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The thresholds for when increased heat load begins to have negative consequences for dairy cows

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  2026 ASABE Annual International Meeting  2600693.(doi:10.13031/aim.202600693)
Authors:   Bjarne Bjerg, Li Rong, Peter Kai
Keywords:   Cattle, heat stress, milk yield, rectal temperature, respiration rate, Temperature Humidity index, THI.

Abstract. High animal welfare, low environment impact and sound production economics are inevitable requirements for future livestock production. New methods to reduce environmental and climate impacts may be less costly if changes are followed by simultaneous changes in the way livestock is housed. E.g. effective air purification methods for removing ammonia from barn air are well known and similar methods for removing methane are under development. However, in cattle housing, these methods are difficult to use because cattle in many parts of the world are kept in uninsulated, naturally ventilated buildings. As a result, the pollutants become diluted in large volumes of air before they can be directed to an air purification system, which reduces the cost-effectiveness of such systems. Insulating the building provides opportunities to achieve a good barn environment with significantly less air exchange than in an uninsulated barn, especially during cool periods. Apart from periods of high solar radiation, insulation of the roof surface, together with the reduced air exchange, will cause an increased barn temperature. An increased barn temperature is beneficial during cold periods, but a disadvantage in warmer periods. To assess the overall effect of insulation and reduced air exchange, it is therefore crucial to know the thermal conditions where additional heat load will have negative impact on the animals.

This study was mainly based on a review of 31 peer reviewed works on effects of heat stress in Holstein cows. The analyses indicated that the cow‘s respiration rate and rectal temperature begin to increase when the Temperature Humidity Index (THI) rises above 60 to 64. For cows producing more than 40 kg of milk per day the milk yield begins to decline when THI rises above 66, and for cows producing less than 30 kg milk per day the corresponding number is 72.

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