Although the chicks are big, we see problems with viability of the chicks, yolk sac absorption, unhealed navels etc. Together with that, we can also see a decreased hatch of fertiles, and then especially by an increase in late deads.

If we think about it, it’s a bit strange why bigger eggs from older flocks should give more problems. Genetically the embryos are identical to the embryos the same flock produced 20 weeks ago. We should expect the embryos to develop more or less in the same way, as they probably will not notice that they are in a bigger egg until the end of incubation, when they fill up the entire egg. An issue could be the decreasing shell quality of older flocks, but even flocks with good shell quality tend to give more problems with chick quality when the breeders get older.

One of the problems we are facing with bigger eggs is the actual temperature of the egg during incubation. The optimum development for an embryo takes place at a very specific temperature inside the egg. This temperature (measured on the egg shell with an infra-red ear thermometer) is optimal between 100.0 and 100.5oF, and should not exceed 101oF. If the temperature inside the egg is too high (and as a consequence the temperature of the shell gets above 100-100.5oF)

When embryos are experiencing higher temperature in the egg, they have more problems with using the yolk and converting it into body tissue. The yolk residue remains big, and the “real” chick (chick without the yolk) is relatively small. If we weigh the chicks, we do not notice this effect, as we are weighing chicks including their residual yolk sac. If we take out the yolk and then weigh the chick, or if we measure the length of the chick as an indicator for its development, we see that bigger eggs give bigger total chick weight, but that a lot of the weight is contributed by the yolk. The actual real chick (without the yolk) is not as big as we expect and is sometimes even smaller than the chick from a breeder flock 10 or 20 weeks younger.

Small chicks with large residual yolks have more problems closing their navels over that big yolk, resulting in bad navels and navel-yolk sac infection. The late deads are increasing, as the high temperatures will kill some of the embryos. The embryos that were not killed had at least a hard time during the last days, resulting in a reduced vitality.

If we set all eggs, big or small, at the same machine temperature, the bigger eggs will experience a higher temperature inside the egg, above the optimum. The reason for that is that the temperature inside the egg is the result of the balance between heat production on one side and heat loss on the other side. Bigger eggs containing bigger embryos will produce more heat, simply because there is more embryo mass in there. On the other side, bigger eggs have more problems of loosing that heat. Eggs loose heat from the surface of the shell to the environment, like the radiator in a car. Bigger eggs have more total shell surface, but per gram of egg the shell surface is reduced, making it more difficult for the eggs to loose heat.

Besides that, bigger eggs are more “packed together” in an incubator, especially when they are in a turned position. This blocks the air flow over the eggs, and air flow is one of the most important aspects in cooling the eggs. Due to this, bigger eggs normally have a higher temperature, especially when the temperature of the machine is not adjusted but set on an “average” egg size.

To overcome these problems, we must give the embryos in the bigger eggs the same temperature as the embryos in the smaller eggs. If we incubate all the eggs at the same machine temperature, bigger eggs will have a higher temperature resulting in a reduced quality. To control this, we have to adjust the temperature profiles, giving embryos in bigger eggs the same temperature inside the shell as smaller eggs. That means that we have to increase the heat loss from bigger eggs in the second half of the incubation period, either by dropping the temperature more severe or by increasing the air flow over the eggs.

If we incubate in a single stage machine, the starting temperature of the eggs, until about 8-10 days of age, is identical for big and small eggs (young and old breeder flocks). Until that moment, eggs do not produce a lot of heat and the temperature in the egg will be close to the temperature of the shell. And as embryos need a similar temperature in the egg, regardless of the egg size, big eggs and small eggs need the same air temperature at the start, when no heat is produced inside the egg yet.

If the incubation process continues, eggs will produce more heat, and temperature has to be dropped. Because bigger eggs produce more heat and have more problems loosing that heat, the temperature has to be dropped more severe on these eggs. This means that if the temperature of small eggs is dropped from approximately 99.7oF at 10 days to 99.2oF at 18 days, we have to drop the temperature in the machine with the big eggs more. It’s a bit difficult to say how much, as this will depend as well on the actual size of the egg as on the type of machine used, but dropping to at least half a degree lower then for small eggs is the minimum.

That means that if small eggs end their air temperature at 99.2oF at 18 days, big eggs should drop to at least 98.7oF at 18 days. At the same time, it might be necessary for the big eggs to start dropping the temperature also a bit earlier, for instance at 9 or 8 days instead of 10 days. It’s important to start dropping the temperature early enough, because ones eggs are overheated, it’s almost impossible to repair the damage. Once the chicks are overheated, the slow down in development and heat production. Then the drop in temperature later in incubation has to be done more moderate and careful, to prevent undercooling.

In the field we normally use a straight line for dropping the temperature. This means that if we start for small eggs at 10 days wit 99.7 and we end at 18 days with 99.2, we actually drop the temperature in a straight line over 8 days to the desired temperature. If for the big eggs we want to drop to 98.7 at 18 days and we start with that from day 8 onwards, we drop 1 degree in 10 days, so 0.1 per day.

How do we know if we dropped the temperature enough?

To get a good feeling if the actual temperature scheme is correct, we must systematically check the chick quality. This can be done by checking the yolk residue, the quality of the navel and by checking the length of the day old chick. If a drop in temperature results in a better developed, longer chicks with less residual yolk and better navel quality, it was the right choice to do. However, checking chick quality and adjusting temperature profiles never stops.

December 2006