Seeing Through The Hatch Window

By Ron Meijerhof, Senior Technical Specialist, Hybro B.V. - The time period between the hatching of the first and the last chick is called the spread of hatch or the hatch window.
calendar icon 11 December 2006
clock icon 5 minute read

Normally, a hatch window of 24 hours is considered optimal, although in reality a window of as much as 36 hours is often observed in practice.

A long hatch window is not recommended, because this means that the first chicks to hatch – sometimes as early as at 19 days – must remain in the hatcher for an extended period. When this happens, the newly hatched chicks are at risk from non-optimal body temperature and de-hydration, if conditions in the hatcher are not carefully controlled.

This control can be difficult to achieve, as the hatched chick requires a different environment to the hatching egg, and further because the chick can be adversely affected by variable conditions in the hatcher, especially as a result of fluctuations in temperature and air velocity.

Another very good reason to strive for a short hatch window, is that we want to start the chicks on feed and water as soon as possible after hatching.

Embryo temperature

The speed of embryonic development is determined by temperature inside the egg, which is in turn determined by the embryo’s heat production - and the amount of heat it loses.

This embryo temperature can vary significantly between different points inside an incubator, and particularly as a result of variances in air velocity and water evaporation in the setter. Differences as high as 3-4°F in the temperature of eggs at an equal stage of incubation but at different places in the machine are not uncommon. Even on one tray, research and practical experience show that differences of 2-3°F can occur.

As the temperature in the egg drives embryo development, so any differences in embryonic temperature will produce different rates of development. Embryos in warmer eggs in the set will develop and hatch more quickly than embryos in cooler eggs, whose development will be slowed down, thereby lengthening the hatch window.

Egg size

By achieving uniformity in embryonic temperature, we might reasonably therefore expect to achieve a short hatch window. However this is not necessarily the case. Because while uniform internal temperature causes all the embryos to develop at the same speed - any difference in egg size creates another factor to take into consideration: it takes the embryos in the larger eggs longer to hatch.

As long as their temperature is equal, the embryos in the larger eggs will develop according to their genetic potential, which is more or less equal for all eggs in the set – and at the same speed regardless of egg size. But as the larger embryos require more time to mature and hatch, we can see that if we mix egg sizes in the machine, we can also expect to extend the hatch window.

Embryo temperature versus egg size

In general, it is harder for larger eggs to lose their heat effectively for two reasons: the egg itself obstructs the air flow to a greater extent than the smaller eggs, and heat transfer is further impeded by a reduced egg mass to egg shell ratio. As a result, embryos in larger eggs experience higher embryonic temperature, and their development is accelerated – which reduces their growth to hatch phase.

In principle, this means that the embryos in the larger eggs will develop more quickly than those in the smaller eggs, so once again reducing the hatch window.

However, where egg sizes are mixed, there is also the danger that some eggs will become overheated, while others will be undercooled. So although we have achieved a short hatch window, the negative effects of non-optimal temperature on chick quality and performance are probably greater than the positive effect of achieving the short spread of hatch, especially if conditions in the hatcher are well controlled for performance and uniformity.

This doesn’t mean that we should accept a large hatch window. But striving for uniformity in embryonic temperature and development should be our first goal. And if, due to unequal egg size, the hatch window is still not as short as we would like, then it is worth looking at how to standardise egg size, as achieving optimal temperature for embryonic development remains the single most important contributor to achieving uniform chick quality.

November 2006

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