Almost every heat stress plan we are shown is a ventilation plan. Fans, inlets, pad cooling, a controller setting somebody changed in June. All of it matters, and none of it explains why two houses running the same air can lose very different amounts of shell quality in the same week. The difference is usually inside the bird: what it was fed, when it was fed, and what its blood chemistry was doing while it panted.
Key takeaways
- Under constant heat, feed intake falls by around 3.5 % per degree Celsius; under cyclic heat, by around 1.5 % per degree.
- Water demand roughly doubles relative to feed: 4 to 5 mL per gram of feed at 32 °C against 2 to 3 mL in comfort conditions.
- Every 10 % rise in relative humidity lowers the effective comfort zone by one to two degrees.
- Electrolyte and vitamin interventions have published dose ranges, and at least one of them, selenium, has a legal ceiling that some published ranges exceed.
What panting actually costs
Birds have no sweat glands. Above the thermoneutral zone, generally described as roughly 16 to 25 °C for chickens, heat is shed by evaporation from the respiratory tract, which means rapid open-mouth breathing and the gular flutter.
That mechanism works, and it has a metabolic bill. Hyperventilation removes carbon dioxide faster than it is produced, and the resulting respiratory alkalosis raises blood pH. The kidney compensates by excreting bicarbonate, and bicarbonate is precisely what the shell gland draws on to deposit calcium carbonate. A layer flock in a heatwave therefore loses shell thickness through two independent routes at once: less calcium eaten, because intake has fallen, and less carbonate available to use what was eaten.
Potassium follows the same path out of the bird. This is the reason electrolyte support is a physiological correction rather than a general tonic, and the reason plain cool water, while necessary, does not restore what panting has removed.
The numbers worth planning against
Heat stress is one of the better quantified problems in poultry production, which makes it possible to plan rather than react.
| Condition | Reported effect |
|---|---|
| Cyclic heat stress | Feed intake down about 1.5 % per °C above comfort |
| Constant heat stress | Feed intake down about 3.5 % per °C |
| 34 °C exposure | 8 to 9 % lower feed intake; up to 17 % lower body weight in a documented 6 h per day regime from day 22 to 35 |
| Severe heat by temperature-humidity index (THI 84 against 67) | Feed intake down 30 %, body weight gain down 51 % |
| 32 °C | Water intake at 4 to 5 mL per gram of feed, against 2 to 3 mL under comfort conditions |
The humidity relationship is the one most often left out of a farm plan. Because evaporative cooling depends on the air’s capacity to accept moisture, a rise of 10 % in relative humidity effectively removes one to two degrees of tolerance. Thirty degrees at 40 % humidity and thirty degrees at 80 % are not the same event, and a pad cooling system is closer to useless in the second case. The housing and airflow side of that trade-off is covered in our guidance on running a house through both thermal extremes; what follows here is the part that happens in the feed bin and the water line.
Feeding for heat, not despite it
The first correction is compositional. Protein carries the highest heat increment of the three energy-yielding fractions, so a ration formulated with a comfortable margin of crude protein makes the bird warmer while it digests. Reformulating on digestible amino acids rather than crude protein, and shifting part of the energy supply toward fat, lowers metabolic heat production without lowering nutrient supply.
The second correction is chronological. Published guidance is to feed roughly 4 to 6 hours before the temperature peak, which in practice means early morning and late evening or night feeding. The bird then digests during the coolest hours and is not adding metabolic heat at the exact moment the house is hardest to cool.
Supplementation: the published ranges, and one legal ceiling
Reported dose ranges from the nutritional literature on heat stress in broilers include sodium bicarbonate at 4 to 10 kg per tonne of feed, potassium chloride at 0.2 to 0.5 % in drinking water, vitamin C at 250 mg per kg of feed or 1 g per litre of drinking water, vitamin E at 100 to 250 mg per kg, zinc at 30 to 40 mg per kg and selenium at 0.3 to 1 mg per kg. Combined vitamin E and C is repeatedly described as more effective than either alone.
One of those figures needs a correction before anyone formulates from it. In the European Union, the maximum authorised content of total selenium is 0.5 mg per kg of complete feed, with organic selenium supplementation limited to 0.2 mg per kg within that total. The upper end of the published research range sits above the legal ceiling. Feed additives are authorised under Regulation (EC) No 1831/2003 with defined maximum levels, and a trial dose is not an authorisation.
The same caution applies more generally: these are ranges from published work, not a prescription for your flock. They belong in a discussion with your nutritionist and, where a health claim or a medicated product is involved, with your veterinarian.
A working sequence around a forecast
- Two to three days before. Flush and check the water lines, verify flow rate at the far end of every line, confirm standby generator and alarm function, and agree the feeding times that will apply. Water line capacity is the failure that hurts most and is the easiest to test in advance.
- The evening before. Move the main feed distribution to the cool hours. Bring forward any handling, catching or loading that was planned for the following afternoon.
- During the peak. No handling, no vaccination, no loading, no unnecessary entry into the house. Keep drinking water cool and available, and watch distribution: birds crowding at the inlets are telling you the air is not reaching the rest of the house.
- Recovery. Intake returns before body weight does. Expect several days of catch-up feeding, and record what happened, because the same houses tend to fail in the same way each summer.
Frequently asked questions
At what temperature does heat stress begin?
There is no single threshold, because humidity and air speed change the effective load on the bird. The thermoneutral zone for chickens is generally given as roughly 16 to 25 °C, and each 10 % increase in relative humidity effectively removes one to two degrees of that margin. Panting is the practical signal that the margin has gone.
Does cutting protein help during a heatwave?
Reducing surplus crude protein while maintaining digestible amino acid supply does help, because it lowers metabolic heat production. Cutting protein without balancing amino acids simply underfeeds the bird at the moment it is least able to compensate.
Is cold drinking water useful or risky?
Cool water is helpful: it raises intake and takes heat with it. What matters more than temperature is availability, because water demand roughly doubles relative to feed at 32 °C and a line sized for comfort conditions can become the limiting factor before any bird shows a clinical sign.
Why does shell quality drop even when mortality does not?
Because respiratory alkalosis reduces the bicarbonate available for shell carbonate, independently of how much calcium the hen ate. Shell quality is therefore an early and sensitive indicator of heat load, well before any mortality signal appears.
Everything above depends on the water line holding up
Intake doubles, the line is unchanged, and the constraint moves from the fans to the plumbing. Water is the part of a heat plan that is cheapest to check and most expensive to discover mid-heatwave.
Drinking water in poultry farming, beyond the supply question
Sources: Nambapana and Jayasena, Heat Stress Management via Nutritional Strategies for Broilers, IntechOpen, 2024, for intake, body weight, water and supplementation ranges; published work on the temperature-humidity index and broiler performance; Regulation (EC) No 1831/2003 on additives for use in animal nutrition and the authorised maximum of 0.5 mg total selenium per kg of complete feed. Consulted August 2026.
Published previously, fully revised on 9 August 2026. This article is general technical guidance for poultry professionals. Ration changes and supplementation must be validated by your own nutritionist within the authorisations in force, and any health intervention remains the responsibility of the veterinarian in charge of your flock. Regulatory references apply to the European Union; verify the rules of your own market.