A hatch rate is a single number that summarises about four weeks of decisions, most of them taken by people who never see the result. That is what makes it hard to fix: by the time the figure disappoints, the eggs that produced it were set three weeks ago and stored for a week before that. The five levers below are the ones that still move that number, listed in the order the egg meets them.
Key takeaways
- Beyond seven days of storage, published trials put the loss at roughly 0.5 % hatchability per additional day.
- The parameter that matters is eggshell temperature, near 37.8 °C, not the setpoint displayed on the machine.
- Target 11 to 12 % egg weight loss between setting and transfer at day 18, and manage humidity to reach it.
- Above 0.5 % CO2 in the setter during the first days, hatchability falls; the tolerance changes as incubation advances.
Lever 1. Storage: the hatchability you lose before setting
Storage decides how much hatchability is available to you before the incubator is even switched on. Eggs held up to a week are typically kept at 18 to 20 °C and 70 to 75 % relative humidity in commercial practice, with the temperature stepped down as the intended storage period lengthens, to roughly 15 to 17 °C beyond a week and 14 to 15 °C past ten days.
The cost of waiting is measurable. Published work puts the loss at about 0.2 % hatchability per day up to seven days, then around 0.5 % per day beyond that, driven mainly by additional early and late embryonic mortality rather than by losses in the middle of incubation. Flock age changes the tolerance: eggs from breeders older than 45 to 50 weeks can start losing hatchability after four or five days, while eggs from flocks under 35 to 40 weeks hold up for eight or nine.
Position matters for a simpler reason. Stored large end up, the air cell stays where the embryo will need it at internal pipping. Stored the other way, you are asking the chick to find it.
Lever 2. Embryo temperature, not the number on the display
The setpoint on a machine is a proxy. What the embryo experiences is eggshell temperature, and the value most consistently associated with good chick quality is close to 37.8 °C held throughout incubation. Air temperature and eggshell temperature diverge as soon as the embryo starts producing significant heat of its own, from roughly day 10, and the divergence grows with egg size, tray position and airflow.
This is why two machines running the same setpoint can hatch differently, and why a handheld eggshell temperature probe used at several positions in the setter tells you more in twenty minutes than a month of logged air temperature. Overheating accelerates development and produces early hatches with poorly closed navels; sustained underheating delays it and leaves weak chicks and unhatched fully formed embryos.
| Parameter | Working target | What a sustained miss looks like at breakout |
|---|---|---|
| Eggshell temperature | Around 37.8 °C | Early hatch and open navels if high, late hatch and dead-in-shell if low |
| Egg weight loss to day 18 | 11 to 12 % of fresh egg weight | Small air cell and wet, sticky chicks if low; dehydrated chicks if high |
| CO2 in the setter, first days | Held near or below 0.3 to 0.4 % | Rising early mortality, worsening markedly around 1 % |
| Storage beyond 7 days | Avoid, or pre-warm and accept the loss | Early and late mortality together, with a delayed and spread hatch |
Lever 3. Moisture loss: manage the outcome, not the humidity setting
Relative humidity is an input; cumulative weight loss is the result you actually need. The working target is 11 to 12 % loss of fresh egg weight between setting and transfer at day 18, which is what produces an air cell large enough for the chick to draw its first breath and start pipping on schedule.
Weighing a sample of trays at setting and again at transfer costs a scale and ten minutes per batch. It is also the only way to tell whether a humidity setting that has been unchanged for five years still suits eggs from a different flock age, a different shell quality or a different season. Eggshell conductance varies between flocks; the humidity number that worked last year is not a constant of nature.
Lever 4. Ventilation and carbon dioxide
Ventilation is where hatcheries most often run on default settings, and CO2 is the variable that exposes it. In the first days of incubation the embryo tolerates little: concentrations above 0.5 % in the setter reduce hatchability, with the effect becoming pronounced near 1 %. Tolerance then changes as the embryo develops, and work published by Bruggeman and colleagues in 2006 showed that a gradual rise to around 1.5 % by day 4, held to day 10, is tolerated by broiler embryos.
The hatcher is a different environment again. Once internal pipping starts, the chick uses both vascular and pulmonary respiration for roughly a day, and a controlled elevated CO2 concentration during pipping is associated with a narrower hatch window rather than with poorer chick quality. What matters is that the concentration is deliberate and measured, not the accidental product of a fan setting nobody has reviewed.
Lever 5. Turning: frequency, angle, and the failure that hides
Turning prevents the embryo adhering to the shell membrane and supports normal development of the extra-embryonic membranes. The classical minimum is three to five turns a day, and modern setters turn roughly hourly through an angle of about 45 degrees either side of vertical. Turning matters most in the first week and can be stopped at transfer.
The failure mode worth watching is mechanical and silent. A turning arm that stalls on one trolley, or a tray not seated correctly, produces a localised block of dead-in-shell embryos that the machine reports as normal. Checking turning physically, tray by tray, is a five-minute task that no sensor replaces. Understanding where in the 21 days those losses fall is the substance of reducing embryonic mortality through precision, and it starts with candling and breakout rather than with a new machine.
Breakout analysis: how you know which lever moved
None of the five levers can be attributed from the hatch percentage alone. A breakout of the residue, sorting unhatched eggs into infertile, early dead, mid dead, late dead, pipped not hatched and contaminated, converts one number into a diagnosis.
The pattern points to the lever. Infertiles send you back to the breeder flock, not the hatchery. Early mortality clustered with late mortality suggests storage. Late mortality with poorly used air cells points at moisture loss or temperature. Pipped-not-hatched concentrated in one machine position is usually ventilation or turning. We would run this on one tray per flock per set before spending anything on equipment.
What operators ask when the hatch drops
Which single measurement should we add first?
Eggshell temperature, sampled at several positions in a full setter. It is inexpensive, it takes one visit, and it separates a machine problem from a loading or airflow problem straight away.
Can pre-warming compensate for long storage?
It reduces the penalty, it does not remove it. Pre-warming and short heating treatments during long storage have been shown to limit the loss, but eggs stored for two weeks will not match eggs stored for three days from the same flock.
Is a lower hatch always a hatchery problem?
No, and assuming it is wastes weeks. Fertility, breeder nutrition and egg handling on the breeder farm sit upstream of everything described here. The breakout is what tells you which side of the fence the loss occurred on.
When should a veterinarian be involved?
As soon as contamination shows in the breakout, or whenever chick quality falls without a matching change in the physical parameters. Bacterial contamination and vertically transmitted infection are diagnoses, not settings, and they belong to the veterinarian responsible for the site.
From five levers to a running record
Storage days, eggshell temperature, weight loss, CO2 and turning are worth little as one-off readings. They become a management tool once each set carries its own figures and you can compare a disappointing hatch against thirty good ones.
Sources: peer-reviewed literature indexed in PubMed on hatching egg storage duration, storage temperature and hatchability; Bruggeman et al., 2006, on carbon dioxide tolerance during early incubation; published work on eggshell temperature and chick quality; Royal Pas Reform technical guidance on egg weight loss at transfer. Consulted August 2026.
Published previously, fully revised on 8 August 2026. This article is general operational guidance for hatchery professionals. Figures given are working reference values, not a protocol: validate them against your own equipment, breed and flock data, and involve the veterinarian responsible for your site for any suspicion of contamination or disease.