Unlocking the yolk sac as a ready-made lunch box - By Ron Meijerhof, Senior Technical Specialist, Hybro B.V., Boxmeer, The Netherlands - A main energy source for the developing embryo is the yolk sac which. As the embryo grows and uses more yolk, so the yolk sac reduces in size. In simple terms, the yolk is actually being converted into embryo, and the more embryonic development we see during incubation, the less yolk there will be left over at hatching.

We know from experience that day old chicks can have a yolk residue of 2-3 grams at hatching, but it is not unknown to see as much as 12-13 grams of yolk residue, depending on the incubation conditions that we create.

This 10 grams difference is a lot if we compare it with the total chick weight, but especially when compared with the original yolk weight. As the original yolk usually weighs 20-25 grams, a difference of 10 grams in yolk residue means that twice as much yolk is used for development. As we are aiming for a well developed day old chick, the remaining yolk sac at the moment of hatching is consequently limited.

Besides the beneficial effects on the development of the embryo, this improved development also means that it is easier for the embryo to close the navel over the yolk, as compared to the size of the total body, the yolk is comparatively small.

However, residual yolk is used by the new born chick as a ready-made food supply for the first four or five days, after which it should be completely utilised. Day-old chicks must learn where to find food and water, and the energy provided by this remaining yolk helps them to cope with the new demands being placed on them as they learn to survive in their new environment.

Therefore it is often assumed that a large residual yolk sac at hatching will be an advantage to the young chick. In principle, the better filled the lunch box, the greater the chick’s chances of survival in these first difficult days – and on this basis, we could also assume that a small residual yolk sac may decrease the chick’s chances.

However when we place chicks in the field, we often see a peak in mortality around 3-4 days. As this normally coincides with the complete consumption of the yolk, we could conclude that these chicks are simply lacking sufficient energy for survival. But if we examine these chicks, we find that the majority – if not all – of them still contain a large residual yolk sac. We also find that the navels and yolk sacs of these birds are inflamed and contaminated, and bacterial examination often indicates e-coli present.

This indicates that these chicks didn’t die due to a lack of yolk, but more probably because conversely, they failed to use this energy source efficiently.

If we feed day old chicks during transportation, we find that they use their yolk more efficiently. We know that physiologically, the chick needs carbohydrates to be able to utilise the fat in the yolk.

This means that the birds must have some nutritional reserves left over after hatching, or access to an alternative food source, to be able to digest the fat and use their yolk reserves successfully.

In both cases, the quality of the incubation process is critical, to ensure that the birds are hatched in good health, with every opportunity for an optimal start in their first few days of life.

Birds that are stressed during incubation or hatching, hatch with a poorly closed navel or that are placed into sub-standard conditions on arrival at the farm, will have difficulty finding feed, will not be able to use their yolk properly and are likely to die of navel yolk sac infection.

Even a large amount of residual yolk will not help these birds to a good start. They may have a full lunch box, but they will not have the strength to find the key to open it.

This column was published in International Hatchery Practice Volume 20 Number 1, 2005

Source: Hybro B.V. - March 2006