The four key points for successful incubation are:

• egg and incubator hygeine
• consistent temperature
• controlled relative humidity range
• egg care

Egg and incubator hygiene

Proper hygiene is essential to achieve good hatching results. Poor hygiene causes chicks to die in their first 10 days of life.

Only clean eggs should be used for incubation. Dirty eggs are peotential carriers of diseases that thrive and multiply in the ideal heat and moisture conditions of the incubator. If you need to incubate dirty eggs, wash them first in warm water (44-49^oC) that contains disinfectant at a rate recommended by the manufacturer (most household disinfectants are suitable), and dry the eggs quickly after washing using separate paper towels.

Do not soak eggs for longer than four minutes to avoid affecting fertility and do not soak eggs in cold water, as it encourages bacterial penetration through the egg shell.

Fumigating eggs immediately after collection also helps with hygiene. A suitable fumigant is formaldehyde gas, which is made by mixing 1 part (by weight) of potassium permanganate (Condy´s crystals) with 1.5 parts (by volume) of formalin (see Table 1 for the correct amounts for each application). Place the chemicals in a dish on the floor of the incubator. Place the Condy´s crystals into the dish first and then pour the formalin over it. Shut the incubator door quickly and vacate the room.

For proper fumigation, run the machine normally with the correct temperature and humidity. After 20 minutes, open the vents or the door and air the machine for a few minutes. Again, vacate the room.

WARNING: Formaldehyde gas forms a dense cloud, which is dangerous for people. Do not inhale. Wear eye protection and rubber gloves when handling formalin.

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Consistent temperature

A hen's normal body temperature varies between 40.5 and 41.7^oC, depending on the bird and her degree of activity at the time. The optimum temperature at the centre of an incubated egg is approximately 37.8^oC. When hatching under a broody hen, the upper surface of the egg may reach 39.2 to 39.4^oC but the egg's centre will not exceed 37.8°C.

In modern fan-forced incubators, the manufacturer's recommended temperature setting is between 37.5 and 37.64^oC. The lethal temperature for eggs is 39.4^oC. The constant and rapid air movement in this type of incubator keeps the eggs' temperature the same as the incubator's.

An embryo's heat production increases as incubation progresses. The temperature increase is greatest during the last two days due to embryo activity. Egg temperature rises up to 2^oC above the incubator's ambient air temperature, which is why the temperature is often lowered by up to 1^oC.

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Controlled relative humidity range

Relative humidity is a measure of the amount of moisture in the air. Althought correct relative humidity is necessary, it can fluctuate more widely than the temperature without serious repercussions. However, low humidity can cause difficulty in small incubators.

An egg loses 11-13 per cent of its inherent moisture during incubation. If humidity is too low, the moisture loss is excessive, hatching is delayed and many embryos fail to hatch even if they go to full term. If humidity is too high, the chicks tend to be forced out early and are wet.

A relative humidity of about 60 per cent during incubation is satisfactory. On day 19, raise the humidity to about 70 per cent for hatching. Then, towards the end of day 21 when all chicks that are likely to hatch have hatched, reduce humidity to 60 per cent. This enables chicks to dry off before being taken out.

Use a wet-bulb thermometer to measure the relative humidity in fan-forced incubators. At an incubator temperature of 37.5^oC, a wet-bulb reading of 29.4-30^oC is desirable. However, a wet-bulb reading of 32.8^oC is best at hatching. (see Table 2 for the correct wet-bulb readings for a given humidity at the temperature of incubation).

Place trays of water inside the incubator to create humid conditions. The water's surface area rather than depth influence humidity. You can control humidity by opening or shutting ventilation holes, using the water tray's position or moving slides over the water tray.

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Egg care

For egg selection, see Incubation - producing and selecting eggs

Rough, careless handling or prolonged delays that result in chilling may cause embryo deaths during the eggs' transfer to the hatching compartment.

Positioning the egg

The ideal position for an egg during incubation is either on its side or with the broad-end axis at 45° to the horizontal. This provides maximum room for the embryo to fully develop and the best exposure of the air cell for adequate respiration. Eggs incubated narrow end up show a higher proportion of dead-in-shell.

Turning eggs

The embryo must be gently and frequently moved within the egg to prevent it settling and sticking to the shell.

The need for frequent turning is greatest in the early stages of incubation. You should turn the eggs at least three times a day and may turn them every 15 minutes. Turn the eggs in the opposite direction each time. Turning the eggs the same direction interferes with the centring action of the chalazae, which can cause high embryonic mortality. No turning is necessary in the last three days of incubation (during the hatching stage).

With non-automatic machines, you need to turn the eggs an odd number of times. This ensures that the embryo does not spend a long, unturned period (usually during the night) lying on the same side. If turning by hand, a pencil mark on the side of the egg is a useful guide.

Fan-forced incubators

For best results, you must meticulously operate the incubator and follow the manufacturer's instructions.

Start the machine about 24 hours before use to enable it to adjust to the correct temperature. For best results, place the incubator in a room with little temperature variation and away from direct sunlight. Ventilation should be adequate and the room free from draughts. Test all equipment before beginning incubation.

There are slight differences between the same makes of incubators and the environment they operate in. Each incubator requires adjustment. To overcome these differences, observe the results from your first few hatches and adjust the controls according to the results. You must maintain uniformity in your incubation program to get most of the clutch to hatch on the recommended day. (If chicks come out too early, the temperature may be too high and vice versa.)

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Incubation faults and causes checklist