Cissy Riedel studied Agricultural Sciences at the Albrecht Daniel Thaer-Institute of Agricultural and Horticultural Sciences (ADTI) at Humboldt-Universität zu Berlin (HU). Her literature study with the title „A brighter future for poultry –How the use of light during incubation can affect behaviour, health and performance in chickens“, was supervised by Barbara Tzschentke and represents the basis of this review. She is now working at the Ministry of Agriculture, Environment and Climate Protection in Brandenburg, Germany.

Barbara Tzschentke is senior researcher at the Institute for Agricultural and Urban Ecological Projects (IASP) at HU and Private Docent at the ADTI. Her research is focused on the influence of incubation factors on the development of physiological control systems and energy metabolism in birds. Recent research projects focus on short-term temperature training of late-term poultry embryos. The aim of this research is to investigate the effects of temperature stimulation on immediate embryonic development, hatching results and the long-term impact on health, robustness and performance, but also to develop a non-invasive control system for the application of temperature training in the field. Barbara Tzschentke is board member of the German Group of the Worlds Poultry Science Association (WPSA). Since 2003 she was involved in the establishment of the WPSA Working Group 12 (Physiology) and since 2015 she acts as chair person of this group.

To contact the authors: barbara.tzschentke@iasp.hu-berlin.de

ABSTRACT

Commercial chicks are conventionally incubated and hatch under darkness. The perception of light through retinal and extra-retinal physiological mechanisms develop in the early phase of incubation. Light during incubation influence the hormonal and neuronal system of the embryo and can also affect post-hatch performance and behaviour. These effects are not fully understood

Light is a complex stimulus, which comprises wavelength (colour), intensity and the distribution of the scotic phases over the 24 hours cycle. The eggshell modulates spectrum and intensity of light perceived by the embryo. Characteristics of the eggshell may be a major cause for differences of breeds in response to light during incubation.

Increasing application of LED lights in poultry production enhance the study of particular wavelength on embryonic development and function. The present state of knowledge can be summarized as follows:

INTRODUCTION

Light during incubation has not been considered as important for the development and liveability of the embryos and the chicks after hatching. Neither the ancient incubation systems in Egypt or China nor the incubators, which have been introduced in Europe in the 18^th century provided light during the hatching period (Sykes, 1991).
In the electric incubators, temperature, humidity, ventilation and turning was regulated automatically but light was not provided. Shafey and AL-Mohsen (2002) assumed that the manufacturer of incubators ignored the influence of light on the embryo.

In recently developed hatching systems like the HatchCare system ( Figure 1. , HatchTech, 2019) or the Patio system (van de Ven et al., 2012) the hatching eggs are exposed to light only during the last days of embryonic development in the hatching system.

Figure 1.
HatchCare-sytem: hatcher with light, feed and water (by courtesy of HatchTech).

In most common incubation systems, however, the entire incubation process, with the exception of candling during the transfer from the setter to the hatcher, takes place in the dark. This includes a very important “critical period” in the embryonic development of regulatory and sensory systems in the last days before hatching (Tzschentke, 2007; Tzschentke and Halle, 2016). Here, environmental influences or stimuli are important not only for the immediate embryonic development of body functions, but also for the entire life after hatching.

The influence of light on embryonic development has been considered sporadically in the 1970ties. Experiments on this topic have been reported more frequently from 2000 onwards.

The results showed, that the developing embryos respond to light and light might have positive effects on behaviour, health and performance of the chicks.

Light is a complex stimulus which can influence the embryo through duration and distribution of the scotic phases throughout the 24 hours day, wave length (colour) and intensity and manifold interactions between these criteria.

Studies on light in chickens are further complicated due to the difference between human and birds in the macroscopic structure of the retina and finally in the perception of light colour and intensity. This problem is being addressed by Kaemmerling et al. (2018).

The effect of light can be confounded with other factors, such as temperature (Yalcin et al., 2022) and egg shell characteristics (Maurer et al., 2015). There exist no systematic studies on this complex matter in poultry.

Most experiments on the effect of light on chicken embryos compared the current practice of continuous darkness (24D : 0L) with a photic period of 12 hours (12D : 12L) using different sources of light, including natural daylight, traditional incandescent and fluorescent light bulbs and LED light with different bands of wavelength.

A few studies compare continuous darkness with continuous light or varying scotic periods. The present review addresses the following topics:

1. Development of embryonic light perception and lateralisation
2. Effect of colour, intensity and rhythm of light on embryonic development and hatched chicks
3. Effect of different light conditions under artificial incubation on behaviour, health and performance

MATERIAL AND METHODS

A literature search was done using Google Scholar, the search system Primus of the Humboldt- Universität zu Berlin, and the electronic data basis of PubMed, Science Direct and Europe PMC, with the keywords incubation, chicken, health, behaviour, performance, animal welfare, avian eye, embryonic development.

Most references have been published from 1971 to 2023. Only a few are from the period of 1930, 1957 and 1961. Information on the light programs, light colour, light intensity and breeds used in the references cited are provided in table 1.

RESULTS

Light perception and lateralisation

Lateralisation is an important event in the function of brain and vision. Lateralisation is the difference in the anatomical and physiological structure of two brain hemispheres which allows the specialisation of neuronal functions in different parts of the brain.

Through the optic chiasm the right hemisphere of the brain is connected to the left eye and vice versa. Consequently, environmental information received by the right and left eye address different brain areas and lead to different behavioural responses.

According to Rogers (2011) and Rogers and Kaplan (2019) the left eye and the right brain hemisphere are controlling behaviour in danger and under stress. They are focused on recognition of social stimuli, maintaining of social hierarchy, and attention to potential predators which lead to fear and stress.

The right eye and the left hemisphere are controlling learned and routine behaviour under stress-free conditions as attention and ability to distinguish objects. Due to the posture of the embryo only the right eye is directed to the egg shell and stimulated by light.

Functional asymmetry of embryonic brains could not be found in chicks hatched in darkness (Chiandetti et al., 2013; Rogers and Kaplan, 2019). Important skills are impaired or not developed. These birds, for instance, cannot distinguish between different objects, show unstable social hierarchies, limited learning ability and higher fear behaviour (Rogers, 2011).

It has been assumed that the light conditions of last three days of incubation are important for the development of lateralisation and the interhemispheric neuronal communication, which is essential for normal development of brain functions and behaviour.

It has been shown, that lateralisation may develop independently of the visual system (Chiandetti et al.,2013) and direct light stimuli on the pineal gland may initiate lateralisation (but not improve interactions between the hemispheres, Roger and Kaplan, 2019) from three days of incubation onwards (Schwean-Lardner, 2018).

Light colour

The use of LED light is developing rapidly in poultry production. LED light not only reduces energy costs but also provides the opportunity to supply light of defined wavelength. It is also suggested that radiation of LED light does not increase the yolk temperature.

El-Shabrout and Khalil (2017) tested 12L : 12D regimes using natural light and yellow LED light in a Fayoumi layer strain. LED illumination did not influence total hatchability, but reduced the time of hatching, increased the chick weight and improved chick quality with regard to activity, navel quality, and leg strength (weakness to stand).

Li et al (2021a) tested white, blue and red light (200 lux) in a 12L : 12 D schedule versus 0L : 24D. There was no significant effect on hatching time, embryonic growth and chick quality.

Archer et al. (2017) compared a light schedule during incubation of 12L : 12D with complete darkness in a White Leghorn layer line, a commercial broiler line and Pekin ducks. Light was provided by a combination of red and white LED lamps with maximum power between 610 and 660 nm wavelength and intensity of 250 lux. Hatch of fertile eggs was significantly better and the incidence of unhealed navels was lower under the 12L : 12D schedule as compared to 0L : 24D.

Hatching in 0L : 24D increased the weight of ducklings significantly but not the weight of layer and broiler chicks. The improvement of the hatching rate through the combination of white and red light was mainly due to a reduced mortality during the first phase of hatching. The authors did not find a similar effect in earlier studies, when only white light or monochromatic red light was used.

Wang et al. (2021) hatched eggs of four different layer breeder stains: White Leghorn, Rhode Island Red, Columbia Rock and Barred Rock under a 12L : 12D monochromatic green light or continuous darkness up to day 18. In all strains green light increased embryonic development and reduced hatch time.

Similar results have been reported by Rozenboim et al. (2013) using green LED light in broiler embryos. The authors suggested that green light penetrates the egg shell und influences the development of muscle cells through retinal and extra-retinal receptors.

Continuous fluorescent green light (24L : 0D) with intensity of 1340 – 1730 lux applied from day 1 to 18 reduced the hatching time by 24 hours, the chick weight by 2 % and improved the overall hatchability in two of 3 consecutive hatches (Shafey and Al-Mohsen, 2002). Hatchability was however reduced by the light treatment in the third hatch.

Continuous fluorescent green light (24L : 0D) with intensity of 1340 – 1730 lux applied from day 1 to 18 reduced the hatching time by 24 hours, the chick weight by 2 % and improved the overall hatchability in two of 3 consecutive hatches (Shafey and Al-Mohsen, 2002). Hatchability was however reduced by the light treatment in the third hatch.

Green light stimulated embryo growth and post-hatch body weight development (Zhang et al., 2012) and breast muscle percentage (Zhang et al., 2016) in a broiler line up to 42 days of age. This effect was mediated through stimulation of Growth Hormone (GH) and Insulin-like Growth Factors (IGF).

Increased post-hatch body weight development in response to green and blue LED light was also reported by Rozenboim (1999; 2013). Green and blue LED light during hatch increased the expression of PBX7 (Paired Box 7) and myogenin which represent important transcription factors for the differentiation of muscle cells (Halevy et al., 2006).

The positive effect short phases of green LED light (15minL : 15minD vs. 0L : 24D) on GH and IGF concentration in the chicken embryo has been confirmed by Dishon et al. (2017). There was, however, no effect of the light treatment on embryo weight, breast muscle weight and liver weight.

Dishon et al. (2021) applied the same light treatment at various phases of incubation. Starting the illumination at day 18 of hatch gave the same results as at any earlier stage of development.

Bai et al. (2019) suggested that the effect of green light on GH and IGF 1 is triggered by melatonin. Blocking pituitary melatonin suppressed the effect of green light on muscle development.

The positive effect of green light on post-hatch body weight has been shown to depend on the genetic line. Wang et al. (2020) tested monochromatic green light (12L : 12 D; 200 lux versus 0L : 24D) on four different breeds. Increased body weight from 8 to 12 weeks of age was found in Rhode Island Red but not in White Leghorn, Columbia Rock and Barred Rock.

Rozenboim et al. (2023) suggested that in broilers the last days of incubation might be a critical period for photo stimulation by green light because of the observed long-term increase in the activity of the somatotropic axis and elevation of post-hatch growth, which could be of high practical relevance.

It would be even more important to use the “right” light in this critical development phase, which is related to the specifics of the bird’s eye and which should, for example, also contain an appropriate UV percentage. Since the body functions throughout life are programmed in this phase, there is a risk of developing persistent false colour vision, which in turn can lead to increased stress.

Light intensity

Broiler breeder eggs hatched under continuous light (fluorescent, 1340 – 1730 lux) from day 1 to 18 reduced hatch time (by 24 h) and increased the weight gain of the embryos and hatchability (Shafey and Al-Mohsen, 2002). The chick weight was slightly reduced.

This effect has been attributed to the shorter time to hatch. Using the same light source and schedule under an increased light intensity (1430 – 2080 lux) showed negative effects on embryonic mortality and hatching rate as compared to darkness in birds with light and medium brown egg shell colour but not in dark brown egg shell colour (Shafey et al., 2005).

Dark brown egg shells obviously attenuate the negative effect of high light intensity. Tong et al. (2018) compared a 12L : 12D green monochromatic light schedule with a 0L : 24D as control. One hour of 1200 to 1500 lux was inserted in the light phase of 100 – 130 lux.

The high light intensity phase was provided in order to imitate the light condition under natural hatching. When the mother hen leaves the nest for a short period for eating and drinking the eggs are exposed to high intensity of natural light.

The relatively short phase of high light intensity led to a reduction of hatchability. Information on the effect of light during incubation on feather pecking is scarce. Riedstra and Groothuis (2005) reported higher gentle feather pecking in White Leghorn hatched under light conditions during the last week of incubation.

Although gentle feather pecking does not damage skin and feather cover, its occurrence represents a risk for the development of severe damaging pecking (Rogers and Kaplan, 2019; de Haas et al., 2021). Therefore, Riedstra and Groothuis (2005) recommended not to use light in the last three days of incubation. These results have to be considered with care as the light intensity in the experiment was high (250 – 1000 lux).

Using lower light intensity, Gnan et al. (2005) did not find a difference in feather pecking between pullets which have been incubated under light or dark conditions. There exist relationships between feather pecking and fear and stress (Grams et al, 2015, Archer, 2017; Archer and Mench, 2017).

Since incubation under light conditions reduces fear, it can be speculated that light influences feather pecking indirectly through reduced fear and stress. Özkan et al. (2022) hatched Brown Nick breeder chicks under a 18L : 6D schedule using white 6500K LED and green 8000K LED with a light intensity between 150 to 250 lux.

Gentle feather pecking, severe feather pecking and aggressive pecking were observed at 16, 24 and 32 weeks of age. Different physiological criteria related to stress (corticotropin releasing hormone, CRH) and serotonin (5-HTT), which are considered to influence feather pecking, were assessed on the brain level. There was no consistent effect of the light treatment on the behavioural and physiological traits over the three observation periods.

But there was a tendency for green light of lower intensity than in the experiments of Riedstra and Groothuis (2005) to reduce severe feather pecking and aggressive pecking and to increase 5-HTT levels at 40 weeks of age.

It can be assumed that the direct effects of light on embryonic development and the long-term effects on postnatal development are more positive when a lower light intensity reaches the embryo, as these conditions correspond better to the natural incubation conditions of chickens in a more dimmed environment.

CONCLUSIONS

Although the results of studies on the influence of light during incubation on embryonic development, hatching performance and post-hatching physiology, behaviour, health and primary performance considered in this review are not uniform – ultimately also due to the mostly non-comparable experimental conditions – light application particularly during the last days of incubation and in accordance with the requirements of the chicken’s eye can have a stimulating effect on embryonic and post-hatching development and environmental adaptability.

Furthermore, light application in a critical incubation period as the last days before hatching is obviously essential for the development of certain brain functions that are crucial for the development of different behaviours, cognitive abilities and stress tolerance.

Improved stress tolerance, which could possibly also reduce health problems and behavioural disorders, would be an important welfare aspect. Further systematic studies in this field under comparable experimental conditions that meet the requirements of the chicken’s eye and the specific conditions during incubation are necessary.

Findings from the natural breeding of domestic and wild chickens could make a major contribution to this.