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Raising awareness of heat stress in dairy cattle

Key takeaways

  • Heat stress in dairy cattle can already occur at 20 C or a Temperature Heat Index (THI) of 62
  • The negative impact of heat stress on health and milk production is significant
  • Dairy Heat Load Index (DHLI) takes the temperature at the cow into account and is therefore more suitable for grazed cattle
  • A warning system based on DHLI but also a parameter based on the presence of heat stress is under development

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Tom Chamberlain, BVSc, PhD, MRCVS, discussed how we can measure the effects of heat stress on cows living in countries with temperate climates and what can be done to avoid loss in performance.

In moderate climates, heat stress can already occur at 20°C

Heat stress in dairy cows is a common problem, even in moderate climates. Due to global warming, it is a growing problem[104]. Milk production[74], and fertility[67,68] in dairy cattle will be negatively affected by heat stress and the incidence of lameness and mastitis will increase[75,80]. Heat stress can be an important potential cause of systemic immune activation[69], which in term can result in involuntary culling of dairy cows[4].

Figure 2, Temperature Humidity Index calculated on the basis of humidity and temperature. If THI exceeds 72, cows can experience mild heat stress and a reduction of fertility. Above a THI of 78, milk production will be reduced. Above 82, very significant losses in milk production will occur in combination with signs of severe heat stress.

Temperature Humidity Index (THI)

A common way to quantify the risk of heat stress is the Temperature Humidity Index (THI), an index which combined environmental temperature and humidity. Threshold values are often based on studies working with cows that are acclimatized to high environmental temperatures and humidity. In those parts of the world, the risk that cows suffer from heat stress starts to occur at THI level of about 72[107] (see Figure 2). Cows in climate zones in which heat stress occurs regularly will adapt and as a result, threshold values will be much lower, can already show signs of heat stress at a THI of 62[74,78]. The THI is based on environmental temperature and humidity. It does not take into account the level of radiation. Therefore, the THI is suitable to quantify the risk of heat stress in housed dairy cattle but has its limitations in grazing cows that are exposed to direct sunlight.

Figure 3: real-time THI and DHLI data, from a farm in the South of England, taken from the website grazemore.co.uk.

Dairy Heat Load Index (DHLI)

With the support of Trouw Nutrition, Dr. Tom Chamberlain, veterinarian and owner of Chalcombe Consultancy, is running a project “Raising the awareness of heat stress problems with dairy farmers”. The project has a number of important targets. The first one is to increase awareness that heat stress can also affect cattle in temperate climate zones such as North West Europe and the coastal areas of Latin America and North America. Next to that, a target is to monitor the risk for heat stress with a parameter that is more suitable for grazing cattle, the so-called Dairy Heat Load Index (DHLI).

Dr. Chamberlain comments: “In grazing situations it is preferrable to work with the Dairy Heat Load Index, which is based upon recordings of humidity and temperature in a so called “black globe”. This black globe is placed where the cattle are grazing, giving you an accurate measurement of the conditions a cow really has to face. We now have real-time data for THI and DHLI available from seven farms in the South-West of England (See Figure 2). The results are available in real-time through our webpage.”

Towards a metric to diagnose heat stress in dairy cows

The third target of the project is to provide farmers with a better tool to help them with assessing heat stress in their herds. Both THI and DHLI are environment and not animal based. As such, they indicate to what extend animals are at risk, but they do not measure the effect of heat stress in animals. The project will develop a metric based on animal measurements. Dr Chamberlain continues: “Previous research workers have developed animal-based metrics, but these have been too invasive or expensive for commercial use. Our intention is to develop a metric that is simple to apply on commercial dairy farms.”

Questions asked during the webinar...

Question Answer
What would be the critical heat stress temperature for heat adapted cows that reside in hot climates such as South Africa? If cows are in hot climates, they adapt to heat more easily. Critical heats stress temperatures would therefore be about 26 °C.
How much does the heat stress contribute to the reduction of milk production in terms of cow milk? As you have seen in the slides by Dr. Chamberlain the average production of heifers will drop by a THI between 62-72 with approx. 2 L/day, with a THI -79 you will loose 2-4 L/day and over 79 THI over 4 L/day.
Is there a difference in het tolerance of different breeds / animals reared in different conditions? and how to deal / with these differences with regard to recommendations? There are indeed breed differences, especially the Zebu and Girolando (Gir x Holstein cross) cope more easily with warmer conditions. Of course, these varieties do have production differences with, for example, Holstein, which also plays a role in the extent to which the varieties can tolerate heat.
Is it good to wash the cow to reduce heat stress? Cows cool down as a result of washing, as long as it does not result in an increase of te relative humidity. Thus, cows should be outdoors or in a well ventialted barn if they are being washed.
A very exciting presentation. In for example natural grazing systems what could be those critical interventions that can reduce heat stress? In a grazing system it's crucial to have enough shadow places, direct sunlight is the biggest risk factor. Provide shade, aim 4 – 6 m2 shade per cow. Maximize eating time / opportunities in evening. ‘Siesta’ management, give best grazing in evenings and offer enough clean and cool water options near grazing areas.
What do you suggest on a nutritional point of view to reduce heat stress? With more feeding moments, fresh and cool feed, cows will tolerate the heat more easily. For rumen health, sodium bicarbonate is a buffer and probiotics to support stressed rumen.

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How we can measure the effect of heat stress in dairy cows

Presentation by Dr. Tom Chamberlain MRCVS during the CRV/Selko Webinar July 28, 2021.
Dr. Chamberlain discussed how we can measure the effects of heat stress on cows living in countries with temperate climates and what can be done to avoid loss in performance.

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