Heat stress in livestock occurs when ambient temperature and humidity together rise so high that an animal can no longer properly regulate its body temperature. For dairy cows, that threshold is reached at a THI value of 68, which corresponds to approximately 22°C at a relative humidity of 50%. Pigs and poultry are even more sensitive. In this article, we answer the most frequently asked questions about heat stress in livestock: from the determining factors and the THI index to concrete symptoms and preventive measures.
What factors determine whether livestock experience heat stress?
Whether an animal experiences heat stress depends not only on temperature, but on a combination of environmental and animal-related factors. The two most important environmental factors are temperature and humidity. Air movement, radiation, barn design, and stocking density also play a role. At the animal level, production level, body weight, coat or plumage, and genetic predisposition all contribute.
High-producing animals, such as dairy cows at peak lactation, generate a great deal of internal heat through their intensive metabolism. They reach a critical body temperature more quickly than low-producing animals. Heavy beef cattle and animals with a dense coat are also more vulnerable. In summary, the following factors together determine whether heat stress occurs:
- Temperature and humidity in combination (see THI index)
- Air movement and ventilation in the barn
- Direct solar radiation or heat radiation from barn walls
- Production level and metabolic heat production of the animal
- Body weight and body surface area
- Genetic predisposition for heat tolerance
- Stocking density in the barn
What is the THI index and how does it work?
The THI index, or Temperature Humidity Index, is a number that expresses the combined load of temperature and humidity as a measure of heat stress in livestock. The index combines the dry-bulb temperature and relative humidity into a single value, allowing farmers and advisors to quickly assess how severe the conditions are for an animal.
The formula most commonly used for cattle 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 dimensionless number that is classified into risk categories.
A THI below 68 is considered no stress for dairy cattle. Between 68 and 72, mild stress is present; between 72 and 80, moderate stress; and above 80, severe stress. At a THI above 90, the situation becomes life-threatening. Importantly, a warm day of 28°C with 80% humidity produces a higher THI than 32°C in dry air. Humidity is therefore at least as decisive as temperature itself.
At what temperature does cattle begin to suffer from heat stress?
Cattle begin to experience heat stress at a THI of 68, which at average humidity corresponds to a temperature of approximately 22 to 24°C. High-producing dairy cows are even more sensitive and may show a decline in production at a THI as low as 65. The thermoneutral zone for cattle lies roughly between 5°C and 20°C.
Above that threshold, the animal must actively dissipate heat. It does so through increased respiration, sweating, and reduced feed intake. Dairy cows reduce their dry matter intake even under mild heat stress, which directly leads to lower milk production, poorer fertility, and a deteriorating energy balance. Beef cattle and dry cows have a slightly higher tolerance, but they too experience negative effects above a THI of 72.
Suckler cows and heavily built beef bulls are particularly vulnerable because their ratio of body volume to surface area is unfavorable for heat dissipation. Ruminants also generate additional heat through rumen fermentation, which increases the internal heat load.
How do pigs and poultry respond to high temperatures?
Pigs and poultry are more sensitive to heat than cattle because they have no sweat glands and their capacity for heat dissipation is severely limited. Pigs begin to experience heat stress at around 25°C, depending on their weight and production stage. Poultry are already vulnerable above 28°C, with fast-growing broilers being the highest-risk group.
Heat stress in pigs
Pigs cool themselves exclusively by panting and through contact with a cool surface. Heavy finishing pigs and lactating sows are the most vulnerable. At temperatures above 25°C, feed intake declines, growth rate decreases, and feed conversion worsens. In sows, heat stress during summer leads to reduced fertility, more returns to estrus, and smaller litters. Piglets weaned during heat stress require extra support because their gut barrier is already under pressure from weaning stress itself.
Heat stress in poultry
Broilers and turkeys respond to high temperatures with increased respiration, reduced feed intake, and lower growth rates. Breast muscle yield and carcass quality decline noticeably. Laying hens experiencing heat show reduced egg production, thinner eggshells, and poorer feed conversion. Because poultry cool themselves exclusively through panting and heat dissipation via the legs, good barn ventilation is the first line of defense.
What symptoms indicate heat stress in livestock?
The most recognizable symptoms of heat stress in livestock are increased respiratory rate, reduced feed intake, lethargy, and elevated body temperature. In severe heat stress, animals may stand rather than lie down, cluster around water points, and show signs of dehydration.
The signs vary slightly by species, but the following symptoms are widely recognized as indicators of heat stress:
- Increased respiratory rate and visible panting
- Reduced feed intake and declining production
- Higher water consumption combined with signs of dehydration
- Lethargy and reduced activity
- Elevated body temperature (rectal measurement above 39.5°C in cattle)
- Diarrhea or changes in manure consistency as a result of osmotic disruption
- Reduced fertility in sows and cows
- Increased mortality in poultry in extreme cases
In cattle, a respiratory rate above 60 breaths per minute is a clear warning sign. In broilers, widespread open-beak breathing and wing-spreading indicate that immediate action is required.
How can heat stress in livestock be prevented or reduced?
Preventing heat stress in livestock requires a combination of barn management, nutritional adjustments, and targeted use of feed additives. The most effective approach combines environmental cooling with support for the animal’s own physiological stress response.
In terms of barn management, the following measures have the greatest impact:
- Increase ventilation speed and ensure directed airflow along the animals
- Reduce stocking density during hot periods
- Provide sufficient drinking water stations with fresh, cool water
- Provide shade in outdoor or exercise areas
- Adjust feeding times: feed during the cooler parts of the day
At the nutritional level, it is important to increase the energy density of the ration without increasing the heat produced by the feed. Rations higher in fat and lower in crude fiber generate less metabolic heat. Adequate electrolytes, particularly potassium and sodium, support fluid balance. Drinking water and feed additives play a concrete role here, as water intake during stress is more reliable than feed intake and the dosage remains fully controllable for the farmer.
How Jodoco helps with heat stress in livestock
Heat stress is a physiological problem that requires a solution at the cellular level. At Jodoco, we developed a product portfolio that addresses exactly that need, with natural betaine at the core of our approach.
Betaine acts as an osmolyte: it protects cells against moisture loss and osmotic stress by stabilizing intracellular water balance. That is precisely the mechanism that comes under pressure during heat stress. Scientific research confirms that betaine improves breast muscle weight under heat stress through enhanced cellular hydration and antioxidant protection. In sows, betaine improves reproductive performance during warm summers. In ruminants, it supports osmotic balance and rumen fermentation.
What we concretely offer for heat stress in livestock:
- Jodobet: natural betaine as a proven osmoregulator for feed, improves heat tolerance in broilers, pigs, and ruminants
- Jodoplume: betaine combined with organic acids via drinking water, ideal for heat or coccidiosis stress in poultry
- Grovax: organic acids, butyrate, and essential oils that support gut health under stress conditions
- Custom formulations for compound feed manufacturers and distributors looking to integrate heat stress management into their product range
Want to know which solution best fits your operation or product line? Contact our team for a tailored conversation.
