M.A. Mitchell1
P.J.Kettlewell2
1Roslin Institute,
Roslin, United Kingdom
2Silsoe Research Institute,
Silsoe, United Kingdom
Modern broiler production systems involve the rearing of largenumbers of birds on geographically dispersed sites and their subsequenttransportation on purpose-built modular vehicles to centralized processingplants. Transport represents a potential risk to bird welfare and productquality. Fuller understanding of the origins of transport stress andcharacterization of the birds' biological requirements allow the design anddevelopment of improved methods of transportation. This review considers therole of the transport thermal microenvironment in the etiology of physiologicalstress.
Physiological modeling has been employed to define acceptable rangesand limits for thermal variables upon the vehicle. Improved broiler transportvehicles (mechanically ventilated) have been developed from this knowledgeusing sound engineering principles, thus matching micro-environments to thebirds' physiological needs.
Introduction
Global production of poultry for meat increases year upon year.Thus, in 2002, world production of broiler chickens was approximately 45.5billion birds and for turkeys 661 million (FAOSTAT, 2003). Whilst the majorproducer nations are the USA, China and Brazil, substantial contributions aremade by many other countries. The European Union had broiler and turkey outputsof 5 billion and 245 million, respectively, during the same period whilstproduction in Australia reached its peaks at 416 million broilers, thecorresponding figures for turkeys being 7.4 million.
A striking feature of the geographical distribution of meat birdproduction is the range of climatic zones and conditions in which highlysuccessful industries have developed. Thus, poultry are reared in temperate,equatorial and sub-artic nations under tropical and arid conditions with a verywide spread of average, minimum and maximum temperatures. This success ispartly attributable to the provision of suitable birds by the major broilerbreeder companies but perhaps the most important factor is the matching of the"broiler house" environments and nutritional regimes to the birds'requirements. This allows expression of the birds' genetic potential for growthand efficiency.
Good control of the "in house" thermal environment isessential and ensures minimization of physiological stress and suppression ofdesirable production traits. In the production cycle of a broiler flock,regulation of imposed thermal loads and thus stress may be ensured throughoutthe growth period. However, at the point of slaughter the journey from the siteof production to the processing plant may constitute a substantial risk to thewell-being or survival of the birds and may compromise productivity throughmortalities, down grading and reduced product quality. A primary cause of theseproblems is the thermal microenvironment to which the birds are subjected intransit (Mitchell and Kettlewell, 1998; Mitchell et al., 2001).
Whilst environmental control in the poultry house is usually wellmanaged and efficient, little attention has been paid in the past to thetransport thermal microenvironment. The vagaries of "passive" vehicleventilation regimes, coupled to an economic necessity to transport largenumbers of birds on each journey, often leads to heat stress even in relativelytemperate external conditions. Similarly, poorly controlled and veryheterogeneous ventilation regimes, resulting in both over ventilation and underventilation under low external temperature climates, can precipitate extremecold stress or paradoxical heat stress in birds in those locations in thebio-load which are most vulnerable (Mitchell et al., 2001; Mitchell andKettlewell, 2002).
In order to address these issues a collaborative research projectexploiting the complementary disciplines of animal physiology, environmentalscience and bioengineering has pursued the objective of matching the broilertransport conditions to the birds' physiological requirements.
Methods
Physiological modeling: All experiments were performed upon 6-week old broiler chickens.Birds were held in commercial transport crates and placed in controlled climatechambers for a period of three hours (typical of commercial journeys).
Chamber temperature andrelative humidity were controlled throughout the experimental period to ± 0.2°Cand ± 5%. Various combinations of temperature-humidity were employed in therange 10-35°C and 30-95%. Temperature and humidity within the transportcontainers were continuously monitored using Tinytalk data loggers. Bird rectaltemperatures were taken immediately prior to and following exposure to eachthermal load.
Blood samples were obtained at these times and pH and pCO2determined immediately by means of an automated blood gas analyzer(Ciba-Corning 238). Thermoregulatory success (deep body temperature) andthermoregulatory effort (blood pH and gas disturbances) were correlated withthe actual imposed thermal load (temperature-humidity combination).
"Apparent Equivalent Temperature" (AET) was used as anindex of thermal load. This parameter is derived from the temperature, watervapour pressure and the psychrometric constant and describes the total heatexchange between a wetted surface and the environment.
q*app.= T + ( e/γ*)
where θ*app = AET
T = absolute temperature (K)
e = water vapour pressure (mbar)
γ* = corrected psychrometric constant (mbar K-1)
γ* = γ (rv/rh)
where rv = the resistance to water vapour transfer (sm-1)and rh = the resistance to heat transfer (sm-1).
Using the AET approach the combinations of temperatures andhumidities, which produce equivalent biological effects, were determined(Mitchell and Kettlewell, 1998; Mitchell etal., 2001). The relationships between change in deep body temperature,blood gas and acid-base parameters, indicators of cell pathology and AET wereestablished. The response patterns to thermal load allow definition oftemperature-humidity combinations imposing mild, moderate and severephysiological stress.
This approach has allowed identification of "safe", "alert" and"danger" combinations of temperature and humidity that equate to mild, moderateand severe physiological stress. The model thus permits the definition ofthermal comfort zones for broilers in transit as presented in Figure 1.
At temperature-humidity combinations yielding AET values between of40-45°C or less thermal stress will be minimal in transit. Attemperature-humidity combinations giving AETs between 40-45°C, moderate thermalstress will occur with some degree of hyperthermia and acid-base disturbances.At AETs of 65°C or greater, physiological stress may be deemed severe andmortalities will increase. Such thermal loads must be considered unacceptable.Under practical conditions, where relative humidities in the transportcontainers rarely fall below 70% because of obligatory water loss from thebirds, it can be recommended that the maximum "in-crate" allowable temperature,compatible with good welfare and productivity therefore, should be 26-27°C.
In parallel studies, a similar modeling approach has been adopted todefine the lower limits for dry bulb temperature consistent with good welfareand productivity in birds exposed to air movement, with both dry and wetfeather cover (Mitchell et al., 2001; Mitchell and Kettlewell, 2002).Further studies have measured metabolic heat and moisture production ofbroilers under a range of thermal conditions upon vehicles (Kettlewell et al., 2001ab). Integration of thedefined acceptable thermal envelope with knowledge of heat and moisture outputhas allowed calculation of ventilation volume flows necessary to dissipate heatand moisture to maintain acceptable "on-board" thermal environmentsunder different climatic conditions. These findings have formed the basis forthe design, development and implementation of mechanical ventilation systemsfor commercial broiler transport vehicles (Kettlewell and Mitchell, 2001ab).
In conclusion, physiological response modeling has been employed todefine acceptable ranges and limits for thermal conditions during thetransportation of broiler chickens. This novel experimental approach hasprovided data specific to transportation conditions and which are directlyapplicable to commercial practice. The optimum thermal envelope for broilercarriage has been defined in terms of the "physiological thermal comfort zones"and factors precipitating or contributing to the incidence of thermal stresshave been identified. The findings have been employed as the sound scientificbasis for improvements in vehicle design (in particular ventilation systems)and transport practices.
References
FAOSTAT (2002) http://apps.fao.org.
Kettlewell, P. J.,Hampson, C. J., Green, N. R., Teer, N. J., Veale, B. M. & Mitchell, M. A.(2001a) In: Proceedings of the 6th International LivestockEnvironment Symposium, Louisville, Kentucky, U.S.A., 21st-23rd May, 2001.Edited by Stowell, R. R., Bucklin, R. & Bottcher, R. W. pp 519-526.
Kettlewell, P. J.,Hoxey, R. P., Hampson, C. J., Green, N. R., Veale, B. M. & Mitchell, M. A.(2001b) Journal of Agricultural Engineering Research 79: 429-439.
Kettlewell, P. J. &Mitchell, M. A. (2001a) Journal of the Royal Agricultural Society of England162: 175-184.
Kettlewell, P. J. &Mitchell, M. A. (2001b) Engineering and Technology for a Sustainable World8: 13-14.
Mitchell, M.A. and Kettlewell, P.J. 1998. Poultry Science 77:1803-1814.
Mitchell, M. A.,Kettlewell, P. J., Hunter, R. R. & Carlisle, A. J. (2001) In: Proceedingsof the 6th International Livestock Environment Symposium, Louisville,Kentucky, U.S.A., 21st-23rd May 2001. Edited by Stowell, R. R., Bucklin, R.& Bottcher, R. W. pp 550-555.
Mitchell,M.A. & Kettlewell, P.J. (2002) In II Congreso International de Produciony Sanidad Animal, XXXIX Simposium de WPSA (Espanol) p63-73.
From Proceedings of the "2004Australian Poultry Science Symposium", new South Wales, Australia.
