Embryonic “temperature training” for  robust chicks

In modern poultry production, the transfer of up to two million chicks per week from a hatchery to broiler farms is becoming typical. More than 30% of the chick’s total lifespan is spent in incubation, placing the hatchery in a critical role for overall performance throughout the poultry value chain. During ‘critical periods’ of incubation, environmental factors, particularly incubation temperature, may influence embryonic development with long-lasting effects on post-hatch performance.

Exposing Ross 308 broilers to short-term, mild temperature stimulation during the last four days of incubation produced the following results: improved hatching rate, increased male to female hatchling ratio, increased body weight at slaughter in male broilers and improved feed conversion rates in male and female broilers.

To improve the protocols for temperature stimulation (“temperature training”) in practice, Humboldt-University, Pas Reform Hatchery Technologies, and the FLI, Federal Research Institute of Animal Health, Institute of Animal Nutrition, in Braunschweig are collaborating in a study of so-called “Circadian Incubation”.

The aim of this collaboration is to improve robustness (adaptation to environmental variations, expressed as good health, welfare and performance) in high yielding poultry breeds by embryonic temperature stimulation (“temperature training”).

Taking a sauna regularly
“Temperature training” is comparable to taking a sauna regularly. It flexes thermoregulatory function, to improve performance in response to temperature variation. The incubation environment creates a similar opportunity to impact post-hatch development, health and performance and could hold an important key to life-long robustness (adaptability to environmental variations).

In broilers, study has shown that such temperature training, or Circadian Incubation, during the last four days before hatching, improves robustness. This has a positive effect on performance throughout the bird’s life. Temperature profiling that includes short-term variation exclusively during the final days of embryonic development may, therefore, be highly relevant to improving robustness, health, welfare and performance in poultry and an important development for the future of commercial poultry incubation.

Efficient birds
Modern poultry production requires robust birds that grow uniformly and efficiently. Efficient birds are resistant to stressful conditions, eg. variation in environmental conditions, using only small amounts of nutrient to maintain basic physiological functions, eg. controlling body temperature.

Robustness is a health criterion that is influenced by incubation temperature management. Changes of just 1°C from optimum have a major impact. Hatching results in turkeys and broilers show, for example, that in turkey embryos, overheating reduces hatchability. The extent to which this occurs depends on the time frame during embryogenesis used and the degree and duration of the changes in temperature. Conversely in poultry, specific body functions may be improved by the manipulation of incubation temperature during different phases of embryonic development. Muscle development, for example, is stimulated by short-term temperature changes during early or later incubation. Changes in incubation temperature may also induce post-hatch adaptation to specific temperature ranges (warm- or cold adaptation).

Rapid growth
Genetic selection for poultry meat yield is strongly directed towards rapid growth and increased feed efficiency. These traits increase metabolic heat production during embryonic development and post-hatch development, with the life-long effect that the bird’s ability to balance energy expenditure and control body temperature under changing environmental conditions is reduced. This can result in major economic losses further along the production chain.

Incubation programmes that can be configured to deliver daily temperature variation are not widely available. Questions of interest to the study therefore included identifying the optimal timing, intensity and duration for temperature stimulation during incubation.

Thermoregulatory system
To identify optimum timing for temperature stimulation during incubation requires extensive investigation of the development of regulatory systems in poultry embryos. Within the hierarchy of regulatory systems, the thermoregulatory system is on a higher level. The goal of temperature regulation in warm-blooded (homoeothermic) animals is the maintenance of stable core body temperature under most conditions. The thermoregulatory system integrates the activities of all physiological systems to maintain constant body temperature.

Under less than ideal climatic conditions, for example, most nutritional energy is used to maintain basic physiological functions and therefore not available for performance. In all poultry species, the thermoregulatory mechanisms develop before hatching (prenatally). This is essential for quick maturation of temperature regulation in the early post-hatch phase, which influences overall performance.

Equipped to respond
Before hatching, the embryo’s regulatory systems develop from open loop systems without feedback control, into closed control systems with feedback mechanisms: a ‘critical period’ in the development of physiological control systems. During this time, manipulation of the incubation environment is known to have life long consequences. The “training” of body functions is only possible when a closed control system can continuously feed back information on the training “experience” to the regulatory centre in the brain.

Our studies concluded that temperature stimulation (“temperature training”) is most effectively applied during the final days before hatching. During this period, the prenatal maturation phase, avian embryos have well-developed thermoregulatory mechanisms and well-developed physiological systems to control other body functions. During this phase, the embryo is equipped to respond to environmental stimuli. These responses produce long-lasting effects, by ‘imprinting’ into body functions.

Imprinting of body functions
‘Imprinting’ describes a fundamental process of life, which occurs during limited periods of embryonic development and early phases of life. The imprinting of body functions during embryonic development has effects that last into adult life. In his classical studies on newly hatched goslings, Konrad Lorenz analysed in 1935 already the development of social binding, applying the term ‘imprinting’ to describe this process. One of his major hypotheses was that imprinting occurs during limited and severely restricted ‘critical periods’ in early life.

Later in 1974/1975, Günter Dörner, a pioneering developmental neuroendocrinologist, developed a general origination concept of the ‘epigenetic’ perinatal programming of the lifetime function of fundamental regulatory systems. In this concept, hormones as well as neurotransmitters and cytotokines (as immune cell hormones) act as carriers of environmental information to the genome during ‘critical periods’. Ultimately, they too are acting as epigenetic factors. In the avian embryo, the development and maturation of physiological control systems is such a ‘critical period’ and therefore sensitive to ‘imprinting’.

Life-long set point
In our hypothesis, during the development of physiological feedback mechanisms, an ‘imprinting’ of the regulatory systems occurs, probably realised at the microstructural level in the brain (‘neuronal imprinting’), as well as by a lasting, environment-induced (epigenetic) modification of the genome (‘genomic imprinting’). During the ‘critical period’ of incubation, the actual level at which physiological parameters are active may pre-determine a life-long ‘set point’ (or ‘set ranges’) for the respective physiological control system. If embryonic body temperature is higher during the ‘critical period’ in the development of the thermoregulatory system, the ‘set point’ of the thermoregulatory system will be imprinted to a higher level for the life of the bird. Reduced incubation temperatures induce the opposite effect.

Because of the integrating role of the thermoregulatory system, it is worth considering that “temperature training” may also induce ‘imprinting’ of other body functions: metabolism, feed intake and body weight, immune system and behaviour, for example.

Imprinting of body functions under suboptimal environmental conditions could be a basis for the perinatal malprogramming of body functions, producing e.g., metabolic disorders and cardiovascular diseases as well as behavioural disorders during later life. It is known from studies in humans (e.g. development of obesity due to gestational maternal diabetes) as well as in poultry (e.g. development of Acites syndrome due to suboptimum incubation conditions).

However we concur, that greater knowledge and better understanding of the mechanism of ‘imprinting’ could be specifically used to induce long-term (epigenetic) adaptation to postnatal environmental conditions.

We found that short-term, warm stimulation, by 1°C over standard for 2 h per day during the last four days of embryonic development, increased the hatch rate by approximately 1.5% in all six incubation trials (Table 1). Under these incubation conditions, a significantly higher percentage of hatched male chicks were achieved. In a subsequent broiler growth trial, the mean daily weight gain of the short-term warm stimulated male broiler chicks was significantly higher than for a control flock and a third group that were kept constantly warm during the last four days of incubation before hatching. In comparison, the short-term, warm stimulation produced a significant body weight increase of 2.9%. Feed conversion among warm-temperature stimulated male and female broilers was significantly lower than among the males and females of both the control and chronic warm incubated groups (Table 2).

Improved adaptability
Short-term temperature stimulation (“temperature training”), applied exclusively during the last days before hatching, has positive, specific effects on all production parameters in male and female poultry. These effects produce improvements that we refer to generally as robustness. The increased proportion of hatched male chicks in relation to all hatched chicks after short-term, warm temperature stimulation or “training” supports this conclusion.

It is known that male embryos are typically more vulnerable to environmental changes than the females. Our hypothesis is, that the particularity of embryonic “temperature training” is an improved adaptability to climatic variation and possibly to other environmental and social influences. Finally, reduced stress levels in the birds may also improve immune response.

Robust chicks
An incubation method that includes temperature stimulation is closer to a natural correspondence with the physiological needs of the embryo, and may therefore also have positive implications in the context of animal welfare and protection. Robust chicks are resistant to climatic variations, which decreases mortality and loss of performance. It is likely that as a result of improved general health and resistance, birds exposed to embryonic “temperature training” also need less medication/vaccination during their lives.

An incubation temperature profile that includes short-term temperature variation is highly relevant to the aim of improving poultry performance and may represent the future of commercial poultry incubation. First applications of “temperature training” for broiler embryos under commercial conditions are underway – and delivering promising results.