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Every broiler farmer loses an occasional bird. The important question is not only, “What percentage mortality is acceptable?” It is also: Which birds are dying, at what age, how quickly, and why?
A flock that loses one bird here and there over several weeks is very different from a flock that loses ten birds in one hot afternoon. A sudden change in the daily pattern can be more important than the final percentage.
If mortality rises suddenly, many birds become weak at the same time, or you see severe respiratory signs, nervous signs, swollen heads, blue combs, bloody diarrhoea or unexplained rapid deaths, restrict unnecessary movement and get veterinary or diagnostic help quickly.
How to calculate broiler mortality
Mortality percentage = number of birds that died ÷ number of birds placed × 100.
For example, if you placed 1,000 chicks and 30 died during the production cycle:
30 ÷ 1,000 × 100 = 3% mortality.
Also record culls separately. A bird you deliberately remove because of severe leg trouble or another welfare problem is not the same as a bird found dead, even though both reduce the number of saleable birds.
The figure that often gives the earliest warning is daily mortality. Cumulative mortality can still look small while today’s deaths are suddenly climbing.
There is no blind “acceptable mortality percentage”
Farmers often want one number that says everything below it is safe and everything above it is bad. Broiler mortality does not work like that.
Age matters. Cause matters. The speed of increase matters. Chick quality, breed or strain, weather, final weight, housing, ventilation, feed, water and disease pressure all affect the result.
A breeder performance objective can give you a useful benchmark for a particular strain under good management, but your own flock record is still important. If a flock normally loses very few birds and suddenly starts losing several per day, that change deserves investigation even before the final percentage looks high.
Broiler mortality during the brooding stage
Everything we discussed in our Early Chick Mortality guide applies directly to broiler chicks.
During brooding, common problems include chilling, overheating, poor chick quality, dehydration, starvation, poor access to feed and water, overcrowding, brooder equipment problems and early infections. At this age, small management mistakes can kill birds quickly because chicks have little reserve.
Do not only look at the thermometer. Watch chick behaviour. Chicks crowding under heat, moving far away from the brooder, piling in corners, crying continuously, failing to spread evenly, or not finding feed and water are telling you something is wrong.
Water can kill a broiler flock faster than many farmers expect
I learnt this the painful way with a small backyard batch of about twenty broilers. The birds were being raised for the Easter period. It was during the hot dry season, and I was away from home.
Someone called to say the birds were panting. My wife had already given them water earlier, but they had finished it. The pen was locked, so the person who noticed the problem could not refill the drinkers. By the time my wife drove home, all the birds were dead.
The lesson has stayed with me: broilers do not joke with water. Fast-growing birds produce a lot of metabolic heat, and in hot weather they depend heavily on water to control body temperature. Feed was still available to those birds, but water was not. That combination was disastrous.
Check drinkers several times during hot periods. Make sure water reaches every part of the house, the storage tank has enough reserve, drinker flow is adequate, and somebody can respond quickly when you are away. See our Heat Stress in Poultry guide for the wider heat-management plan.
Overcrowding: do not use one birds-per-square-metre figure for every broiler
Broilers need space, but stocking density should not be reduced to one blind number for every strain and every house. A Ross flock, Cobb flock, Arbor Acres flock, Anak flock or Oba Marshall flock may differ in growth pattern and final body weight. Housing and climate also matter.
Current breeder guidance itself shows why one universal number is risky. The 2025 Ross Broiler Management Handbook recommends that stocking density be matched to final live weight, climate, housing and ventilation. For open-sided houses with poor environmental control it gives a maximum of about 20–25 kg live weight per square metre, reducing to about 16–18 kg/m² during the hottest periods. Cobb’s management guide also bases density on final live weight and housing system and advises lower density in warm climates and houses without strong mechanical ventilation.
Notice that these breeder recommendations are expressed mainly as live weight per square metre, not simply birds per square metre. Ten small broilers and ten heavy seven-week broilers do not place the same load on the house.
Nigerian research points in the same practical direction. Studies using densities such as 10, 12 and 14 birds/m² have produced different performance and welfare responses depending on strain, season and ventilation. Other Nigerian work has found that heat sensitivity, including respiratory response, increases as stocking density rises.
So I would not tell a farmer, “Every broiler must be stocked at exactly X birds/m².” Use the current breeder or hatchery manual where one is available, consider the expected market weight, climate and ventilation, and get help from a poultry extension worker, animal technologist, veterinarian or experienced poultry adviser if you are unsure.
As the birds grow, reassess the space. A house that looked comfortable at three weeks can become hot and crowded at six or seven weeks.
Late-cycle mortality: heavy birds, heat and poor ventilation
I saw this clearly when I was coordinating a farm raising about 5,000 broilers for Christmas. The flock looked fine earlier in the cycle. Around the seventh week, close to marketing, heavy mortality started.
The birds were big, but their size alone was not the real cause. The bigger problem was the house. Air movement was poor. When you walked among the birds, you could feel the heat trapped inside the pen and see birds panting.
As broilers get heavier, they produce more heat and require more air, more drinking-water capacity and more effective floor space. If the house cannot remove that heat, late-cycle mortality can rise very quickly.
This is why stocking density, ventilation and market age must be planned together. Do not wait until birds are heavy before discovering that the house cannot carry the heat load.
Coccidiosis and gut disease
Coccidiosis remains an important cause of broiler losses. Watch for reduced appetite, poor growth, weakness, diarrhoea, dehydration and, with some Eimeria species, blood in the droppings.
Do not diagnose coccidiosis from blood alone and do not treat every diarrhoea as coccidiosis. Use flock history, droppings, post-mortem findings and laboratory or veterinary support where necessary.
Our complete coccidiosis guide explains diagnosis, litter control and treatment principles, while the Eimeria lifecycle guide explains how the parasite keeps cycling between birds and contaminated litter.
Gumboro and other infectious diseases
Infectious bursal disease, commonly called Gumboro, is another important broiler disease during the growing period. Age can help with suspicion, but do not diagnose Gumboro from age alone. Sudden depression, ruffled feathers, diarrhoea, dehydration and rising mortality can overlap with other diseases.
The same principle applies to Newcastle disease, bacterial infections and avian influenza: mortality is a signal, not the diagnosis.
Rapid mortality with severe respiratory or nervous signs, swollen heads, cyanosis or other signs compatible with a reportable disease needs urgent professional attention and appropriate movement control.
Sudden Death Syndrome: the broiler that was fine and is suddenly dead
Broiler farmers sometimes find a healthy-looking, fast-growing bird dead suddenly, often lying on its back with the legs upward. This pattern is commonly associated with Sudden Death Syndrome (SDS), also called flip-over disease.
Veterinary references describe SDS mainly in rapidly growing broilers. Birds may appear normal immediately beforehand, then suddenly extend the neck, flap or convulse and die. Many are found on their backs.
But do not assume that every bird found on its back died from SDS. Heat stress, heart problems, suffocation, trauma and infectious disease can also cause sudden death. If the number is increasing, post-mortem examination is more useful than naming the disease from the position of the carcass.
Merck Veterinary Manual notes that SDS is associated with fast-growing broilers and cardiac arrhythmias and that reducing excessive early growth can reduce incidence.
What to check when mortality suddenly rises
| Check | What you are looking for |
|---|---|
| Bird age | Brooding problems, disease timing, late-cycle heavy-bird problems |
| Daily death pattern | One-off loss, gradual increase or sudden spike |
| Water | Empty tank, blocked line, low flow, hot water, inaccessible drinkers |
| Feed | Feed outage, wrong feed, spoilage, contamination or sudden change |
| Temperature and ventilation | Panting, heat build-up, cold stress, poor airflow, high humidity |
| Stocking pressure | Birds outgrowing available floor, feeder, drinker or ventilation capacity |
| Droppings | Persistent diarrhoea, blood, abnormal colour or major flock-wide change |
| Respiratory or nervous signs | Possible serious infectious disease |
| Carcass and post-mortem findings | Evidence that helps separate disease from management causes |
When broiler mortality becomes an emergency
- Deaths rise sharply compared with the previous day or normal flock pattern.
- Several birds die within a short period during hot weather.
- Water or ventilation fails.
- Many birds stop eating or drinking.
- You see severe respiratory or nervous signs.
- There is widespread bloody diarrhoea or severe dehydration.
- Sudden deaths are appearing across several parts of the house.
- You suspect poisoning, contaminated feed or a reportable disease.
Do not respond by throwing several medicines into the water at once. First correct obvious environmental failures, restrict unnecessary movement if an infectious outbreak is possible, preserve useful samples where advised and get diagnostic help.
Keep mortality records that actually help you
Record the following every day:
- bird age and date;
- number found dead;
- number culled;
- where in the house the birds were found;
- feed and water intake or any interruption;
- temperature, ventilation or heat event;
- important clinical signs;
- action taken; and
- post-mortem or laboratory findings where available.
A good record can show patterns you will never see from the final mortality percentage alone.
Financial effect of broiler mortality
A broiler that dies at seven weeks has already consumed far more money than a chick that dies on day one. By late cycle you have invested chick cost, feed, water, heating or cooling, labour, vaccines, medication and floor space into that bird.
That is why mortality should be reviewed beside FCR, average sale weight, feed cost and cost per surviving bird.
Use our Mortality and Survival Tracker for daily calculations and the Broiler Cost Calculator to see what mortality is doing to the business.
How to reduce broiler mortality
- Buy healthy, traceable chicks from a reliable hatchery.
- Prepare brooder heat, feed and water before chicks arrive.
- Watch chick behaviour and correct brooding problems early.
- Never allow broilers to run out of drinking water, especially in hot weather.
- Match stocking density to strain guidance, expected final weight, season and ventilation capacity.
- Increase available space, drinker capacity and ventilation as birds grow.
- Keep litter dry and control coccidiosis.
- Maintain strong poultry biosecurity.
- Follow a suitable vaccination and flock-health programme.
- Investigate unusual deaths early instead of waiting for the percentage to become large.
Sources and further reading
- Aviagen — Ross Broiler Management Handbook 2025
- Cobb-Vantress — Cobb Broiler Management Guide
- Asaniyan & Akinduro — Stocking density, environment and broiler performance in Nigeria
- Olaniyi et al. — Stocking density and thermo-physiological response of broilers
- Merck Veterinary Manual — Sudden Death Syndrome of Broiler Chickens
- World Organisation for Animal Health — Avian Influenza
Important: This article combines practical broiler-management experience with published breeder guidance and research. Mortality percentage is a monitoring tool, not a diagnosis. When deaths are rising, birds are severely affected or a serious infectious disease is suspected, seek qualified veterinary or diagnostic help.
