Poultry farming is a sector where genetic progress gives the fastest results and feed conversion ratio (FCR) is calculated to the millimetre. But high metabolic rate and rapid growth make broilers and laying hens very sensitive to environmental stress. At the top of the list is heat stress.
Chickens have no sweat glands, and their bodies are covered by a thick layer of feathers. When house temperature rises above the comfort zone (about 20-24 °C), birds start breathing fast (panting) to lose body heat. This mechanism protects them at first, but within a few hours it can lead to respiratory alkalosis, a serious drop in feed intake and heavy economic losses.
Why Respiratory Alkalosis Develops Under Heat Stress
As temperature and relative humidity rise, chickens can increase their breathing rate from about 25 to over 200 breaths per minute. This excessive ventilation removes large amounts of carbon dioxide (CO2) from the blood. Carbonic acid in the blood falls and blood pH shifts to the alkaline side. This is called respiratory alkalosis, and its consequences follow one another in a chain:
- Bicarbonate depletion: To balance blood pH, the kidneys excrete bicarbonate, along with potassium and sodium, in the urine. Free bicarbonate in the blood falls.
- Thinner eggshells: The main component of the shell is calcium carbonate. Even when calcium is sufficient, a lack of carbonate (bicarbonate) disrupts shell formation. Shells become thinner, and the share of cracked or broken eggs can rise from around 2% to 8-12%.
- Increased gut permeability: Blood flow is redirected from the internal organs to the comb and legs to shed heat. The gut lining does not get enough oxygen, the junctions between gut cells loosen and toxins enter the blood. This can cause systemic inflammation and sudden deaths.
How House Temperature Affects Performance
As house temperature rises, performance changes as follows:
- 20-22 °C (comfort zone): Feed intake is normal. FCR is optimal at 1.45-1.55 and eggshell strength at 38-42 N.
- 25-28 °C (mild stress): Feed intake falls 4-8%, FCR worsens by about 0.05 points and the shell starts to thin slightly.
- 29-32 °C (moderate to severe stress, panting): Feed intake falls 15-22%, FCR worsens by about 0.15 points and shell strength drops by about 18%.
- Above 33 °C (critical): Feed intake can fall by as much as 30-45%, and mortality and the share of broken eggs rise markedly.
These are general averages and vary with humidity, breed and age.
Nutritional Strategies Against Heat Stress
However strong the fan-pad and tunnel ventilation systems are, it is hard to keep a flock stable in very hot periods without nutritional measures. The basic strategies that work in the field are:
- Reduce metabolic heat production: Excess crude protein and carbohydrate release heat when digested. In summer rations, supporting energy with vegetable oils and lowering crude protein by 1-2 points with synthetic amino acids reduces the body's heat load.
- Keep dietary electrolyte balance (dEB): The Na + K - Cl balance in the ration should be kept at about 250 mEq/kg. Sodium bicarbonate and potassium chloride supplements help correct alkalosis.
- Betaine and antioxidant support: Natural betaine helps retain water in gut cells and reduces cellular dehydration. Vitamins C and E help limit the effect of the stress hormone corticosterone.
Frequently Asked Questions
Does changing feeding times help on hot days?
Yes. Giving feed in the coolest hours of the day (04:00-07:00 in the morning and 20:00-23:00 in the evening) keeps the metabolic heat released during digestion from coinciding with the hottest hours of the day.
Why does drinking water temperature matter for chickens?
Chickens avoid water above 25 °C. Insulating the water line and keeping water at about 18-20 °C helps lower body temperature and raises water intake.
Does CoolVital Poultry block water lines?
CoolVital® Poultry is a water-soluble micro-granule. It is designed not to leave sediment in nipple drinkers, dosing pumps and water filters. Even so, we recommend checking the dosing pump and filters at first use.
How is heat stress recognised in a poultry flock?
Panting with an open beak, spreading the wings away from the body, falling feed intake, rising water intake, thin and cracked eggshells and sudden deaths are the main signs.
