The Physiological Zero. Why do they die early.

WHY DO THEY DIE EARLY? UNDERSTANDING PHYSIOLOGICAL ZERO

We will describe how LATE dead-in-shell is all about the time of death, not its cause — a time of stress and hypoxia just before primary pip and again before secondary pip. But what about the EARLY failure of fertile eggs?

Embryonic development slows and stops as the egg temperature drops below what has traditionally been called ‘physiological zero’. At this point, activity within the embryo is greatly reduced but remains reversible. When the temperature rises again, development continues.

We see ‘physiological zero’ mentioned in the incubation literature as a range of temperatures from 12 to 20°C. The old definition placed it at 21°C, below which there was considered to be no embryonic development.

More recently, that simple benchmark has been replaced by the concept of ‘embryonic diapause’ or ‘dormancy’. This recognises that some activity within the cells continues at a very low level, while the major changes in the shape and structure of the embryo are paused.

For comparison, embryonic dormancy is well described in other species, including turtles, marsupials and even deer, where development is slowed or arrested as a strategy for coping with unfavourable environmental conditions.

THERE IS A CRITICAL THRESHOLD

In the chicken, embryonic development slows after laying as the egg cools towards room temperature, about 22°C. During this cooling period, the embryo develops from gastrula stage IX to stage XIII. Physiological zero is now restricted specifically to stages XII–XIII of development.

The important point is that there is a critical threshold beyond which development can no longer be safely paused.

If the embryo develops beyond this stage and the primitive streak begins to form, prolonged cool storage will slow its development and eventually cause the embryo to die.

This is why unwanted heat before incubation matters. An egg left too long in a warm nest, beneath a lazy nest-sitter, or in storage where daytime temperatures become too high can continue developing beyond that critical threshold.

RECONDITIONING EGGS STORED FOR LONGER PERIODS

Commercial hatcheries apply a temperature treatment when eggs are to be stored for more than seven days. This mimics the mother hen sitting on the older eggs while laying each subsequent egg, during which the contents of the older eggs are ‘reconditioned’.

The eggs are placed in a cold incubator and warmed. Heating is stopped at 32°C, the door is opened and the eggs are allowed to cool gradually back to room temperature before being returned to storage.

There is an obvious caution here for the amateur.

If the process is taken just a little too far, the embryo develops the primitive streak. The egg is then cooled and the embryo dies.

The same thing can happen unintentionally to eggs in storage when daytime temperatures exceed 32°C for several hours.

PRACTICAL STORAGE

  • Collect eggs from the nest soon after laying and allow them to cool gradually to room temperature.
  • Watch out for lazy nest-sitters.
  • Store eggs ideally below 21°C and for not more than seven days.
  • Keep the relative humidity as high as possible — 80–85% RH. If storage conditions are dry, or the eggs are stored in cardboard cartons, wrap them individually in cling film to prevent moisture loss.
  • For storage longer than seven days, store eggs point-end up.

The important distinction is between an embryo whose development has been safely paused and one that has already passed the point where that pause is possible.

Unwanted heat can take the embryo across that threshold before incubation has even begun.

Bob Peel, Greatlander