A balancing act: managing heat loss to maximise incubation performance
By Ron Meijerhof, Senior technical specialist, Hybro B.V. - Embryonic development is determined by embryonic temperature inside the shell. It is often assumed that this temperature is fully dependent on the air temperature around the egg. Yet, while this is an important factor, it is not the only factor to consider.
5 June 2006
5 minute read
Embryonic temperature, which is measured at cell temperature (ideally 100.0-100.5 F) is in fact the result of a fine balance between embryonic heat production and heat lost to the environment outside the shell.
One side of the balance: heat production
Metabolic heat is a product of embryonic development. At the start of incubation, there is hardly any heat production - and it is not until day four that any real signs can be detected. From days eight/nine onwards, heat production becomes so prolific that embryo temperature will rise to unacceptable levels if we conditions are not managed.
At day 18, heat production is at its highest level. Once the embryo starts pipping, heat production rises again, due to this increased activity.
Different breeds produce more heat than layers, and high yielding breeds produce more than classical breeds. Males normally produce more heat than females, even within one line. The effect of a higher embryonic temperature is not, however, equal for all breeds and lines. Layers produce less heat than broiler breeds, so we can expect to see lower increases in embryonic temperature - but the effect of a 1o increase in layer embryos is much more dramatic than the same increase in a broiler breed.
The other side: heat loss
To maintain embryonic temperature at the desired level of 100.0-100.5 oF, we must remove enough heat from the shell to compensate for the amount of heat being produced. There are four key factors we should consider:
Air temperature. A difference in temperature between shell and air will force heat to flow either towards the shell (when air is warmer than the shell) or from the shell (when shell is warmer than air). The greater the difference between the two, the more heat will be transferred.
Humidity. Humidity influences heat loss in two different ways. Dry air retains very little heat, because heat is actually carried by water molecules. In conditions of high humidity (more water molecules in the air), more energy is stored per unit of air. So at a given temperature difference, humid air will remove heat from the egg. In a 'normal', that is relatively narrow, range of humidity, this is not a major factor. However, when we incubate at high altitude (low pressure), there are less water molecules in the air, even at the same relative humidity as at low altitude, and it will be more difficult to achieve heat loss from the eggs. Eggs evaporate water at a constant rate, to a total of 12-14% of their initial weight. This evaporation uses energy, so moisture loss will, therefore, reduce embryonic egg temperature. Conversely, when relative humidity is high, less water is evaporated so less heat will be lost. So, while high relative humidity will increase heat loss through increased heat capacity, it will also decrease heat loss due to decreased evaporation.
Air velocity. At the same temperature difference, objects lose more heat if air velocity is high, which is why tunnel ventilation for broilers is so effective 0 it is like creating a 'wind chill' factor. Eggs in areas of high air velocity then will clearly lose more heat than eggs incubated at low air velocity. The difference in embryonic temperature can be as high as 3-4 oF at the end of incubation, which indicates that if air velocity differs within the machine, embryo temperature will vary, no matter how uniform the air temperature is.
Water spray. Any incubator will add water to compensate when air is too dry. As water needs energy to evaporate, this will cool the eggs closest to the spray. As we have seen, evaporation inherently has a cooling effect, but spraying large amounts of water will decrease uniformity in embryo temperature, as the water is usually sprayed locally, and when in areas of high air velocity this will naturally compound the effects of heat loss.
