Incubation

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Fertile Eggs

The nucleus of the female cell is a small light or white speck about the size of a pin head, and is located on the top side of the yolk. Here the microscopic male sperm cell finds lodgement and the cells are united to form the embryo.  A fertilised egg is characterised by a white ring 3-4 millimetres in size on the yolk surface (germ cell), whereas an infertile egg is characterised by a single white speck of about 2-3 mm diameter.

Fertile Egg Storage

Fertile eggs should be clean and dry and stored between 12-15 degrees Celsius at a relative humidity of 75% with the small end down.

Eggs should be turned by 90 degrees once to twice daily.  Optimal hatchability is achieved  in fresh eggs less than 10 days old, but reasonable hatchability can be obtained in eggs up to 14 days of age. Fertile eggs should maintain a relatively constant weight with minimal weight loss during storage.Temperatures above 25 degrees Celsius and can initiate cellular replication of the germ cell on the yolk of the fertile egg and will increase embryonic mortality and reduce hatchability.  Temperatures below 10 degrees Celsius can inactivate the germ cell.

Principles of Artificial Incubation of Fertile Eggs

The four main essentials of incubation of good quality fertile eggs are the;

• Correct and even temperature controlled by a thermometer or thermocouple.
• Correct humidity controlled by ventilation rate and water application
• Correct oxygen and carbon dioxide concentrations controlled by ventilation
• Turning of fertile eggs by approximately 90 degrees several times per day by manual or automatic means.

The most important point is to closely adhere to the incubator manufacturer’s operation instructions.

Incubation Temperature Range and Variation

The temperature requirements for incubation are described in Table 1. (below) and most incubators have a temperature variation of 0.2-0.4 ⁰ C for effective incubation and high hatchability.

Tolerances to temperatures above recommended temperatures (Table 1.)  are only about 1⁰ C for high temperature, before there is significant embryonic mortality, and tolerances to low temperatures are similar.  Embryos are much more susceptible to temperature variation in the early and late phases of incubation.

Incubation Relative Humidity Range and Variation

The maintenance of consistent relative humidity is more difficult during incubation and can only be maintained constant by ventilation rate, using adjustable ventilation apertures and by surface water and water sprays during incubation. The tolerances to different ranges of humidity are greater than temperature, but there are negative consequences observed with humidity below 40% and above 90%.  Good hatchability is maintained by maintaining relative humidity at approximately 50-65% until the last 3 days of incubation and should then increase to between 70-90% just prior to hatching.

Ventilation and Carbon Dioxide/Oxygen Concentration

Embryonic growth is optimised at an air concentration of carbon dioxide of 0.4% and embryonic growth is depressed and mortality increased with carbon dioxide concentrations above 1%. The normal atmosphere contains 21% oxygen and 0.04% carbon dioxide. The hatched chick is most susceptible to oxygen deviation followed by the pipped chick and the embryo in the intact egg, and this implies that ventilation rate and carbon dioxide concentration is most critical in the late phase of incubation.

Egg Turning and Rotation

Egg rotation or turning is required to ensure that the embryo developing on the yolk does not adhere to the shell membrane.  This phenomenon of adherence to the shell membrane commonly occurs during fertile egg storage and during early incubation (first week). The turning process allows the embryo to revolve and slide in the inner white and provides access to additional nutrients for embryonic development.  Egg turning should be undertaken 3-6 times per day and an uneven number of rotations are better so that the eggs are not in the same position for a longer period.  Most incubators rotate the eggs by approximately 90 degrees.

Table 1.  Incubation Period and Operating Conditions for Fertile Eggs of Domestic Birds in Forced Draught Incubators

Requirement	Fowl	Turkey	Duck	Muscovy Duck	Goose	Pheasant	Guinea Fowl	Quail
Incubation Period days	21	28	28	35	28	23-28	28	23-24
Incubation Temperature °C	37.6	37.4	37.5	37.5	37.4	37.6	37.6	37.6
Wet Bulb Temperature °C	29.4-30.5	28.3-29.4	28.8-30	28.8-30	30-31.1	30-31.1	28.3-29.4	28.8-30
Relative Humidity %	56-62	51-56	53-60	53-60	60-65	60-65	51-56	53-60
No Turning after day	18	25	25	31	25	21	25	21
Incubation Temperature last  3 days °C	37.4	37.2	37.3	37.3	37.2	37.4	37.4	37.4
Wet Bulb Temperature °C Last 3 days	32.2-34.4	32.2-34.4	32.2-34.4	32.2-34.4	32.2-34.4	33.3-35	32.2-34.4	32.2-34.4
RelativeHumidity %	70-83	70-83	70-83	70-83	70-83	76-90	70-83	70-83

For still air incubators add approximately 1 ⁰ C to operating temperatures recommended in table 1. For still air incubators the thermometer is normally located at the top of the incubator and there is a marked temperature gradient from the top of the incubator to the bottom.

Candling of Incubated Fertile Eggs

After 5-8 days of incubation the eggs should be examined using a candling light to examine the embryo for the “spider web like” blood vessel development and a dark spot. Infertile eggs are obviously clear with no evidence of blood and early embryonic death is noted by the presence of a blood ring surrounding the yolk. Infertile and early dead embryos are removed at this stage. Candling can also be undertaken at 18 days of age, where the embryo is clearly visible with a distinct dividing line between the embryo and the air cell.

In large commercial incubators candling is not normally undertaken and there is a high reliance on fertility and egg hygiene to maintain viable embryos.

Weight Loss during Incubation

The progressive weight loss of fertile eggs can also be objectively monitored to improve incubation success and the data below (Figure 1.) is a good guide.   Weight loss patterns should be monitored and the ideal weight loss (13%) achieved if possible.

Main Incubation Failures

Modern electronic technology has been developed to manage temperature and humidity variation during incubation and these parameters are monitored consistently with records of incubation settings able to be analysed in relationship to hatchability and embryonic mortality. Analysis of dead in the shell embryos is also a valuable problem solving tool in refining incubation practice.

Early hatched, weak chicks, unhealed navels, unabsorbed yolk sacs, crooked toes, crossed beaks and a high proportion late dead in the shell, indicates hot incubation.

Late hatching of large soft chicks which are slow starting, and chicks with wry necks, is characterised by cool incubation.

Large numbers of unhatched, unpipped chicks and live trapped embryo’s is characteristic of high humidity. High humidity is also characterised by large chicks, coated with albumen.

Shells chipped most of the way round egg shell , but chick exhausted trying to get out, and chicks glued to the shell indicates low moisture or low relative humidity. Small weak chicks with large air cells also characteristic of low humidity.