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Monitoring Heat Stress

Reduce the impact of heat stress

 Help reduce heat in cows.

How to help your herd during the warmer months.


An animal is said to be heat stressed when an interplay of environmental temperature, relative humidity, solar radiation, air movement and precipitation combine to prevent an animal from exchanging body heat with the environment.


Heat stress is known to cause reduced dry matter intake (DMI); reduced rate of weight gain; reduced fertility in dairy cattle; reduced milk yield and eventual death of the animal. Amongst the factors influencing how much heat animals can effectively exchange with the environment, only ambient temperature and relative humidity has been widely studied as a practical monitoring tool for heat stress determination, because all other factors are not readily assessed. Ambient temperature of 29°C and relative humidity of 90% has been shown to reduce milk yield by as much as 33%. Therefore a 7,000 litre cow could potentially lose 2,310 litres if heat stress is sustained and goes unnoticed.


The index currently used to monitor and reduce heat stress induced losses is the temperature humidity index (THI). This is a single value representing the combined effects of air temperature and humidity associated with a level of thermal stress. The physiology of cattle makes THI determination important for both welfare and production reasons. Bovines can tolerate high ambient temperatures at lower relative humidity, by sweating and panting, however this capability becomes compromised when the air becomes laden with moisture, preventing the effective exchange of body heat. The comfort zone of cattle has been determined to be between 5°C and 25°C, temperatures outside this zone impact negatively on cow comfort.


The amount of moisture in the air is critical as it affects the rate of heat loss via evaporation through the skin and the lungs. When the mean daily air temperature exceeds the thermal comfort zone of an animal, the amount of water vapour in the air becomes critical in the maintenance of animal body temperature. THI has implications for cattle comfort and therefore animal welfare. For instance humans are able to dissipate about 190% of body heat generated by metabolism, whereas cattle are only able to dissipate 105%. Therefore it is necessary to assess the impact of air temperature and relative humidity in order to be assured that cattle are free from the kind of stress that would affect their productivity.


Several formulas have been proposed to describe THI; the one most commonly used in the UK is the National Research Council (NRC) 1971 formula, where temperature is measured in °C and relative humidity in %. Utilizing data from the US, Dairy Co Housing Guide (Dairy housing - a best practice guide, 2012, pg68) stipulates that animals in environments where THI is between 72-79 are said to be experiencing ‘moderate heat stress’; 80-99 ‘severe heat stress’ and >= 100 very severe heat stress or ‘dead cows’. However growing numbers of current research advocate a threshold of 68 rather than 72 as the lower limit of moderate heat stress in Holstein Friesian dairy cows where most of the research has been undertaken. A lower threshold is also a practical level for monitoring the impact of heat stress as other factors (as explained earlier) influence the level of discomfort for an animal.


SmaXtec Climate Sensor heat map


There are several ways of reducing heat stress in animals, such as installation of fans, fog misters, sprinklers and provision of shade. These methods have been proven effective. However when utilising misters and sprinklers, caution has to be exercised to prevent the build-up of moisture. Combining sprinklers with a fan would help reduce humidity levels. When providing shade, the material used has to be one that prevents solar gain.

Heat stress produces discomfort in cattle; monitoring can save farmers money from losses associated with a depressed physiological system. In addition, the welfare of the animal is compromised, which is not a good portrayal of the industry, therefore its monitoring and abatement would lead to a healthy, happy cattle and a positive bottom line.

For more information on monitoring heat stress with smaXtec, please call Moletech on 01392 873265

Source Details

Victor Ogedegbe, Moletech

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