Effects of day length on performance traits of commercial poultry
Vision is likely the most important sensory tool available to domestic poultry. Poultry birds (broiler and layers) rely on vision more than any other sense (Prescott et al., 2003). Light is the primary zeitgeber in the control of biological rhythms (Kristensen et al., 2006), and to be able to do this, it must be perceived by the brain. Light is the primary tool used by the body to entrain circadian rhythms (Rattenborg et al., 2005). Constant or near-constant lighting programs had been a traditional choice of farmers, primarily with the thought that it could provide constant visual access to feeders and water troughs, which ultimately resulted in maximize feed consumption and growth. Although various lighting programs involving more darkness exposure have been studied and recommended, nearly continuous day length is still common in commercial broiler production. The 1 hour (h) of darkness in the 23 light (L):1 dark (D) schedule has not been provided as a period for rest or sleep (Savory and Duncan, 1982).
Light is an important management technique in broiler production and is composed of three aspects, light wavelength, light intensity and photoperiod length & distribution (Lewis et al., 1998). In environmentally controlled housing systems, birds are exposed to artificial light. So source, intensity, spectra and regimen of light had major impact in modern broiler management (Rozenboim et al., 1999). Long dark periods enhanced the growth more than continuous light but extended darkness without light provision resulted in impaired growth (Deaton et al., 1970). Gorden (1994) concluded that maximum benefit could be obtained by rearing broilers under lighting regimen of 16 h per day.
A lack of darkness in many species had negative consequences on biological or physiological reactions in the body. The exposure of young mice to continuous light altered body circadian rhythms, and had a lasting effect on these rhythms rest of their life (Ohta et al., 2006). Also true in birds, this could have important implications in newly hatched chicks, which generally given long day length lighting programs after placement during brooding.
The mechanism of action in the bird is different than other species, as light perception can occur via the retina as in other species, and at appropriate light intensities, can penetrate through the skull to directly affect photoreceptors in the pineal gland and the hypothalamus (Prescott et al., 2003). The pineal gland also serves as a circadian clock in the body (Minneman and Wurtman, 1976) while the hypothalamus serves a vital role in initiating the sequence of events required for reproduction and growth. This starts with the release of growth hormone and gonadotropins, which in turn affects luteinizing hormone and follicle stimulating hormone produced in the pituitary gland (Lewis and Morris, 2006). When light comes in contact with photoreceptors in the retina and...