Much is written on the issues surrounding housing and environment issue of African poultry, but because no single ‘hard and fast’ situation exists there is not one single solution that can fit all
This is due to the sheer size of the African continent experiencing such a huge range of climate and weather and because production is now split between traditional free range and open-house farming and closed large and more intensive poultry production systems. Last but not least, there is the related, compounding and sometimes confounding factor of relative humidity which cannot be divorced from temperature on its own.
Conditions are so varied. For instance, poultry raised on farms in lowland coastal Nigeria or Kenya and within the Equatorial belt will experience year round temperatures of 30+ ºC, and relative humidity levels persistently over 80 per cent. On the other hand those in Sudan in the north, and South Africa in the south, will experience summer temperatures climbing to over 40°C while falling to 0ºC in winter. Seasons apart, diurnal (day/night) temperature fluctuation is wide while humidity can fluctuate just as widely during the twenty four hour day and at different times of the year. Poultry farming at elevation such as the East African Highlands complicates even more.
Avian biology and heat stress
Be that as it may, there are specific physiological issues around birds that make poultry more susceptible to heat stress than other non-avian livestock. Acute complications for domesticated chickens with heat stress compared to most other farm animals is down to a naturally high body temperature of around 41°C and slightly higher at night. Natural body temperature of chickens is close to the temperature at which cellular enzymes (protein in nature) start to denature (lose functional shape).
This leaves little leeway for temperature between normal body function and organ failure and death, and is why domesticated chickens are generally much more tolerant of low temperature than high temperature. In other words there is much less temperature slack to ‘play with’ at the heat stress end compared to the cold stress end.
All animals have mechanisms enabling them to dissipate heat but in birds these are comparatively inefficient and ineffective. Poultry is at a comparative disadvantage when temperature is very high and even more so with accompanying high humidity. The air is already saturated with water vapour making it all that harder for birds to lose heat through evaporation of liquid body moisture into atmospheric water vapour.
Heat loss methods
Birds will attempt to maintain their normal body temperature using a combination of so called ‘sensible’ heat loss methods, whereby heat goes from body to environment through radiation, convection and conduction. In addition, heat may also be lost to some extent by evaporation of water from the skin tissues. This is called latent heat loss because liquid water takes heat from the bird’s body (latent heat of evaporation) for the change of state (liquid into vapour).
One particularly significant mechanism in heat regulation is controlling blood flow to the peripheral tissues for rapid heat loss, and particularly the comb, wattles and legs. The mechanism is called peripheral vasodilation and functions by increasing blood flow to transfer heat from the deep-seated body tissues and organs to the surface tissues, from where it is lost to the environment.
Heat generation and loss in birds is all a question of balance. For birds to remain in good working and productive order heat loss via these routes must be equal to the heat produced within the bird’s body from digestion, absorption and metabolism of feed. In this way the bird can balance its energy and heat production to maintain body temperature within normal range.
Consequences of heat stress
Definite and distinct changes in bird behaviour become apparent as ambient temperature within the poultry house rises. Key indicators to watch out for are:
The bird’s wings are held away from the body to promote heat loss;
Marked reduction in the activity of birds as they strive to limit heat production of heat in the body from muscular effected movement;
A noticeable increase in consumption of water accompanied by a fall off in feed intake.
Around three-quarters of feed ingested by birds and metabolised into energy will end up as body heat and will therefore have to be discharged into the environment. For this reason reduced feed intake is a crucially important physiological safety mechanism for the bird when confronted with heat stress and the need to reduce it.
Conduction, convection and radiation
With a rise in temperature birds suffer reduced ability to lose heat by conduction, convection and radiation and will therefore attempt to lose heat by panting because this assists evaporation of water from the moist epithelial linings of the respiratory system. Initially the most important part of this evaporative process is rapid passage of air in and out of the mouth.
But if temperature climbs even higher water may also evaporate from air sacs within the lungs during this so-called panting. This lowers blood carbon dioxide level to bring on a condition called respiratory alkalosis, a condition that can seriously impair broiler performance, especially when accompanied by decreased feed intake, due to the decreased potassium and minerals balance.
A point can be reached when birds’ are unable to get rid of heat as quickly as it is produced. The bird’s metabolism essentially gives in and the bird gives up, lying prostrate and gasping on the floor, leaving it weaker and susceptible to death from respiratory, circulatory and/or metabolic imbalance.
Open and closed house management
To produce poultry in the tropics farmers must ensure and maintain a good working environment inside the poultry house. This provides birds with adequate opportunities to dissipate excess body heat and stay comfortable and stress free.
House ventilation is clearly the key through continuous and adequate exchange of air between inside and outside, efficient dissipation of heat from the bird’s body by close control of air speed and sufficient evaporative cooling of the air being drawn into the house.
Proactive ventilation is clearly not so important for open-house systems because air-circulation is largely free and unimpeded. Producers should still follow basic guidelines on house location and construction including orientation, white painted roofs for maximum heat reflection and sufficient overhang of roofs. Producers should judiciously plant trees for shade without impeding wind movement and plant low-profile cover crops like cucurbits (pumpkin and cucumbers) around the house to mitigate ground temperature without impeding air flow.
Closed house systems are in a completely different situation with higher overall numbers and intensities of birds and potentially massive build-up of toxic waste vapours including ammonia as well as high temperature and humidity.
In these circumstances properly installed and operated tunnel ventilation will provide sufficient air exchange and adequate acceleration of air movement over the birds’ bodies. Air flow at optimum speed, known as wind-chill or cooling effect, assists heat dissipation and the loss of heat by convection.
Absolute temperature measurement from a standard thermometer is insufficient to predict whether or not a bird is comfortable within its environment, because combination of absolute temperature and speed of air flow over the bird determines comfort. This becomes particularly important at higher humidity levels when any actual loss through panting is reduced because the air is already saturated with water vapour. It is compensated for by extra heat loss through convection by increasing speed of air flow over the birds.
Evaporative cooling through pad cooling systems to reduce temperature of incoming air is an important tool in reducing the absolute temperature of the poultry house. The effectiveness of an evaporative cooling system will depend on the relative humidity of air drawn into the system. Cooling effect is greatly reduced at 70+ per cent humidity and such systems not recommended once relative humidity reaches 80 per cent.
That said tunnel ventilation and evaporative cooling in combination will provide the necessary control of poultry house environments prevailing in tropical Africa. Indeed proper use and control of such systems is the key to profitable poultry production in such climates.
In the many situations where open housing is still employed natural ventilation is optimized via the design and orientation through exploitation of the local topography and environment. Internal standing fans giving some measure air flow control are established as essential equipment for this type of house.
Bird management in closed environments
Manipulation of stocking density is essential in hot climates to minimise heat stress. Reduction in stocking density minimises number of birds producing heat over unit area and correspondingly decreases quantity of that has to be eliminated from the house to maintain equitable temperature. Attempts should be made to predict, plot and plan for hot weather periods depending on the type of house in use. During hot weather periods maximum stocking density for market-ready broilers should not exceed 25 kg live weight/m2 for controlled environment houses and 17 kg/m2 for open-sided houses. These are ‘general rule of thumb’ figures that vary depending on capacity and efficiency of the cooling systems used.
Dr Terry Mabbett