Most birds are seasonal breeders and their reproductive cycle is controlled by changes in day length called as Photoperiod. Through its effect on the hypothalamus, light is responsible for the control over gonadotropin releasing hormone production by pituitary, and this is ultimately responsible for control of ovulation in the hen and spermatogenesis in the rooster.
Circadian Clock & Reproduction Cycle
Birds perceives this photoperiod in two ways:
1) The retina of a bird's eye can perceive a great variety of information about the intensity, colours, and direction of light. The information is transmitted to two different areas - to the part of the brain responsible for vision (visual cortex) and to the pituitary gland.
2) UVA is the specific type of light that the Harderian Gland senses. This is one of the triggers for reproductive system. There is a special tissue that surrounds parts of the avian eye socket called the Harderian Gland. It can sense the photoperiod. The below diagram is showing how the normal circadian clock triggers for reproduction and migratory behaviour.
Birds are not “stimulated” by the entire period of light, but rather by two important parts of this period. Birds are sensitive to the time of initial “Light-on “and subsequently during a period 11-13 hour later which is called the “Photosensitive Phase”.
A short day is not stimulatory, whereas a long day initiates or maintains the cascade of hormonal release that control ovulation or spermatogenesis. For practical purposes, the minimum stimulatory day length is around 12 hours, and there is little benefit to going much beyond 16-17h total light.
Seasonal Effects on Light Stimulation and Egg Production
The natural pattern of light: darkness varies depending upon latitude, season and pollution. The daylength at “maturity” will be dictated by the season of the year. Studied conducted by Lopez & Leeson (1992) revealed, as latitude increases the variation in the maturity and % peak production is noticeable at same managemental conditions.
Effect of Light Stimulation & Body weight on Reproductive performance
There is a correlation between mature weight and age at maturity. Underweight pullets should not be light stimulated until the standard weight (around 2.0kg) is achieved, regardless of age. A useful thumb rule with breeders is to ensure hens are at least 2.1kg and 20 weeks of age before light stimulation.
Both pullets and cockerels will be grown on continuous light for 2-3 days, and then day length reduced to 8-12h of constant light, up to 20 weeks of age. During this stage, shorter the daylength, the greater the saving in feed cost, because birds are less active and spend more time in darkness.
Age Related complications in Light Stimulation
Poor breeder performance will always result from an ill-designed or implemented light program, while a good lighting program is merely part of an overall management strategy. Over time, breeders become photorefractory to light, meaning that the endocrine system becomes less active, and output of gonadotrophins from the pituitary decline. It is known as geriatric phenomenon.
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