Temperature Humidity Index (THI) is a measure that has been used since the early 1990s. It accounts for the combined effects of environmental temperature and relative humidity and is a useful and easy way to assess the risk of heat stress.
Research has identified THI values above which heat stress begins in dairy cattle. Cows that suffer heat stress experience serious consequences on their productivity and on the quality and quantity of their milk. This happens because a large part of the energy coming from food is used to maintain constant body temperature and it is therefore not free to be used for other body processes like milk production, growth, conception and maintaining pregnancy. High producing cows are producing massive amounts of energy if they become heat stressed the fastest way for them to cool themselves down is to stop consuming feed(lowing their DMI). That has a huge impact on their production, growth, reproduction and conception rates.
Heat stress is caused by a combination of temperature, relative humidity, solar radiation, air movement, and precipitation. The majority of studies on heat stress in livestock focus on the two main environmental stressors: temperature and relative humidity (RH).
Multiple studies indicate a stress threshold of 67 THI, so animals are experiencing heat stress at a THI of 68 and greater. The levels of stress were separated into:
- Mild (68 to 71 THI)
- Moderate (72 to 79 THI)
- Severe (80 to 89 THI)
- . (90 THI or greater)
Some facts about THI,
- 68 THI: Where mild heat stress begins in dairy cows, although research is proving that it is more like 65.
- When the THI exceeds 72, cows are likely to begin experiencing heat stress and their in-calf rates will be affected.
- When the THI exceeds 78, cows milk production is seriously affected.
- When the THI rises above 82, very significant losses in milk production are likely, cows show signs of severe stress and may ultimately die.
- 2 hours/day more a cow will spend standing during heat stress when the THI is greater than 70.
- 4 lbs of milk/day that will be lost due to increased standing time.
- 7-25% increase in the maintenance energy requirement of cows during heat stress.
- 80 THI: Where moderate to severe heat stress begins
- 8-12%: decrease in dry matter intake (DMI)
- 10-25%: decrease in milk production
- 35-50%: of the decline in milk yield that is explained by decreased DMI
- 1,000 - 2,000 lbs: reduced milk yield in the lactation after dry cows experience heat stress.
Evaporative cooling systems, like the CowKühlerZ are the most effective means of controlling moderate to severe heat stress in dairy cattle. Timed mist over the stalls or bedding pack with high-velocity air movement is the most effective method to cool cows. The timed, fine mist allows the water to evaporate, which in combination with the variable speed fans creates evaporative cooling a "chilling effect" which allows cows to dissipate more heat and maintain optimal core body temperature. At higher temperatures, fans alone are not adequate to allow the cow to cool down.
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