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Factors Influencing Bird Heat Production in Tunnel-ventilated Broiler Houses

21 April 2015

US Poultry and Egg Association logo

New research at the University of Georgia reveals trends in bird heat production and loss that can help broiler growers and integrators to minimise heat stress in their birds during hot weather.

The objective of the current study – by Drs Brian Fairchild and Michael Czarick and funded by the US Poultry and Egg Association – was to evaluate heat removal from tunnel-ventilated broiler houses. This study used the commercial broiler house as a large calorimeter, which allowed bird heat production to be explored as a function of temperature, relative humidity (Rh) and air velocity.

The total, sensible and latent bird heat production was measured in a number of houses with market age broilers. Sensible heat loss, which results in an increase in house air temperature, is the heat dissipated by birds through heat transfer to the surrounding air. Latent heat loss is the heat lost from a bird through the evaporation of moisture from its respiratory system resulting in an increase in house humidity.

The following trends in bird heat production/loss were observed, which can be used by growers and integrators to minimise heat stress during hot weather.

1. During typical hot summer conditions, bird heat represents 97 per cent of the total heat production in a broiler house. This confirms that there is little to gain by increasing house insulation above what is typically recommended since the majority of the heat is being produced by the birds.

2. The total heat production per bird increased linearly with air speed for average air velocities between 350 and 525 feet per minute. The roughly 30 per cent increase in heat production is a result of the combination of increased bird weights and increased heat removal as air speeds increase.

3. The total heat production, on average, was partitioned into 40 per cent sensible and 60 per cent latent. This indicates that even in the presence of air velocities between 350 to 525 feet per minute that market age birds lose the majority of the heat they produce through the evaporation of moisture from their respiratory system. It is important to note that a 40:60 ratio of heat production is normal for birds and does not necessarily mean the birds are panting.

4. At high air velocities (450 to 550 feet per minute), latent heat production/loss decreased rapidly between the temperatures of 75 to 85°F as relative humidity rose above 85 per cent. Producers must be careful not to use evaporative cooling excessively even though it may result in lower house temperature because bird heat loss can be adversely affected.

5. The proportion between sensible and latent heat loss can vary with bird size, temperature, relative humidity and air velocity.

For birds in the 4-lb. range and an air velocity of 600 feet per minute, the ratio between sensible and latent heat production was closer to 50:50 than 40:60 (air temperature = 75 to 85°F). The higher proportion of sensible to latent heat production indicates that the combination of slightly higher air speed (600 versus 500 feet per minute) and smaller bird size can lead to more effective bird cooling. Since a 50:50 ratio was not seen in any of the houses with larger birds, it can be surmised that larger birds (6 lbs and more) could have possibly benefitted from higher wind speeds (over 525 feet per minute).

Temperatures below 75°F with air velocities of 600 feet per minute were shown to result in excessive heat production by the small bird (4 lb.), which could result in reduced bird performance.

6. Total bird heat production varied diurnally (day/night). The amplitude of the diurnal variation was influenced by the lighting programme and fan operation.

Shorter day lengths resulted in a decrease in night-time heat production and a corresponding increase in day-time heat production which could result in increased potential for bird heat stress.

Increasing night-time air velocity resulted in an increase in night-time heat production and a corresponding decrease in daytime heat production. This in part could help to explain why other studies have shown increased bird performance during hot weather when high air speeds are maintained during the night.

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April 2015 

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