry/transition cows and in collecting yards, around robots, water access (indoors and in grazing paddocks), cow comfort to promote lying times, reduction of solar radiation from sky lights in sheds and for grazing cows consider provision of shade and walking distances for cows to and from grazing.

The nutritional aspects can be wide ranging, including increasing the rations energy density, buffering of the rumen, addition of live yeasts such as Levucell Titan SC to improve rumination activity, aid rumen efficiency and fibre digestion, trace mineral supplementation such as protected forms of Zinc, AvailaZn from Zinpro and regular provision of fresh feed.

At some point during the summer of 2023, cows will inevitably experience heat stress, the severity of which is yet to be seen. Can we afford to risk the health and productivity of our cows by not planning ahead and considering all of the options available to us?