The crucial role of ventilation in performance and thermoregulation of domestic fowl
Recent decades have seen significant progress in the genetic selection of fast-growing meat-type broiler chickens and turkeys. However, fast growth has been accompanied by inferior development of the visceral systems, which contributes to difficulties in coping with heat stress. This situation, in which growth rate and therefore heat production increases from year to year, demands an efficient and economical means to improve the acquisition of thermotolerance by fowls in hot climates.
It is important to understand the physical aspects of excess heat dissipation by poultry, in relation to the improvement of thermotolerance. This paper focuses on air velocity as a principal parameter that dramatically affects sensible heat loss and its contribution to the ability of acclimated poultry to maintain a favourable energy balance efficiently under hot conditions.
The studies reviewed in this paper demonstrated that:
a. Air velocity plays a major role in broiler energy balance at high ambient temperatures.
b. The optimal air velocity for maximising growth performance varies with the ambient temperature and at ambient temperatures below 30 deg C. Even quite low air velocities can cause chilling, which adversely affects broiler performance.
c. Air velocity affects turkey performance.
d. Genetic selection has improved growth performance, but to some extent at the expense of the broilers' ability to maintain a favourable energy balance.
The driving force for sensible heat loss by convection and radiation is the difference between the surface and ambient temperatures. Convective heat flux depends on the temperature difference between the body and the air, the area of contact, and the heat transfer coefficient. The average heat transfer coefficient depends on the body's geometry, the air's physical properties and the flow regime.
Air velocity dramatically affects the body weight of broiler chickens exposed to different environments. There is an ambient temperature turning point in the response of broiler chicken performance to ventilation. This turning point is about 30 deg C, below which increased air velocity probably caused chilling, which led to increased energy expenditure on maintenance, and consequently reduced growth performance. It can be further speculated that at low ambient temperature, even a low air velocity may negatively affect the broilers' ability to balance their energy losses.
In a previous study in which broilers were subjected to a temperature of 35 deg C, heat losses by radiation and convection were calculated (Yahav et al., 2004). The study showed that although heat loss by radiation did not differ among treatments, heat loss by convection increased significantly and linearly with increasing air velocity.
The main conclusion is that, under each and every set of environmental conditions, a specific fine-tuning of ventilation must be determined. It will be based on the age of the chicken, the density of the flock and the ambient conditions such as ambient temperature, relative humidity, ammonia and dust.
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Article made possible through the contribution of Australian Poultry Science Symposium (APSS)2007.