The THI index, or Temperature Humidity Index, is a measure that indicates how severe the heat load is for livestock based on the combination of temperature and humidity. The higher the THI value, the greater the risk of heat stress. Dairy cows, pigs, and poultry in particular are sensitive to high THI values, even at temperatures that still feel comfortable to humans. In this article, we answer the most important questions about the THI index and what you as a livestock farmer can do.
How is the THI index calculated?
The THI index is calculated based on air temperature and relative humidity. The most commonly used formula is: THI = (1.8 × T + 32) − [(0.55 − 0.0055 × RH) × (1.8 × T − 26)], where T is the temperature in degrees Celsius and RH is the relative humidity as a percentage. The result is a number that reflects the combined effect of heat and moisture on the animal.
What this calculation makes clear is that humidity is at least as important as temperature itself. A day of 28°C with 80% humidity produces a higher THI than a day of 32°C with 30% humidity. Livestock can dissipate heat through evaporation, but at high humidity levels that mechanism becomes far less effective. The THI value accounts for this.
For practical use, there are convenient online calculators and apps that allow you to quickly calculate the THI based on current weather conditions at your farm.
Which THI values are dangerous for livestock?
Livestock begin to experience heat stress at a THI of 68 or higher. Dairy cows are the most sensitive: at a THI above 72, milk production drops noticeably. At a THI above 80, severe heat stress occurs with a risk of health problems. Pigs and poultry have comparable threshold values, although sensitivity varies by species and production type.
The following zones serve as a guideline for dairy cattle:
- THI below 68: No heat stress, animals function normally
- THI 68 to 72: Mild heat stress, first signs of reduced comfort
- THI 72 to 80: Moderate heat stress, visible decline in production and feed intake
- THI 80 to 90: Severe heat stress, health risk increases significantly
- THI above 90: Life-threatening heat stress, immediate intervention required
For pigs, the critical threshold is slightly lower than for cattle. Sows in gestation or lactation are particularly vulnerable at higher THI values. In poultry — and especially in broilers and laying hens — production losses can occur at a THI as low as 70.
What are the consequences of heat stress for production and health?
Heat stress in livestock leads to a wide range of production and health problems. Animals reduce their feed intake, use more energy to regulate their body temperature, and produce less. In dairy cows, milk production drops; in pigs, growth and carcass quality deteriorate; and in poultry, egg production and breast muscle weight decline.
The consequences extend beyond production figures alone. At the cellular level, heat stress causes oxidative stress and disrupts the osmotic balance in cells and the intestines. This has a direct impact on intestinal integrity: the tight junctions that seal the intestinal wall are compromised, allowing harmful substances to pass through the gut wall more easily. This increases the risk of infections and inflammation.
In sows, heat stress has a particularly significant impact on reproduction. High temperatures during the breeding period or late gestation can reduce the number of live-born piglets and lower birth weights. In ruminants, rumen function deteriorates, dry matter intake decreases, and the risk of ketosis increases — particularly in the period around calving.
How do you recognize heat stress in livestock?
Heat stress in livestock can be recognized by a combination of behavioral changes and physical signs. The most noticeable indicators are an increased respiratory rate, heavy panting, reduced feed intake, and animals drinking more water while moving less. Cows stand more frequently to improve heat dissipation through the udder and avoid lying down.
Other recognizable signs include:
- Animals seeking shade or clustering near ventilation openings
- Milk production drops visibly without any other obvious cause
- Feed remains in the trough longer, or feed intake shifts to the cooler hours of the day
- More cases of loose or wet manure due to disrupted intestinal function
- Reduced activity and decreased estrus expression in sows and cows
- In poultry: spread wings, open-beak breathing, and reduced egg production
It is important to recognize that animals already experience heat stress internally before external signs become clearly visible. Regular monitoring of the THI on your farm, combined with close attention to behavioral changes, gives you an early warning to intervene in time.
What measures reduce heat stress at high THI values?
At high THI values, there are structural, management-related, and nutritional measures that can reduce heat stress in livestock. The most effective approach combines multiple strategies simultaneously, as no single measure is sufficient on its own to fully offset the impact of extreme heat.
Environmental and management measures
Ventilation is the first priority. Adequate air movement through fans or natural ventilation significantly lowers the perceived temperature for animals. Cooling with misting systems or shower installations can provide additional relief, but be aware: at high humidity levels, misters will raise the THI further if air movement is insufficient. Also ensure adequate shade and ample drinking water, and adjust feeding times to the cooler hours of the day.
Nutritional support during heat stress
Alongside environmental measures, nutrition plays a crucial role. Animals experiencing heat stress have an increased need for osmotic support at the cellular level. Natural betaine is a scientifically supported choice in this regard: as an osmolyte, it protects cells against dehydration and stress, supports intestinal integrity, and helps maintain technical performance under warm conditions. In addition, drinking water additives can contribute to better hydration and electrolyte balance — especially at times when feed intake drops but water intake remains reliable.
When should you start heat stress prevention?
Heat stress prevention should begin before THI threshold values are reached, not only once animals are already showing visible symptoms. As soon as weather forecasts indicate that temperatures could exceed 25 degrees Celsius combined with a relative humidity above 60%, it is time to activate preventive measures. In practice, this means that in the Belgian and Dutch context, you should already be alert from April onward.
The damage caused by heat stress accumulates quickly. In dairy cattle, it can take several weeks for production to fully recover after a period of heat stress. In sows, the effects of heat stress during gestation can still be felt weeks later in litter results. Early intervention is therefore always more cost-effective than recovery after the fact.
A solid prevention plan should include at least the following elements: regular THI monitoring at the farm level, pre-defined threshold values at which you switch to heat stress protocols, and a nutritional strategy that you initiate a few days before a heat period so that animals are optimally prepared.
How Jodoco helps with heat stress prevention
At Jodoco, we understand that heat stress is one of the most underestimated production risks in modern livestock farming. Our products are specifically developed to protect animals at the cellular level against the effects of heat stress, with natural betaine as the central active ingredient.
What we offer for heat stress prevention:
- Jodobet: Natural betaine as an osmolyte and methyl donor, supports cellular hydration and intestinal integrity at high THI values
- Jodoplume: Combination of betaine and organic acids for administration via drinking water, ideal for heat stress or coccidiosis stress in poultry
- Solvax: Drinking water solution to support electrolyte balance and recovery during heat stress
- Scientifically supported dosages, tailored to species and stress level
- Technical support and customized advice for livestock farmers, nutritionists, and distributors
Would you like to know which approach best suits your farm’s situation, or how to proactively manage heat stress this season? Contact us and we’ll be happy to think it through with you.
