Single stage incubation is the most natural choice

By Dr. Marleen Boerjan, Pas Reform Hatchery Technologies - What are the incubation needs of the developing embryo? To achieve the best results from commercial incubation we should focus on temperature in the setter, the most critical factor of embryonic development.
calendar icon 31 October 2005
clock icon 12 minute read

Eggshell temperature, the most critical factor of embryonic development

Good hatchability is no accident. Nature has created a heat-dependent process that draws on maternal body temperature and the production of metabolic heat, to produce healthy chicks in three distinct and critical phases of incubation.

Nature has the answers – and our aim in the hatchery must be to replicate these most natural of conditions, to produce healthy chicks that meet the evergrowing demands of commercial poultry markets around the world.

It is widely recognised that genetic improvements in poultry have resulted in an enormous diversification of breeds – all of which require specific incubation conditions. It is clear that embryo metabolism is changing as a result of selection for production traits – and that further changes will continue to emerge, as genetic advancement continues.

However, it is only recently that the majority of incubator manufacturers have recognised the value of improving and adjusting incubation technologies, to better meet the needs of the growing embryo.

The hatchery forms an essential component of the poultry production chain, in that together with egg-related (genetic) factors, it is this process that determines the quality and vitality of the day old chick – a factor that ultimately determines the quality and technical performance of the final poultry product.

The development of a vital day old chick is a complex process that can roughly be divided into three phases: a phase of cell differentiation, a phase of growth and a hatching phase – each requiring specific incubator conditions.[1]

Temperature and embryonic differentiation

Definition of incubation temperature

Embryo temperature - the body temperature of the growing embryo
Eggshell temperature - the temperature of the surface of the egg
Air temperature - the temperature of the air close to the eggs
Incubator temperature - the set point of the incubator.

Embryonic differentiation is characterised by the formation of different tissues that will develop into the chicken’s final organs in the growth phase (Figure 1 - below).

This first phase of cell differentiation starts in the hen, when the single-cell oocyte divides many times so that, in the un-incubated egg the embryo consists of about 30,000 cells. These 30,000 cells are organised as a plate of cells, known as the early gastrula, which floats on the top of the yolk.

After laying, the temperature of the egg decreases and the development of the embryo ceases or stops completely if the temperature falls below the physiological zero (25-27°C). Embryonic differentiation continues only when the temperature of the egg rises.

Figure 1 - Growth curve and the incubation pattern of the eggshell temperature for chicken (Gallus gallus) in setter for optimum hatchability and a uniform hatch

Embryonic development is a continuous process that can roughly be divided into three different phases, each requiring specific incubator conditions. Typically, differentiation of organs occurs in the first days of incubation and growth, followed by maturation of the organs in later phases of development. As the embryo grows, its metabolic rate increases and this is accompanied by increased heat production. Consequently, the natural pattern of the embryo and eggshell temperature shows an increase towards the end of incubation. For optimum development in the incubator, the eggshell temperature should be within a range of 37.6-37.9°C (99.7-100.2°F) during the first two-thirds of incubation and 38.1-38.8°C (100.6-101.8°F) during the last days in the setter.

The differentiation phase is characterised by a ‘folding’ of the early gastrula, to form a three dimensional structure in which premature organ structures of the head and heart can be recognised within 36 hours. Movement of cells mediates this folding process, whereby the cells in the early gastrula ‘travel’ from one side to the other - and this process is highly temperature dependent. In the differentiation phase, it is not only the embryonic structures that develop, but also the extra- embryonic tissues - such as the amnion and chorio-allantois, both essential structures in the transport of oxygen and nutrients from the yolk to the embryo.

In this stage of development, the embryo floats to the top of the egg, where it is nearest to the eggshell, and normal, synchronised differentiation occurs only when the eggshell temperature is in the range of 37-38°C (98.6-100.4°F).

If temperature is in the range from 27°C to 36°C (80.6-96.8°F), uneven differentiation of the various tissues results and abnormal development occurs as a consequence. Embryonic differentiation is even less tolerant to temperatures above 38°C for longer periods, when exposed brains and eye abnormalities have been recorded. Interestingly, it has been shown that broiler embryos are even more sensitive to high temperatures during the differentiation phase than layer embryos

Embryonic growth and temperature

During differentiation the premature organs are formed and relatively minor changes in the size of the embryo are seen. However the rate of growth is greatly increased during the last half of incubation. Embryonic growth is characterised by an increase in mass while the development of the organs continues (Figure 1).

As we have seen in the first phase of embryonic development, temperature can also have a profound effect on the growth of an embryo – at best speeding up or slowing down growth, and at worst, affecting the growth and left/right symmetry of skeletal parts and the lungs, as shown when broiler embryos were exposed to heat (39.6°C; 103.3°F) and cold (36.9°C; 98.4°F) for periods as short as six hours each day.

Even within the normal temperature range of 37-38°C, differences in temperature induce different rates of development and growth.

Within the growth phase we recognise two different periods during which the developing embryo responds differently to variations in temperature.

The first part of the growth phase, which for a chick embryo starts at about day seven, sees an increase in the size of organs and the embryo as a whole. This phase is characterised by a rapid increase in embryo mass ( Figure 1), which, within the 37-38°C range, is highly dependent on incubation temperature. Higher incubation temperatures applied during this phase increase the growth rate and thus shorten the incubation period. Conversely, lower incubation temperatures result in longer incubation periods because they produce slower growth rates.

In the second part of the chick embryo’s growth phase, which starts at about day 17, ( Figure 1), growth decreases because the maturation of tissues and organs takes place. For this reason, this phase is sometimes called the maturation phase.

Maturation of the organs is characterised by the accumulation of dry matter and, thus, the loss of tissue water.

In addition during maturation, the organs become responsive to specific signals such as heat or cold stress. In this phase, the absolute growth of the embryo is decreased and growth rate is inversely related to temperature.

Managing incubator temperature for optimum development As described above, the development of a vital chick is a process of cell differentiation and growth controlled by the incubation temperature provided by the brooding hen during natural incubation.

To mimic these conditions during artificial incubation, precise control of the incubator temperature is absolutely critical.

In a commercial incubator, many other eggs surround each egg in the same stage of development – all of which must be warmed to start embryonic differentiation and to promote continuing development. During the first week of incubation, Pas Reform accepts an average eggshell temperature difference between trays of 0.1°C (0.2)°F – and it is our aim to maintain homogenous temperature distribution, as the spread of hatching is determined by any temperature variation in this warming period.

As the embryo grows, its metabolic rate increases and this is accompanied by increased heat production. Consequently, the pattern of the eggshell temperature shows an increase towards the end of incubation ( Figure 1).

For example, the temperature of the embryo in a single turkey egg rises to 38.4°C (101.1°F) on day 23, while the eggshell temperature parallels the embryo temperature, at a slightly lower level of 38.3°C (100.9°F). The temperature of the air surrounding this single egg fluctuates between 37.5-37.8°C (99.5-100.4°F), so that in this case, the heat produced by this single egg is removed efficiently.

In an incubator, however, every fertilized egg produces metabolic heat at the same level and will ultimately cause the temperature of the air surrounding the eggs in a tray to rise to unacceptably high levels if no cooling is applied. In a modern incubator, cooled air flows over the eggs to avoid this overheating effect, and for optimum hatchability and chick quality an average eggshell temperature difference within one section of 0.25°C (0.5°F) during the last week in the setter is acceptable.

Single stage versus multi stage incubation

Based on excellent scientific research, Harry Lundy (1969 in Fertility and Hatchability of Hen’s eggs-Carter, TC and Freeman BM, eds, Edinburgh 143-176) summarised the incubation conditions needed for optimum chick development.

When he wrote his review in 1969, it was common to set eggs of several embryonic ages in one incubator: the so-called multi-stage incubation. In multi-stage incubators, the temperature, humidity and ventilation are set at a fixed point. The advantage of multi-stage incubation is its simplicity both with respect to the control system of the incubator as well as the management of incubation. The main disadvantage however, is that the multi-stage incubation environment cannot, by its nature, create optimum conditions for every egg set. For example, in a multi-stage incubator, the average eggshell temperature may vary from 37.5°C (99.5°F) for the youngest embryos, to 39.5°C (103.1°F) for the later embryonic stages - , so it is difficult to find a temperature set point such that eggshell temperature is correct for each embryonic stage. Consequently, in multistage incubation, it is impossible to optimise both hatchability and chick quality, especially when dealing with variable egg quality.

It is clear therefore, that single-stage incubation maximizes hatchability and chick quality - because incubation temperature, humidity and ventilation can be adjusted for each embryonic age and batch of eggs.

In a single stage incubator the incubator set points are adjusted such that the average eggshell temperature follows the natural pattern and thus maximises the quality of the ultimate product.

For example, in Cobb eggs, it has been shown that chicks hatched from eggs incubated at 37.2°C (99°F) until day 16 and then at 38.3 (100.9°F) from day 16 to hatch produced the highest body weight at 44 days, compared to eggs hatched at lower and higher incubation temperatures.

For optimum development the eggshell temperature should follow a natural pattern of 37.6-37.9°C (99.7- 100.2°F) during the first two-thirds of incubation and 38.1-38.8°C (100.6-101.8°F) during the last days in the setter, as shown in Figure 1. Minor variations in this pattern are permissible due to differences in egg types.

Eggshell temperature as the leading parameter

Single stage incubation requires incubators to be equipped with heating, cooling, ventilation, humidifier and turning mechanisms that are controlled accurately and independently. The uniformity and power of heat transfer from the incubator¡¯s temperature to the mass of eggs is a key aspect of incubator performance, because to achieve a uniform hatch, the eggs must be warmed rapidly and homogeneously. Homogenous temperature is best facilitated in an incubator divided into separate units, each with its own climate control. Results, not to mention the hatchery manager¡¯s job, will be greatly enhanced with the tools to design and monitor customised incubation programmes that accommodate the specific requirements of the developing embryo in different egg types. In addition the temperature control system must be accurate so that unacceptably large deviations or fluctuation in temperature around the set point, or ¡®overshoots¡¯ are avoided. Again, the hatchery manager must have the facility to adjust the incubator temperature to keep eggshell temperatures at the desired level. In the design of the incubation program, the average eggshell temperature [2] of a representative sample of eggs should be the leading parameter.


It is now understood that genetic improvements in poultry have resulted in an enormous diversification of breeds, all of which have very specific incubation conditions. The embryo metabolism is changing through selection for production traits. For optimum cell differentiation and growth the embryo is dependent on specific eggshell temperatures. It is therefore essential that the hatchery manager have the ability and facilities to control the set points of temperature, humidity and ventilation independently and as accurately as possible.

In tests, Pas Reform recognised the importance of temperature for optimum embryonic development and defined eggshell temperature as the leading parameter for the design of incubation programs.

For optimum hatchability and chick quality we have found ¨C and therefore advise - that the eggshell temperature follows a natural pattern within a range of 37.6-37.9°C (99.7-100.2°F) during the first two-thirds of incubation and 38.1- 38.8°C (100.6-101.8°F) during the last days in the setter. Minor variations in this pattern may occur because of differences in egg types. In this way the hatchery manager can predict the time and uniformity of the hatch.

Single stage incubation facilitates optimum incubation programming, per batch and egg type, according to the natural temperature pattern. A single stage incubator should be divided into small, separate units, each with its own climate control.


1 In this article we discuss how temperature influences the differentation and growth of the embryo. The influence of temperature during the hatching phase is beyond the scope of this article.

2 The average eggshell temperature of a representative sample of eggs must be measured systematically by means of an infrared ear thermometer calibrated for measurements between 37 °C and 40 °C. It is essential that the eggshell temperature is measured in a running incubator, since the eggshell temperature will decrease or increase immediately, depending on the embryonic stage, once the incubator stops or the eggs are taken out of the incubator.

References available on request.

Source: Pas Reform Hatchery Technologies

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