Barometric pressure declines with altitude, as does the partial pressure of oxygen and absolute humidity. Fresh ventilating air will tend to be colder and drier than at sea level.

Oxygen Availability

The oxygen content of air is always 21 per cent but reduced partial pressure at altitude provides less oxygen from a given volume of air. This pressure reduction results in lower levels of oxygen for the embryo, which is partially compensated by the embryo's higher capacity for binding oxygen to blood haemoglobin (Dragon et al., 1999).

At altitudes above 2000 metres, it can help to inject oxygen into the setter and the hatcher, to raise the oxygen level from 21 to 23-25 per cent. The main drawbacks of using oxygen are cost and safety. Its use may, therefore, be limited to hatching parent stock.

Water Loss

It is reasonable to assume that the drier air at altitude will result in increased moisture loss from the eggs. However, it is important to realize that breeder flocks adapt to altitude by producing eggs with a lower effective pore area. This offsets increased diffusion and therefore water vapour loss through the egg shell at any altitude remains the same as at sea level (Rahn et al., 1977).

Conclusions

The following three scenarios are considered:

1) Eggs produced at sea level: hatchery at altitude (1000-2000 metres)
Of the three scenarios, this is the least desirable because it will definitely result in reduced hatchability. Eggs produced at sea level have a relatively large effective pore area and will therefore lose more water at higher altitudes. To compensate, setters and hatchers should be operated at a higher relative humidity. This is best achieved by pre-conditioning the inlet air to a relative humidity of 75 per cent, with a temperature of 24-28ºC (optimum). At the same time, increase the ventilation rate from normal for sea level, to accomodate the reduced oxygen levels.

2) Eggs produced at same altitude as hatchery (1000-2000 metres)
In general, this will give good results. Ventilation rates should be higher than normal for sea level. During humid external conditions, increase ventilation even more, as humidity reduces oxygen levels in the air still further. This higher ventilation rate may cause reduced humidity in the setters and hatchers. To avoid constant humidifying, humidity set points should be lowered and the resulting more than optimal weight loss, e.g. 14-15 per cent, is preferred in this case.

3) Eggs produced at altitude; hatchery at sea level
Generally, this will give good results. The set points for relative humidity need to be reduced to achieve optimum weight loss as the eggs have a reduced effective pore area.

Advice

Exact set points for relative humidity are dependent several factors including altitude and egg shell conductivity (age of the flock, nutrition, genetics). It is therefore recommended that relative humidity set points are fine-tuned by weighing trays of eggs before setting and again at transfer at 18 or 18.5 days.

Optimum weight loss for good hatchability and chick quality is indicated in the table.

Table 1. Optimum weight loss for good hatchability and chick quality based on experience
Age breeder flock	Optimum weight loss (%)
Young flocks	10-11
Medium flocks	11-12
Old flocks	12-13

Alternatively the size of the aircell provides an indicator of weight loss. If during an egg-breakout it is observed that too many wet, fully developed embryos have failed to pip, this indicates insufficient weight loss and/or a shortage of oxygen. In this case, set points for relative humidity should be reduced and/or ventilation rate should be increased.

References
Dragon et al., 1999. Journal of Experimental Biology, 220:2787-2795
Rahn et al., 1977. Reduction of Pore Area of the Avian Eggshell as an Adaptation to Altitude.

For further information or advice, please contact the Pas Reform Academy.

August 2008