Understanding the role of CO₂ in commercial incubation

There is no question that the most critical element involved in the successful incubation of the avian egg is temperature.
calendar icon 15 October 2019
clock icon 6 minute read

Replicating the experience, not the environment

To understand the important role of CO₂ in commercial incubation, we must first look at the situation in nature. What is happening in the chicken’s nest? Of course, the levels of CO₂ utilised in the commercial incubator are not those seen in the nest environment, nor should they be.

Here is why:

In the nest, a small number of eggs are laid in a still air environment. Once the clutch is complete, the brooding hen will initially constantly sit on the eggs, ensuring good heat transfer through direct contact with the brood patch. By covering the eggs and sealing them off from the environment, the CO₂ builds up. She will only break this contact when standing to turn the eggs to ensure, amongst other things, good uniformity. This high level of attentiveness (and therefore high CO₂) only reduces once the eggs become exothermic. At this point the hen will leave the nest and take the opportunity to feed, which makes the CO₂ drop, but she remains close to the nest to ensure the eggs are sufficiently cooled. She checks them every so often, if they’re beginning to cool too much, she’ll bring them back together and maybe sit again. Conversely, if they are still too hot, then she moves them towards the outer edge and hopefully exposes them to more fresh air.

Only during the critical hatching phase will the hen’s attentiveness and the CO₂-level become heightened again. Clearly the rate of diffusion of gases and fluids will vary depending on whether the hen is effectively smothering the eggs or leaving them in the open air.

It is our objective to replicate this nest experience in the commercial incubator. The challenge here is to achieve a good uniform temperature on a much larger scale. This is done through a temperature controlled constant airflow in and around the eggs.

To mimic the attentiveness of the mother hen, we cannot physically replicate the smothering effect on a large scale in an incubator. This is near to impossible for practical reasons, especially because we want to maintain good temperature uniformity. We will control the rate of diffusion of gases and fluids by controlling the environment around the eggs. Higher levels of CO₂ in the external environment will result in less flow from the egg through its eggshell, as if the hen was sitting on the nest. Much lower levels of CO₂ during the exothermic phase replicate the lowering of the parental attentiveness. By using these environmental conditions, we can now certainly replicate the experience of the attentiveness of the hen during the hatching process. This way the embryo has the same experience as if it were in a clutch of eggs in a hen’s nest.

Stress or stimulus

The broody hen has several ways of ensuring eggs laid days apart emerge from the shell within a relatively narrow timeframe. Some of these methods we’ve yet to fully understand (such as vocalisation). However, it has been shown that by returning to the heightened levels of attentiveness and sitting on the hatching eggs, the temperature increases and the gas diffusion rate decreases, which stimulates the emerging chicks to escape from the egg. We mimic this in the commercial incubator. One could say it gives the chick ‘stress’. Stress is a word commonly used today and can sound negative because it is often used incorrectly. The word 'stress' in this case refers to elements acting as a stimulus. This is a perfectly natural action that offers the stimulus that the hatching chick would experience in nature.

Using nature as a reference

Evolution has enabled the survival of so many varied species on this planet. In every case, natural selection has produced a creature that utilises elements to its advantage or has evolved a defence mechanism against challenges. It is therefore logical to say the embryo doesn’t simply survive despite such elements as differing diffusion rates due to the varied degree of attentiveness, but rather has evolved to utilise the effect to its benefit.

It’s important to look to nature and firstly note what the broody hen is doing. It is just as important to try and understand what she is trying to achieve. From this we can replicate the optimum conditions and effectively be the perfect parent.


Optimising CO₂ in your commercial incubator will not necessarily give you a huge increase in hatchability, although you should see some improvement. What we are aiming for here is optimum, uniform chick quality ensuring excellent welfare and post hatch performance.

Roger Banwell

Senior incubation expert
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