What is the Temperature Humidity Index (THI)? A guide for livestock & agriculture

The 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.

How to calculate the Temperature Humidity Index

However, most studies focus on the two main environmental stress factors: temperature and relative humidity.

There are several formulas used to calculate the THI, but one of the simplest and most commonly used is the following:

THI =0.8*T + RH*(T-14.4) + 46.4

Where:

• T is the ambient temperature in degrees Celsius
• RH is the relative humidity expressed as a decimal proportion (e.g., 75% RH becomes 0.75)

This formula helps to understand the principle behind the index, namely that the concentration of water vapor in the air can significantly reduce the animal’s ability to dissipate heat through evaporation from the skin and lungs.

Cattle can tolerate much higher temperatures at lower relative humidity because they are able to dissipate excessive heat more effectively by sweating.

However, during hot and especially humid conditions, the natural ability of cattle to dissipate heat is compromised due to the lowered ability to utilize evaporative cooling.

In everyday practice, THI is often calculated using ready-to-use tables or digital tools, which allow for an immediate assessment of risk conditions.

Interpreting the THI chart for livestock

To accurately assess the level of heat stress in livestock farms, it is essential to correctly interpret the THI table. This tool makes it possible to understand the animals’ condition and take timely action to protect them.

The main threshold levels are as follows:

• Comfort Zone (THI < 68).
  Animals are in an ideal condition and show no signs of heat stress.
• Mild stress (THI 68-71).
  The first symptoms appear, such as a slight reduction in feed intake or activity.
• Moderate Stress (THI 72-79).
  Stress becomes evident: growth and milk production decrease, while respiratory rate increases.
• Severe Stress (THI 80-89).
  Animal show severe symptoms such as laboured breathing, excessive salivation, and drastic drops in productivity.
• Danger Zone (THI > 90).
  The risk of mortality is real: immediate intervention with effective cooling measures is required.

To address these critical conditions, many farms adopt a ventilation system for cattle farms, designed to improve animal welfare even during the hottest days.

Why THI is crucial in agriculture and greenhouses

Although THI was originally developed to assess heat stress in animals, the underlying principle-namely the combination of temperature (T) and relative humidity (RH)-is equally important for agricultural crop quality.

Monitoring and managing these two parameters help prevent damage, optimize yields, and improve crop quality.

Here are some practical applications in different contexts:

• Greenhouses.
  When humidity is too high or too low in relation to temperature, plants can suffer. Excess humidity promotes fungal diseases, while too little humidity can inhibit transpiration and slow growth.
• Mushroom farms.
  Mushroom cultivation requires extremely precise control of temperature and humidity. Even small variations can compromise the production cycle.
• Cannabis cultivations.
  In this sector, climate control is essential: properly managing T and RH makes it possible to maximize both yield and final product quality.

Beyond the index: key factors in heat stress management

THI is a useful indicator for assessing heat stress, but it is a static tool. It represents the risk at a specific moment in time, without accounting for other factors that can either worsen of mitigate the effects of heat on animals.

For effective heat stress management, it is also important to consider:

• Duration of the heat wave.
  Consecutive days with high THI values have a cumulative effect on animal welfare, especially in more sensitive breeds such as Holstein Friesian cattle.
• Ventilation and air movement.
  Proper airflow can reduce the negative effects of a high THI by improving body heat dissipation through evaporation.
• Direct solar radiation.
  THI is calculated in shaded conditions and therefore does not account for direct sunlight. Exposure to solar radiation can increase the perceived temperature and intensify heat stress, even if the index value remains unchanged.

In addition, it is important to interpret THI values in relation to the type of climate (dry, humid, subtropical) and local microclimatic conditions, which are increasingly affected by climate change.

In the coming years, commonly used threshold levels may need to be updated to reflect new environmental standards.

Frequently asked questions

What is a comfortable THI level for dairy cattle?

A THI value below 68 is considered optimal for dairy cow welfare. Higher values can lead to heat stress, with negative effects on both health and productivity.

Does the THI apply to poultry as well?

Yes, THI can also be used for poultry, but with different thresholds and interpretations compared to cattle, depending on the physiology and thermal sensitivity of avian species.

How does ventilation help manage a high THI?

Ventilation promotes the evaporation of body heat and enhances natural cooling, reducing the negative effects of a high THI.

What is the difference between the Heat Index and THI?

The Heat Index refers to how hot conditions feel for humans, while THI is used to assess heat stress in animals, taking into account their physiological responses.