The THI index, or Temperature Humidity Index, is a measure that indicates the severity of heat stress on 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 are particularly 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 producer, 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 in percent. The result is a number that reflects the combined effect of heat and humidity on the animal.
What this calculation makes clear is that humidity is at least as important as the temperature itself. A day with a temperature of 28 degrees and 80% humidity results in a higher THI than a day with a temperature of 32 degrees and 30% humidity. Livestock can lose heat through evaporation, but this mechanism is much less effective when humidity is high. The THI value takes this into account.
In practice, there are handy online calculators and apps that allow you to quickly calculate the THI based on the current weather conditions at your farm.
What 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: when the THI exceeds 72, milk production drops noticeably. A THI above 80 indicates severe heat stress with a risk of health problems. Pigs and poultry have similar thresholds, although sensitivity varies by animal species and production type.
As a guideline, the following zones apply to dairy cattle:
- THI below 68: No heat stress; animals are functioning 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 action required
THI Thresholds for Heat Stress in Livestock by Species at a Glance
| THI Zone | Dairy Cattle | Pigs | Poultry |
|---|---|---|---|
| Under 68 | No stress, functioning normally | No stress, functioning normally | No stress, functioning normally |
| 68 to 72 | Mild stress; monitor closely | Mild stress; keep a close eye on sows | Mild stress visible; monitor production |
| 72 to 80 | Decline in production is evident; take action | Growth and feed conversion are deteriorating | Noticeable loss in production; take immediate action |
| 80 to 90 | Severe stress, immediate action required | Severe stress, reproduction affected | Severe stress, high mortality possible |
| Above 90 | Life-threatening situation, activate emergency protocol | Life-threatening situation, activate emergency protocol | Life-threatening situation, activate emergency protocol |
For pigs, the critical threshold is slightly lower than for cows. Pregnant or lactating sows are particularly vulnerable at higher THI values. In poultry—and especially in broilers and laying hens—production losses can occur even at a THI above 70.
What are the effects of heat stress on production and health?
Heat stress in livestock leads to a wide range of production and health problems. Animals reduce their feed intake, expend more energy to regulate their body temperature, and produce less. In dairy cows, milk production declines; in pigs, growth and carcass quality deteriorate; and in poultry, egg production and breast muscle weight decrease.
The economic consequences of heat stress are tangible and can quickly mount up. International research shows that when the THI exceeds 72, dairy cows produce, on average, 0.2 to 0.4 liters less milk per THI point above the threshold value. For a herd of 100 dairy cows, an average loss of 1 to 2 liters per cow per day during a five-day heat wave quickly adds up to a production loss of hundreds of liters. Add to that the longer recovery time—since milk production in dairy cattle does not fully recover until several weeks have passed—and the financial damage quickly becomes apparent. In market hogs, heat stress leads to a measurable decrease in daily weight gain, while laying hens lay noticeably fewer eggs per heat period. Exact figures per farm depend on breed, stage of production, and the duration of the heat stress period, but the order of magnitude makes it clear that early intervention is always more cost-effective than recovery afterward.
The consequences extend beyond mere production figures. At the cellular level, heat stress causes oxidative stress and disrupts the osmotic balance in cells and the intestines. This has direct consequences for intestinal integrity: the tight junctions that seal the intestinal wall are compromised, allowing harmful substances to penetrate the intestinal 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 season 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 around the time of calving.
How can 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 signs include an increased respiratory rate, heavy panting, reduced feed intake, and animals drinking more water but moving less. Cows stand more often to improve heat dissipation through the udder and avoid their resting area.
Other recognizable signs include:
- Animals seek out the shade or cluster around ventilation openings
- Milk production is visibly declining without any other apparent cause
- Feed remains in the trough longer, or feed intake shifts to cooler hours
- More cases of loose or watery stool due to impaired bowel function
- Reduced mobility and less draftiness in sows and cows
- In poultry: outstretched wings, open-mouth breathing, and reduced egg production
It is important to realize that animals are already experiencing internal heat stress before the external signs become clearly visible. Regular monitoring of the THI on your farm, combined with attention to behavioral changes, provides you with an early warning signal so you can take timely action.
What measures can be taken to reduce heat stress when THI values are high?
When THI values are high, there are structural, management, and nutritional measures that can reduce heat stress in livestock. The most effective approach combines multiple strategies simultaneously, because no single measure is sufficient on its own to fully mitigate the impact of extreme heat.
Environmental and Management Measures
Ventilation is the top priority. Adequate air movement through fans or natural ventilation significantly lowers the perceived temperature for animals. Cooling with misters or shower systems can be a helpful supplement, but be aware: in high humidity, misters will further increase the THI if air circulation is insufficient. Also ensure there is sufficient shade, plenty of drinking water, and adjust feeding times to the cooler hours of the day.
Nutritional Support for Heat Stress
In addition to 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 osmolyt, it protects cells from dehydration and stress, supports intestinal integrity, and helps maintain performance under hot conditions. In addition, drinking water additives can contribute to better hydration and electrolyte balance, especially at times when feed intake decreases but water intake remains consistent.
When should you start taking steps to prevent heat stress?
Heat stress prevention should begin before the THI thresholds are reached, not only after the animals are already showing visible symptoms. As soon as weather forecasts indicate that the temperature may rise above 25 degrees Celsius combined with relative humidity above 60%, it’s time to implement measures. In practice, this means that in Belgium and the Netherlands, you need to be on alert as early as April.
The damage caused by heat stress builds up quickly. In dairy cattle, it takes several weeks for milk production to fully recover after a period of heat stress. In sows, the effects of heat stress during gestation can still be noticeable in litter performance weeks later. Early intervention is therefore always more cost-effective than recovery efforts afterward.
A good prevention plan should include at least the following elements: regular THI monitoring at the farm level, pre-set thresholds that trigger the implementation of heat stress protocols, and a nutritional strategy that you begin several days before a heat wave so that the animals are optimally prepared.
Frequently Asked Questions About the THI Index
What is the THI index?
The THI index, or Temperature Humidity Index, is a measure that reflects the combined effect of temperature and humidity on livestock. The value is calculated using the formula THI = (1.8 x T + 32) – [(0.55 – 0.0055 × RH) × (1.8 × T – 26)], where a higher result indicates a greater risk of heat stress in livestock.
What is a dangerous THI value for dairy cattle?
Dairy cattle begin to experience heat stress at a THI value of 68. When the THI exceeds 72, milk production drops noticeably, and above 80, severe heat stress sets in, posing clear health risks. Above a THI of 90, the situation is life-threatening and requires immediate intervention.
What are the signs of heat stress in livestock?
Common signs of heat stress in livestock include an increased respiratory rate, heavy panting, reduced feed intake, and a noticeable drop in milk production with no other apparent cause. In poultry, spread wings and open-mouth breathing are recognizable signs, while pigs and cows move less and seek out cooler spots in the barn.
As a livestock producer, at what THI level should I take action?
The recommendation is to implement measures before the THI thresholds are reached. As soon as the expected temperature exceeds 25 degrees Celsius combined with humidity above 60%, it’s time to activate your heat stress protocol. This ensures your animals are optimally prepared when the THI value actually rises.
How Jodoco Helps Prevent Heat Stress
At Jodoco, we understand that heat stress is one of the most underestimated production risks in modern livestock farming. Our products are specifically designed to protect animals at the cellular level against the effects of heat stress, with natural betaine as the key active ingredient.
What we offer for heat stress prevention:
- Jodobet: Natural betaine, acting as an osmolyt and methyl donor, supports cellular hydration and intestinal integrity when THI levels are high
- Jodoplume: A combination of betaine and organic acids for administration via drinking water, ideal for treating heat stress or coccidiosis in poultry
- Solvax: Drinking water solution to support electrolyte balance and recovery from heat stress
- Scientifically based dosages, tailored to the animal 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 business situation or how you can proactively address heat stress this season? Contact us, and we’d be happy to help you find a solution.
