Optimal broiler slaughter day — when to sell to earn the most
Every extra day of fattening means a bigger chicken, but also a more expensive kilogram of gain — because with age feed use per kilogram (FCR) rises and another day of cost is added. We show how to find the day when the margin stops growing: you gather the weight curve, daily FCR, the buying price and the daily cost, then compare the value of the gain with the cost of each further day. It is plain arithmetic — once the daily cost exceeds the value of what the bird gains that day, keeping it longer starts to eat into profit.
verifiedFrom the team that has organised work on poultry farms for years.
What we calculate and why
The optimal slaughter day is the day you end fattening with the highest profit from the whole batch, not with the heaviest bird. These are two different things: a broiler keeps growing almost to the end, but every further day is more and more expensive. Our goal is to find the moment after which adding days stops paying off — because the value of the gain on a given day falls below the cost of keeping the bird for that day.
Why a later slaughter does not always mean more profit
A young chicken uses feed very efficiently — early on it needs little feed per kilogram of gain. With age that efficiency drops: the same kilogram of meat costs more and more feed. This is exactly what FCR (feed use per kilogram of gain) shows — how to calculate it is described in the guide on FCR in broilers — how to calculate it. The older the bird, the higher the FCR, so every extra kilogram is more expensive than the last.
The formula in words — daily margin
The rule is simple: for each day you calculate the value of the daily gain (weight gained that day × buying price per kilogram) and subtract the cost of that day (feed used that day + daily upkeep: energy, litter, labour, depreciation). The result is the daily margin. As long as it is positive and high, you keep the birds. Once the daily cost starts to exceed the value of the gain, the daily margin falls to zero and below — a signal that keeping the birds longer lowers the batch profit.
What else affects the decision
The plain daily margin is the foundation, but not the whole truth. The weight the slaughterhouse expects also matters (a contract often sets a bird-weight range), as do the weight-class threshold and the cost of delay — towards the end of fattening stocking density rises (kilograms of live weight per square metre), which worsens welfare and raises mortality risk. The full batch profitability picture is given by the batch margin calculator, and you can work out the whole flock’s weight for collection in the collection biomass calculator.
Run it calmly with DlaFerm.pl
Calculating day by day by hand is tedious, and the data is easy to lose. In DlaFerm.pl you keep a digital Flock Card where you record control weights and feed use, and after sign-up the calculators work out the daily margin and point to the day after which it stops growing. So you decide on slaughter based on numbers, not by eye. You can create a farm account for free.
How to calculate the optimal slaughter day — six steps
From gathering the data to a ready decision on the collection day. We show the maths on a concrete example so you can see where the daily margin stops growing.
Gather the input data
You need four things: the body-weight curve (weight on successive days — from control weighings or breed tables), daily or cumulative FCR, the buying price per kilogram of live weight, and the daily upkeep cost (feed, energy, litter, labour, depreciation). Weights and feed are most convenient to gather in the digital Flock Card, so you get a curve from your own flock, not only from tables.
Write down the formula and the rule
For each day: daily margin = (daily weight gain × buying price) − (feed used that day × feed price) − fixed daily upkeep cost. Daily gain is the weight difference between two successive days. You keep the birds as long as the daily margin is clearly positive; the optimal day is the last one before the daily margin falls to zero. You calculate the FCR needed here as in the guide on FCR in broilers.
Plug in example data
Assume a buying price of 6.50 PLN/kg of live weight, a feed price of 1.80 PLN/kg and a fixed daily cost of 0.06 PLN per bird (energy, litter, labour, depreciation). Day 35: weight ~2.10 kg, daily gain ~62 g, daily feed ~150 g. Day 39: weight ~2.45 kg, daily gain ~50 g, daily feed ~175 g. Day 42: weight ~2.65 kg, daily gain ~38 g, daily feed ~190 g. These are illustrative figures — your flock may have a different curve.
Calculate the margin day by day
Day 35: gain 0.062 kg × 6.50 = 0.403 PLN, minus feed 0.150 × 1.80 = 0.270 PLN, minus 0.06 = +0.073 PLN per bird. Day 39: 0.050 × 6.50 = 0.325, minus 0.175 × 1.80 = 0.315, minus 0.06 = −0.050. Day 42: 0.038 × 6.50 = 0.247, minus 0.190 × 1.80 = 0.342, minus 0.06 = −0.155. You can see the daily margin goes from positive (day 35) to below zero between day 35 and 39.
Read off the optimal day
The optimal day is the last one with a positive daily margin — in the example the margin stops paying shortly after day 35–37, because from then the daily cost exceeds the value of the gain. This does not mean you slaughter exactly that day: the result must be set against the weight the market and slaughterhouse want. You check the full batch profitability in the batch margin calculator.
Use the result in the slaughterhouse contract
Translate the optimal day into specifics: what weight the flock will reach on that day and whether it fits the contract range and the weight class the market expects. You work out the whole flock’s weight for the collection date in the collection biomass calculator. Sometimes it pays to slaughter a day or two earlier (better FCR, better welfare), even at a slightly lower individual weight.
Slaughter age, prices and costs — what to watch out for
The daily margin maths is the foundation, but a good farmer also looks at age norms, weight classes and welfare towards the end of fattening. Here are six things worth keeping in mind.
Typical broiler slaughter age: 35–42 days
A standard fast-growing broiler usually goes to slaughter between day 35 and 42, at a live weight of about 2.0–2.8 kg — depending on the breed line and market requirements. A lighter chicken (a so-called small chicken, about 1.6–1.8 kg) is sometimes collected earlier, a heavy one later. The exact growth rate and target weights are given by the management tables of the given line (e.g. Aviagen/Ross, Cobb) — a good reference point for your curve.
Buying price and weight class
The price per kilogram can differ across weight classes — the slaughterhouse often pays a different rate for light, medium and heavy chickens, and deducts for weight outside the contract range. So the optimal day from the margin maths must be set against the slaughterhouse price list: sometimes adding a day to reach a higher weight class pays off, and sometimes the opposite — crossing a threshold lowers the rate. The contract figures take precedence over theory here.
The daily upkeep cost rises to the end
Towards the end of fattening the bird eats the most while its gain is the smallest — the worst ratio in the whole cycle. On top of feed there is the fixed daily cost: heating or ventilation, litter, labour, electricity, house depreciation. These costs accrue regardless of how much the bird gained, so every “spare” day is a real expense. Do not omit the daily cost in the calculation — without it the daily margin comes out overstated.
Welfare and stocking density at the end
The heavier the birds, the higher the stocking density (kilograms of live weight per square metre). Welfare rules set a maximum density, and exceeding it is not only a penalty risk but also worse conditions: higher mortality, poorer leg quality and more rejects at slaughter. Sometimes collecting part of the flock earlier (so-called thinning) solves the density problem — that is also part of the slaughter-day decision.
Common mistakes in the calculation
The most common errors are: counting only weight instead of margin (a heavier bird ≠ more profit), ignoring the rising FCR, taking a “cycle-average” feed price instead of the real end-of-cycle one, and ignoring the fixed daily cost. It is also a mistake to stick rigidly to one figure from a table — your flock’s curve may differ from the standard. It is safest to calculate on your own control weights and feed use from the Flock Card, and the reports to flock records in IRZplus DlaFerm.pl will send for you, if you want.
Calculate on numbers in the app
Hand-made tables drift easily, and the slaughter-day decision is taken once per batch and weighs on the whole profit. In DlaFerm.pl you have weights, feed and results in one place, and after sign-up the calculators work out the daily margin and point to the optimal day. You can set the result against the EPEF index for broilers, which combines weight, FCR, mortality and age into one batch score.
Frequently asked questions about the optimal broiler slaughter day
At what age are broilers slaughtered?add
A standard fast-growing broiler usually goes to slaughter between day 35 and 42, at a live weight of about 2.0–2.8 kg. A lighter chicken is collected earlier, a heavy one later. The exact day depends on the breed line, your flock’s growth curve and the weight the slaughterhouse and market expect. It is best to confirm it with daily-margin maths on your own data.
Does a heavier chicken always mean more profit?add
No. A broiler keeps growing almost to the end, but each further kilogram costs more and more feed, because FCR rises with age. On top of that there is a fixed cost for every day of upkeep. So what counts is not weight but margin from the whole batch. The optimal day is the one after which the cost of keeping the bird for another day exceeds the value of the gain the bird achieves that day.
How do I calculate the daily margin?add
For a given day: daily margin = (daily weight gain × buying price per kilogram) − (feed used that day × feed price) − fixed daily upkeep cost. Daily gain is the weight difference between two successive days. As long as the margin is clearly positive, you keep the birds; the optimal day is the last before it falls to zero. Take the figures from your own weighings and feed use, not only from tables.
Where do I get the weight curve and FCR?add
The most reliable data comes from your own flock: regular control weighings and a record of feed use. The breed-line management tables (e.g. Aviagen/Ross or Cobb) give a good reference point, but your curve may differ due to feed, house climate and flock health. In DlaFerm.pl you record weights and feed in the digital Flock Card, so you calculate the curve and FCR on real numbers from the farm.
Is it worth keeping broilers longer to hit a higher weight class?add
Sometimes yes, sometimes no. If the slaughterhouse pays clearly more for a heavier class, adding a day or two can pay off despite worse FCR. But if you exceed the upper limit of the contract range, the rate may drop or deductions appear. The decision must rest on the specific slaughterhouse’s price list, not a general rule — and be set against the daily margin and welfare at higher density.
How does DlaFerm.pl help choose the slaughter day?add
In DlaFerm.pl you keep a digital Flock Card with control weights and feed use, so you have your own weight curve and FCR. After sign-up the calculators work out the daily margin and point to the day after which it stops growing, and you set this against the collection weight and the slaughterhouse price list. So you decide on numbers from your own farm, not by eye.
Calculate the optimal slaughter day with DlaFerm.pl
Want to choose the slaughter day on numbers, not by eye? In DlaFerm.pl you keep a digital Flock Card with weights and feed, and after sign-up the calculators work out the daily margin and point to the optimal collection day. Create a free farm account.
Phone
+48 796 258 151