As temperatures are rising, heat stress is a common and growing concern in ruminant production and experts are now fully aware that it is not limited to two summer months or tropical regions. Heat stress could be an issue for long periods. Cattle naturally change their behavior to help alleviate stress, which can disrupt digestion, increase inflammation, reduce performance, and impact their health.
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.
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 gathered unique data on farms across Scotland and the North of England, 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, which took place from 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:
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Heat stress occurred on 50% of the days (46 days out of 91)
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Up to 17.9% of the total time cows experienced heat stress
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Highest THI recorded was 86 THI on 19th July 2022
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Single longest continuous period above 68 THI was 20 hours, 40 minutes on 14th / 15th August 2022
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Up to 64 minutes/cow/day reduction in rumination (11% reduction)
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Losses of up to 1.5kgs milk/cow/day
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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.
With the summers getting warmer, the Dairy Tech Team at Davidsons actively encourage farmers to seriously consider the impacts of heat stress on their herd.
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 hot spells, cows will inevitably experience heat stress. 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?
Would you like one of our Dairy Tech consultants to visit and evaluate the impact of heat stress on your herd? Dairy Tech can suggest small adjustments to help keep your cows comfortable during hot weather. Call us today at 01501 820473 to connect with your local consultant.
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