Thermal manipulation during a chick's embryogenesis is based on the following hypotheses:
a. It is possible to induce long-lasting physiological memory based on epigenetic adaptation
b. A definition of long-lasting memory is: An alteration in the hypothalamic threshold response to
changes in the environment
c. During sensitive periods in embryogenesis, thermal manipulation involving exposure to high
temperatures for specified periods will elicit improved thermotolerance during the birds' life span.
It is well documented that thermal manipulation during the first week post-hatch results in improved lifetime thermotolerance. Thermal manipulation during embryogenesis has been shown to achieve improvement in thermotolerance up to 10 days of age. However, recent studies have shown that broiler chickens do not exhibit any thermoregulatory advantage during thermal challenge at later ages.
These results raise the following questions:
a. Could it be that it is not possible to induce long-lasting thermoregulatory
memory by thermal manipulation during embryogenesis?
b. Could fine tuning of thermal manipulation result in long-term improvement in thermotolerance?
Further research is currently underway, aiming to shed light on these questions, and there is evidence that fine tuning might lead to achievement of the targeted goal.
Recent decades have seen significant development in genetic selection of meat-type poultry (broilers and turkeys). This has led to rapid growth, with increased feed efficiency and metabolic rate, and hence has provided the poultry industry with large, rapidly growing birds. Such development necessitates parallel increases in the size and enhancement in the efficiency of functioning of the cardiovascular and respiratory systems. However, inferior development of such major systems has led to a relatively low capability to balance energy expenditure and body water balance under extreme environmental conditions. Hence, acute exposure of chickens to extreme conditions (hot or cold spells) has resulted in major economic losses (morbidity, mortality and development of metabolic disorders).
It is estimated that the global mean surface temperature increased by 0.8 and 1.7 deg C during the 19th and 20th centuries respectively (US National Climatic Center, 2001). Scientists expect that the average global surface temperature will rise by 0.6-2.5 deg C during the next 50 years. This situation, in which growth rate (accompanied by increased metabolic heat production) increases from year to year and the future projected increase in global surface temperature, demands an efficient means of economically improving thermotolerance in broiler chickens.
Birds are homeotherms, and therefore, are able to maintain their body temperature within a narrow range. An increase in body temperature above the regulated range, as a result of exposure to environmental conditions and/or excessive metabolic heat production, may lead to a cascade of irreversible thermoregulatory events that could be lethal for the bird.
To sustain thermal tolerance and avoid the deleterious consequences of thermal stresses, three direct responses are elicited: the rapid thermal shock response, acclimation, and thermal manipulations based on epigenetic adaptation during the perinatal period.
The last strategy is based on the assumption that environmental factors, especially the ambient temperature, have a strong influence on determination of the 'set-point' for physiological control systems during 'critical developmental phases'.
For more of the article, please click here.
Article made possible through the contribution of Australian Poultry Science Symposium (APSS) 2007.