Balancing the eggs

The development of the embryo is determined by the temperature inside the shell, the embryo temperature. This temperature will dictate the development, but with that also the hatchability and the quality of the day-old chick.

It is often assumed that this temperature is fully dependent on the temperature of the air. Although air temperature has a large influence on it, it is not the only factor and in some situations not even the most important factor to consider.

We want to keep the embryo temperature at a level of 100.0 to 100.5°F. This temperature inside the egg is the result of a balance between the heat production of the embryo on one side and the heat loss of the egg towards it's environment on the other side.

During development, the embryo produces metabolic heat. This heat production depends on the moment of incubation. At start of incubation, the heat production hardly exists. The first signs of heat production can be observed around day 4. From day 8-9 onwards the heat production becomes so high that the embryo temperature will rise if we don't react on it. At day 18, the heat production is at its highest level. Once the embryo's starts to pip, the heat production rises again, due to the increased activity.

Not every breed produces the same amount of heat at a given moment in incubation. Broilers produce more heat then layer type birds, high-yielding breeds produce more heat then classical breeds, male lines do normally produce more heat then female lines and within one line males produce more heat then females.

Although there are differences in breeds, the effect of a higher embryo temperature is not equal for all breeds and lines. For instance layer breeds produce less heat then broiler breeds and therefore the embryo temperature will increase less, but the effect of 1 degree increase in layer breeds is much more dramatic then for broiler breeds.

To keep the embryo temperature at the desired level of 100.0-100.5°F, we have to remove enough heat from the shell to compensate for the heat production. There are in principal four factors that influence the heat loss.

In the last decade, there has been done a lot of research in this area. By now we know that embryo temperature in commercial setters and hatchers is often quite variable, mainly due to differences in air velocity. This raises two questions:

     1. Is it of practical importance, in other words, does it affect the embryo or the chick.
     2. If it is important, can we do anything about it.

To answer the first question, yes, it is a very relevant factor affecting both hatch and chick performance. Practical experience shows that controlling embryo temperatures between acceptable ranges can result in a better hatchability and above all a better chick quality. Especially the influence on yolk uptake and closure of the navels is high, resulting in differences in first week mortality due to navel/yolk sac infections and e-coli infections. Research has shown that differences in embryo temperature away from the optimum, result in a significant difference in hatchability, but also in growth and feed conversion of broilers at 6 weeks of age. Also the development of the total embryo as well as specific organs like the heart muscle are influenced, resulting in for instance a difference in ascites susceptibility.

The second question is more difficult to answer. With a single stage machine, we can at least adjust the machine temperature to match the heat production of the embryo. Although this gives a explicit and nowadays well recognised benefit over multi-stage machines and will result in a better chick quality, not all single stage machines are equal.

Because the major important factor for embryo temperature is air velocity and to a lesser extent the place and function of the humidifier, a substantial part of the problem is in the design of incubators and hatchers, which is challenging incubator manufacturers. Only by creating an equal air velocity over all egg and avoiding local evaporation of water, a uniform embryo temperature can be achieved. Without control on these points, the laws of physics simply don't allow a uniform embryo temperature and with it a uniform and optimal development of the embryos. 

Although a lot can be done to improve the uniformity and control of embryo temperature in existing machines, the fundamental issue has to be solved in the designing stage of new machines. A simple check on the uniformity in embryo temperature of 18 day incubated eggs at different spots in the machine will show how well the design of a machine handles the factors that influence heat loss of the eggs.

Incubators that provide a uniform embryo temperature will allow us to create a more optimal environment for all the developing embryos, resulting in a better hatchability, a better chick quality and a better bird performance.