Rearing chickens at high altitudes

The term “altitude” is commonly used to mean the height above sea level. The high altitude areas are characterised mainly by low atmospheric pressure resulting from two competing forces, namely gravity which causes the air to be as close as possible to the ground, and the heat content of air which causes the molecules to bounce off each other and expand. In this case, the number of oxygen molecules per breath is reduced, but to a varying extent depending on the magnitude of elevation.

Birds reared in high-altitude areas often encounter a number of production, health, and mortality problems. These may not be solely attributed to the O2uptake per se, but may also be attributed to other climatic conditions linked with high altitudes such as low temperature and humidity. Much of these problems could, however, be alleviated when proper management and feeding conditions are duly adopted. This is particularly true in areas of very high altitudes, where the adaptive mechanisms of birds may not be sufficient enough to cope with such adverse conditions.

High altitude and hatchability
Research has shown that hatchability of chicken eggs is reduced by 10% at altitude of 305 meters, and by 30% at 2,130 meters above sea level. Three hatching-related factors come into play at high altitudes:

1. Reduced O2 availability or atmospheric O2 tension.

2. Excessive loss of carbon dioxide (CO2).

3. Excessive water/weight loss by the incubating embryos.

Field experience indicates that hatchability at high altitudes may improve if the setter cabinet temperature is increased by 0.15ºC and the relative humidity is kept constant at 60%. It is also important to improve O2 supply to the embryos during incubation. The most practical way to do this and to restore normal hatchability at high altitude is to inject O2 directly into the setter cabinet. An O2 concentration around 23% will significantly increase hatchability.

Alternatively, opening the setter air inlets more and/or for longer periods will allow more air flow into the setter box so that the embryos will be exposed to more O2. The washing of eggs to remove cuticle may also help improve hatchability at high altitude by increasing permeability of the eggshells and passive diffusion of gases in and out of the egg. This approach, however, does increase the risk of contamination and weight/moisture loss during incubation, although it has been tried successfully in some cases.

Reduced fetal growth
At high altitude, the prevailing low oxygen concentration has detrimental effects on embryonic growth and development of chickens. This could be attributed to the decreased efficiency with which the yolk sac is converted to embryonic tissues, and/or the decreased fetal resource uptake by making the yolk sac unusable.

The extent to which fetal growth is reduced is so much influenced by the maternal residential location. In one study, the fetal mass was reduced by 45% when the breeder flocks were kept at sea level and their eggs incubated at high altitude, and by only 22% when the breeder flocks were kept at high altitudes and their eggs were also incubated at high altitudes, suggesting hereditary effects on adaptability to lower oxygen concentration. Adaptation can develop through a greater ability of the embryo to form hemoglobin in the red blood cells and through the development of a greater heart volume to compensate for the lower oxygen level, in addition to the greater utilisation of the yolk sac which acts as a nutrition resource, as indicated above.

Growth performance
High altitudes also affect growth rates at post-hatching stages. This is probably due to the decreased temperature at high altitudes which often leads to high metabolic rates, with a resulting increased demand for oxygen. At normal altitudes, the increased metabolic rate could be a positive factor affecting growth, but at very high altitude the lower oxygen uptake would conflict with the higher oxygen demand. This would inevitably result in decreased performance in terms of growth and other biological functions.

The reduced post-hatching growth of chickens could further be attributed to the incidence of ascites and other disease problems induced by high altitude, as will be discussed later.

Health and body condition
The low partial pressure of oxygen at high altitudes causes hypoxia (inadequate oxygen supply) with lower lung activity and higher arterial pressure. This affects the working of the heart and may lead to heart failure and accompanying symptoms called ascites. In order to solve this problem, addition of oxygen has been applied with success, but the costs of this solution may be too high. Increasing the ventilation rate could provide more oxygen, but as the humidity level at high altitudes is naturally low, humidity is lowered further and other problems may arise.

The low humidity at high altitudes may affect the bird’s, health and body condition through different ways. For example, the mouth and nose become increasingly more irritated, and skin becomes so dry and itchy and may sometimes develop cracks. The mucus membranes lining the nasal and throat passages may also become dry and cracked, thereby giving a direct path for germs and bacteria to enter the blood stream and cause a number of disease problems.

Physiological adaptation
During continuous exposure to high altitude, birds develop several physiological responses to make it possible to live in a low O2 environment (Table 1). The increases of red blood cell number (RBC) and amount of hemoglobin (Hb) are the most important hematological acclimatisation which appears after 1-2 weeks. As a result, there is an increase of oxygen-carrying capacity of blood which acts as a compensatory mechanism to the stimulus of reduced oxygen saturation. Further, at high altitude, there is a decrease in mean cell volume (MCV), so the total surface of RBC is enlarged, which is advantageous for hemoglobin to bind oxygen, and helps prevent the increase of blood viscosity that results from polycythermia.

Adequate oxygen transfer
The reduced environmental oxygen availability at high altitude stimulates higher respiration frequency (ventilation), which plays an important role in maintaining adequate oxygen transfer to the blood. In one study, the respiration rate per minute at sea level was 31.0, and was increased to 35.3 at 3,200 meters altitude. This mechanism also increases the exhalation of CO2 and thereby serving to counteract the acid-base problems arising from hyper ventilation.

At high altitude, birds may also develop special mutations that alter the amino acid residues in Hb and hence increase O2 affinity. The higher affinity in this case may increase oxygen saturation of blood and hence compensates for the reduced partial pressure of oxygen. This latter aspect, however, is considered to be a characteristic of only those birds which are genetically adapted to high altitudes and may not be generalised to all breeds of chickens.

Feeding and management
For high-altitude chickens, diets should be duly supplemented with anti-oxidants such as vitamin E. This helps improve performance by alleviating much of the oxidative stress problems caused by reduced oxygen partial pressure, increased ultra-violet light, and increased metabolic rates at high altitudes.

A study was conducted in a Himalayan area north of India, where altitude varies from 3,050 to 3,600 meters above sea level, with the atmospheric oxygen pressure being 30% short relative to the sea level. In this study, broiler chickens were fed on 200 mg of vitamin E per kg of feed for six weeks, and had better performance compared to the control chickens receiving diets devoid of vitamin E (Table 2).

The improved performance with vitamin E supplement could be attributed to the improved feed intake and utilisation of the supplied nutrients, particularly protein which is essential for health, body weight gain and survivability. The vitamin also improves blood characteristics, mainly due to its effects on hematopoietic organs and erythropoiesis, thereby increasing RBC and Hb levels and helps adaptation to high altitudes.

The supply of B vitamins such as thiamine, riboflavin and niacin should also be considered at high altitudes. These vitamins help improve performance by releasing energy from the feed and hence compensate for the low energy intake observed under high altitude stresses.

Animal origin sources
It is also important to supply adequate amounts of iron at high altitudes where atmospheric pressure is lower and there is less oxygen in each breath. In this case, iron would increase the oxygen-carrying capacity of blood and facilitates its utilisation by the cells, and would hence improve performance and adaptation to high altitude.

It should be noted here that most conventional cereals and legumes used in poultry feeding contain non-haem iron which is poorly absorbed and utilised. Therefore, the diet of high-altitude birds should be duly manipulated so as to contain haem-iron sources such as those from animal origin, supplying at least 80 mg of iron per kg of feed, with the protein and energy components to be kept at optimal ratios for each production stage.

Reduced metabolic rate
In high altitude cases where the ascites problem is likely to prevail, it may be beneficial to adopt the feed restriction system (50% of the energy required to support normal growth rate), especially for birds exposed to low ambient temperature from three weeks onwards (Table 3).

This is probably due to the reduced metabolic rate during the restriction period, suggesting a possible acclimatisation effect on development of ascites under cold/high altitude conditions. Alternatively, the avoidance of low temperature by slightly heating the houses could be an effective means of reducing the incidence of ascites at high altitude, but may not be recommended for high altitude farmers who cannot afford to heat their broiler houses during cold seasons.