ANALYSIS - The amount of light that poultry receive in their housing, its intensity and its colour all play a crucial role in the health, welfare and performance of the bird.
The issues that surround lighting in poultry houses and how the birds react are discussed in a series of papers by the late Peter Lewis, Research Fellow at the University of KwaZulu-Natal in South Africa, and the late Trevor Morris, past President of the UK Branch of the World Poultry Science Association and past senior vice president of the Association in their book, 'Poultry Lighting - The Theory and Practice'
In their papers, they show that the response to the day length or photoperiod varies according to species and type of stock. The photoperiod is the period of light and dark given to the birds over 24 hours.
They say that although photoperiods are often classified as stimulatory (long day) or non-stimulatory (short day) poultry respond "most dramatically to changes in the photoperiod".
For example they say that the main effect of photoperiod during the rearing phase of pullets is its control over the timing of sexual maturation.
The constancy of the photoperiod can also have an effect on laying hens and their productivity and even the weight of the eggs.
The authors also discuss intermittent lighting programmes and how they can be used for breeding turkeys and broilers but are rarely used during the rearing period for growing pullets - largely because there is little opportunity to save on electricity and also because the lighting programme needs to maximise feed intake.
The illuminance or light intensity during the rearing period of pullets showed small changes between genotypes in their response to changes in intensity. It also can have a bearing on the speed of development to sexual maturity.
In their papers, Lewis and Morris also examine the way birds respond to colour.
Unlike humans, who have three types of cone in the retina of the eye, which perceive the red, green and blue colours as well as white, they point out, birds have a fourth cone.
"This, together with their optically clear lens and humours, allows poultry to be responsive to radiation below 400nm and to 'see' in ultraviolet light," they say.
They show how they react in growth rate, male and female reproduction and behaviour.
The reports show that long photoperiods, continuous illumination, continuous darkness, low illuminance and coloured light can have a deleterious effect on the integrity of the eye.
The reports add that there are adverse effects through long daylengths on adrenal function and immunosuppression in poultry.
Lewis and Morris show how poultry producers have changed from tungsten-filament lamps to more energy efficient longer lasting light sources in recent times and the different characteristics of the light that is now commonly used in poultry houses can produce different responses in the birds.
One conclusion that is drawn is that: "Evidence from studies involving growing pullets, laying hens, broilers, growing turkeys, breeding turkeys and geese suggests that, irrespective of the light's spectral composition or illuminance, growth and reproductive performance are similar under modern energy efficient and conventional incandescent sources of illumination."
However, the study also says that there is also inconsistency in the evidence on how different light sources affect different species and types and there is also little evidence that different sources have any adverse effect on welfare or growth but leg integrity in one broiler trial under fluorescent lights was found to have been reduced.