It might feel like mid winter and a long way away from being able to feel the heat of the sun on your back, but in the next four or five months, dairy cows will inevitably experience some form of stress from the high temperatures of summer shine.

Heat stress. What is it? Is it having an impact on the health and productivity of dairy cows each summer? And should farmers 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 above 68 THI means cows are experiencing heat stress which can affect 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 many believe is only only seen for short periods of time each summer in the North of England and Scotland

According to the Davidsons Dairy Tech Team, 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.

To assess the real on -farm severity and impacts of heat stress in different housing environments, the Dairy Tech team in conjunction with Lallemand Animal Nutrition gathered unique data on farms across Scotland and the North of England, during the summer of 2022, see Graph 1.0. The data, from more than 35,000 data points looked at temperature and humidity over the three-month period, June 1, 2022-August 31, 2022.

The findings, on farms where cows were housed and grazed, were a real eye opener to producers 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 July 19, 2022

* Single longest continuous period above 68 THI was 20 hours, 40 minutes on August 14 and 15, 2022

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

* Losses of up to 1.5kg 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 is encouraging farmers to seriously consider the impacts of heat stress on their herd. Start to put plans in place for the summer ahead and also to consider any longer term investment to reduce the impact of heat stress.

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

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.

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 producers afford to risk the health and productivity of our cows by not planning ahead and considering all of the options available?