growth performance

The assumption and practice of subjecting broiler chickens to continuous or nearly continuous light regimen to maximise their growth performance through free and long-hour access to feed and water have been proven not only to be false, but can also reduce the bird's ability to grow to their full potential within a stipulated period. Schwean-Lardner and Classen (Reference SCHWEAN-LARDNER and CLASSEN2010) reported that birds given 20 hours light (20L) had a higher live-weight gain when compared to those on constant light for 23 hours (1.74 kg vs. 1.70 kg) at 31-32 days of age, with feed intakes of 2.68 kg/bird and 2.61 kg/bird in 20L and 23L groups respectively. The same trend was observed at 38-39 days of age (live-weight 2.34 kg vs. 2.29 kg and feed consumption of 3.87 and 3.78 kg/bird) and 48-49 days old (live-weight of 3.27 kg vs. 3.16 kg and feed consumption of 6.15 and 5.89 kg/bird). This shows that provision of near continuous light for long time access to feed by the birds will not necessarily result in increased feed intake, but can adversely affect the growth performance. Fidan et al. (Reference FIDAN, NAZLIGÜL, TÜRKYILMAZ, KARAARSLAN and KAYA2017b) observed improvements in weight gain in a group of broilers subjected to increasing duration of photoperiod at 42 d compared to the birds on 23L: 1D treatment (2947.36 g vs. 2916.72 g) even though there was no significant difference in the feed consumed by the two groups. Subsequently, the distribution of photoperiod (light hours) and scotoperiod (dark hours), which is one of the components of lighting programme for broilers, has been reported to influence growth performance. For example, Duve et al. (Reference DUVE, STEENFELDT, THODBERG and NIELSEN2011) conducted an experiment to compare the effects of the split darkness of 8 h (4 h in the first half and 4h in the second half of the day) and continuous 8 h darkness. The authors reported significantly increased feed intake (3222 g/bird) and weight gain (2086 g/bird) in broilers subjected to split scoto-period compared to those on continuous 8 h darkness (feed intake, 3072 g/bird; weight gain, 2004 g/bird respectively) at 36 d of age. Also, El Sabry et al. (Reference EL SABRY, YALÇIN and TURGAY-İZZETOĞLU2015) reported improved feed intake and body weight gain in birds reared under the split darkness of 14L:4D : 2L:4D compared to constant photo and scoto-period of 16L:8D though, with the interaction of chicks’ parent stock age. As a result, the authors suggested that split darkness programme could be used to improve the live weight of broiler chicks from young breeders. In addition, Yang et al. (Reference YANG, XING, WANG, JINLONG, YAN, BANGHONG and ZHANG2015) investigated the effects of intermittent lighting (i.e. intermittent 2L:2D or 4L:4D) and continuous lighting (CL) on Ross 308 broiler strain and discovered that birds on treatment 4L:4D at 42 d had the highest bodyweight gain (2531 g) with the least average daily feed consumption (149.4 g) compared to 2L:2D (BW 2466 g and ADFI of 150.2 g) and CL (BW 2496 g and ADFI of 156.8 g) respectively. Improved body weight observed in 4L:4D regime could be as a result of energy conserved by the birds during the longer period of rest (low activity in dark hours). In addition, better FCR was observed in birds on 2L:2D (2.30) compared to CL, which had almost all-day access to feed and water. This supported the observation of Rutz and Bermudez (Reference RUTZ and BERMUDEZ2004) which stated that intermittent lighting synchronised better feed intake with feed passage through the digestive tract of broilers. The scoto-period, which is characterised by reduced activity, corresponded with the non-feeding period, whereby the passage rate of feed digesta was reduced and there were delays in emptying the digestive tract of birds (Ramão et al., Reference RAMÃO, NUNES, BRUNO, TSUTSUMI, SILVA and POZZA2011; Summers and Leeson, Reference SUMMERS and LEESON1979; Duke et al., Reference DUKE, DZIUK and HAWKINS1969). This will allow optimum digestion, nutrient availability and utilisation, contrary to what has been seen when the feed passage rate is fast due to continuous feed intake, as observed during continuous photoperiods (Unpublished report). This report is consistent with the findings of Abbas et al. (Reference ABBAS, ALM EI-DEIN, DESOKY and GALAL2008), who reported significantly improved bodyweight gain by 10% with lower feed consumption and, hence, better FCR, in birds exposed to intermittent light of 2L:2D when compared to the near continuous light group. Conversely, Parvu et al. (Reference PARVU, ANDRONIE, SIMION and AMFIM2014) showed that when birds were given intermittent lighting of 16 h light (2L:1D) and combined programme of 16 h light (6L and six period 2L:1D) from one-day-old had respectively 10.5% and 8.5% decreased bodyweight compared to the group subjected to 23L:1D at 21 days. The same trend was observed for feed intake. However, this could be as a result of differences in the start-up lighting programme adopted by the researchers.

Providing continuous lighting for broilers may be advantageous in their early life (1 d -7 d) to allow free access to feed and improve feed intake, which is essential for the early intestinal growth and development. It has been reported that the intestinal microbial community is established in chicks shortly after initial feeding (Naqui et al., Reference NAQUI, LEWIS and HALL1970) which aids early intestinal development. Deep et al. (Reference DEEP, RAGINSKI, SCHWEAN-LARDNER, FANCHER and CLASSEN2013) reported high mortality rate and low weight gain in chicks subjected to extremely low light intensity (0.1-0.5 lux) within first seven days post-hatch as a result of the inability of the chicks to feed. Poor feeding activities and intakes, and poor intestinal and immune system development at an early stage may have negative effects on final productivity of broilers reared intensively.

health, welfare and mortality

Lighting programmes have been reported to have major implications for the welfare of broiler chickens (Schwean-Lardner et al., Reference SCHWEAN-LARDNER, VERMETTE, LEIS and CLASSEN2016). For example, many species, including poultry, possess a circadian rhythm which is a biological process that displays an endogenous oscillation of about 24 hours. In simple terms, it is a 24-hour internal clock of living organism and cycles between sleepiness and alertness at regular intervals. The circadian rhythm, which is controlled by the hypothalamus in the brain, is influenced by external cues called Zeitgebers which include light, temperature and redox cycles (Kristensen et al., Reference KRISTENSEN, AERTS, LEROY, WATHES and BERCKMANS2006; Rattenborg et al., Reference RATTENBORG, OBERMEYER, VACHA and BENCA2005; Middleton et al., Reference MIDDLETON, ARENDT and STONE1996). However, during the scotoperiod, the eyes send a signal to the hypothalamus which, in turn, alerts the pineal gland to secrete melatonin (a neuro-hormone that makes the body feel tired and initiates sleep). Melatonin regulates the production of other hormones (including those related to growth and immune function), stimulates lymphocyte proliferation (Zheng et al., Reference ZHENG, MA, GU, YUAN, SHI, GUO and ZHAN2013; Pang et al., Reference PANG, PANG, POON, WAN, SONG and BROWN1996; Phillips and Berger, Reference PHILLIPS and BERGER1992) and assists in scavenging oxidising free radicals (Reiter et al., Reference REITER, TAN and GALANO2014; Tomás-Zapico and Coto-Montes, Reference TOMÁS-ZAPICO and COTO-MONTES2005) in tissues. During the cellular metabolic process, the reactive oxygen species (ROS) are produced which react and destroy important lipid and protein molecules within the cell hence, posing a health challenge to the birds. Based on this, near continuous lighting for broilers suppresses melatonin production which can negatively affect the health and welfare of the birds as a result of stress induction, alteration of brain function, reduction of immune function, increased pathological diseases and reduction of alertness, all of which could arise from sleep deprivation or fragmentation (Everson et al., Reference EVERSON, THALACKER and HOG2008; Boerema et al., Reference BOEREMA, RIEDSTRA and STRIJKSTRA2003). Birds' instincts show that some dark periods are required for their proper wellbeing. This is supported by the findings of Coban et al. (Reference COBAN, LACIN and GENC2014), whereby birds progressively subjected themselves to some hours of darkness in a self-photoperiod treatment with no significant effect on live weight gain when compared to birds on continuous lighting treatment.

Schwean-Lardner et al. (Reference SCHWEAN-LARDNER, VERMETTE, LEIS and CLASSEN2016; Reference SCHWEAN-LARDNER, FANCHER, GOMIS, VAN KESSEL and CLASSEN2013) measured the health of a flock of broilers by the mortality and culling rate and observed that photoperiod augmented with dark periods significantly reduced both metabolic and skeletal disorders in broilers which in turn, lowered the mortality and morbidity rate. Moreover, the dark period addition can be beneficial to bird's health by delaying early high weight gain to a later period, because fast growth condition of commercial broilers negatively impacts broiler's health and is associated with leg problems, sudden death syndrome and ascites (Zheng et al., Reference ZHENG, MA, GU, YUAN, SHI, GUO and ZHAN2013; Schwean-Lardner et al., Reference SCHWEAN-LARDNER, VERMETTE, LEIS and CLASSEN2016). As reported in the study of Schwean-Lardner et al. (Reference SCHWEAN-LARDNER, FANCHER, GOMIS, VAN KESSEL and CLASSEN2013) in which groups of birds were subjected to light treatments of 14L, 17L, 20L and 23L between 7 d and 32 d, the rate of mortality caused by metabolic and skeletal disorders decreased with increasing level of augmented darkness (i.e. metabolic- 1.23 1.20 1.82 and 1.73%; skeletal- 0.38 0.25 0.50 and 0.71% for 10 h, 7 h, 4 h and 1 h dark periods in the treatments, respectively). This is consistent with the observations of Mlaba et al. (Reference MLABA, CIACCIARIELLO and GOUS2015). Furthermore, Zheng et al. (Reference ZHENG, MA, GU, YUAN, SHI, GUO and ZHAN2013) observed the influence and implication of light treatment on serum melatonin concentration, lipid peroxidation in serum and liver and antioxidant status of broilers. They reported significant increases in serum melatonin in groups of broilers under intermittent light (INL) of 18L and 6D compared to a CL group at 21 d (INL I 16L:2D: 1L:2D: 1L:2D 3.8 pg/ml; INL II 17L:3D: 1L:3D 3.85 pg/ml vs. CL 24L 3.7 pg/ml). Besides, the researchers observed that INL significantly improved the total antioxidant capacity at 21 d in the liver, in relative units (INL II 0.69, INL I 0.68) and serum (INL II 5.01, INL I 4.71) compared to the CL (liver: 0.53, serum: 4.56). This may have arisen as a result of the important role of melatonin in antioxidant status of broilers by directly eliminating free radicals and the stimulation of some enzymes that metabolise free radicals and their breakdown products from the system (Tomás-Zapico and Coto-Montes, Reference TOMÁS-ZAPICO and COTO-MONTES2005). Melatonin, which is secreted during the scoto-period in broilers to initiate sleep, is a non-enzymatic antioxidant because of its tendency to eliminate potential ROS from cells (Tomás-Zapico and Coto-Montes, Reference TOMÁS-ZAPICO and COTO-MONTES2005).

Another important indicator of broiler's welfare status is the extent of mobility, because inability or difficulty in walking due to severe pain or paralysis affects access to feed and water (Škrbić et al., Reference ŠKRBIĆ, PAVLOVSKI, LUKIC, PETRICEVIC, MILJKOVIC and MARINKOV2012), and, hence, performance. This, at most times, is determined by gait scoring. Schwean-Lardner et al. (Reference SCHWEAN-LARDNER, VERMETTE, LEIS and CLASSEN2016; Reference SCHWEAN-LARDNER, FANCHER, GOMIS, VAN KESSEL and CLASSEN2013) used a technique described by Garner et al. (Reference GARNER, FALCONE, WAKENELL, MARTIN and MENCH2002) to assess mobility and footpad dermatitis subjectively. This method consists of gait scores ranging from 0 depicting perfect mobility and 5 depicting the total loss of mobility, but the range of 0 to 2 was used for footpad assessment. However, results of their assessment showed that birds subjected to longest hours of darkness had the least mean values of gait score at 28 d and these values increased with decreasing hours of dark periods (10D: 0.62, 7D:0.81, 4D:0.93, 1D:1.16). The same trend was also experienced at 35 and 45 d of age.

Škrbić et al. (Reference ŠKRBIĆ, PAVLOVSKI, LUKIC and PETRICEVIC2015) and Fidan et al. (Reference FIDAN, NAZLIGÜ, TÜRKYILMAZ, AYPAK, KILIMCI, KARAARSLAN and KAYA2017a) evaluated the incidence of footpad dermatitis and hock burns using different photo-period duration. Their observations were consistent with the findings of Schwean-Lardner et al. (Reference SCHWEAN-LARDNER, FANCHER, GOMIS, VAN KESSEL and CLASSEN2013; Reference SCHWEAN-LARDNER, VERMETTE, LEIS and CLASSEN2016) whereby increasing photoperiod instead of CL reduced the frequency of leg health associated problems thus, improved broiler welfare. Furthermore, Schwean-Lardner et al. (Reference SCHWEAN-LARDNER, FANCHER, GOMIS, VAN KESSEL and CLASSEN2013) concluded that lack of darkness itself in broiler production system negatively affects skeletal health, especially in the legs, rather than the fast growth rate associated with a long light period, as it is commonly believed.

carcass characteristics

The practical assessment of various characteristics of an animal's carcass part and organs (in poultry this includes head/eyes, neck, breast, back, wing, thigh, drumstick, kidney, gizzard, liver, etc.) is one of the major ways to study the effects of management regimes. The carcass characteristics of broilers concerning their performance are influenced by the length of photo-period and its distribution adopted during the rearing phase. However, the effects are not consistent among different studies. For example, Sun et al. (Reference SUN, LI, LI, CHEN, BAI, XUE and CHEN2017), observed a significant increase in the breast muscle yield from chickens subjected to intermittent lighting of 16L:2D: 2L:4D compared to those on constant 16L:8D regimen, but similar when compared with CL treatment. Conversely, intermittent lighting treatment effects reported by Das and Lacin (Reference DAS and LACIN2014) caused lower breast yield in broilers when compared to CL (42.62% vs. 44.76%). It is noteworthy to state that the intermittent lighting regimen defined in these two studies were different. In addition, differences in the strains of male broilers used could be a factor in the conflicting results. Nevertheless, different photoperiod duration (CPL- 23L:1D for 42 d; IPL- 23L:1D from 1 to 8 d, 14L:10D from 9 to 15 d, 16L:8D from 16 to 22 d, 18L:6D from 23 to 29 d, 20L:4D from 30 to 36 d, followed by 23L:1D from 37 to 42 d) and light intensity (bright intensity BLI or dim intensity DLI) (BLI: 20 lux for 42 d; DLI: 5 lux from 1 d to 8 d, 2.5 lux from 9 d to 15 d and 1.25 lux from 16 d to 42 d) interactions were seen to have effects on whole leg yield as a percentage of live-weight, with CPLxDLI interaction having the highest value (38.44%) which was significantly different from CPLxBLI (36.61%), IPLxBLI (37.87%) and IPLxDLI (37.73%) values respectively (Fidan et al., Reference FIDAN, NAZLIGÜ, TÜRKYILMAZ, AYPAK, KILIMCI, KARAARSLAN and KAYA2017a), despite no effect for photoperiod length alone on carcass characteristics. This result was consistent with the findings of Downs et al. (Reference DOWNS, LIEN, HESS, BILGILI and DOZIER2006), who reported no significant effect in the cold carcass yield of birds exposed to different lighting programme, but did see an increase in leg yield percentage with increased photoperiod length. Coban et al. (Reference COBAN, LACIN and GENC2014), reported no significant effect on carcass percentages when birds were exposed to 24 hours of CL, CL of 16l:8D and self-photoperiod. However, Schwean-Lardner and Classen (Reference SCHWEAN-LARDNER and CLASSEN2010) reported increased carcass yield with increasing length of photoperiod, although this effect is age-dependent (i.e., no significant effect of light on carcass yield at age 31-32 d but significant increase in carcass yield was recorded at 38-39 d and 48-49 d). At 48-49 d, carcass yield, as a percentage of the live-weight, in broilers exposed to 20L was significantly higher (72.5%) followed by the group exposed to 23L (71.5%) then, 17L (71.0%) and 14L (70.5%) being the lowest. Furthermore, these researchers observed an increase in breast meat yield and decreased whole leg percentage (especially, the left drum meat yield) with increasing photoperiod length, in parallel to what was obtained from the studies of Downs et al. (Reference DOWNS, LIEN, HESS, BILGILI and DOZIER2006) and Fidan et al. (Reference FIDAN, NAZLIGÜL, TÜRKYILMAZ, KARAARSLAN and KAYA2017b). In addition, photoperiod duration was observed to have a significant effect on the percentage weights of blood, feather, and gizzard relative to the BW of broiler (Coban et al., Reference COBAN, LACIN and GENC2014; Das and Lacin, Reference DAS and LACIN2014).

growth performance

The highly specialised visual system of poultry has been proven to be essential for increased productivity of broiler chickens (Mendes et al., Reference MENDES, PAIXÃO, RESTELATTO, MORELLO, DE MOURA and POSSENTI2013). Light is a potent and critical climatic component of broiler housing facility, as it influences many behavioural, physiological and metabolic processes in birds. Different wavelength outputs from light in the visible spectrum determine its colour (Riber, Reference RIBER2015). Senaratna et al. (Reference SENARATNA, SAMARAKONE, ATAPATTU and NAYANARASI2008), stated that the colour of light is one of the important features of light in the broiler's environment. Yang et al. (Reference YANG, YU, PAN, YING and ZHOU2016) and Rozenboim et al. (Reference ROZENBOIM, BIRAN, CHAISEHA, YAHAV, ROSENSTRAUCH, SKLAN and HALEVY2004) suggested that short wavelength monochromatic light colours, light colour switching, and mixture systems can be used to manipulate and stimulate broiler growth and metabolism. Nonetheless, the short wavelength lights (green and blue) are said to make chickens calmer and reduce their level of activities (Prayitno et al., Reference PRAYITNO, PHILLIPS and OMED1997; Rodenboog et al., 2001) which could explain improved growth performance reported by researchers. For example, green and blue LED mixed light system was reported to have a significant improvement on broiler body weight, FCR, circulating glucose levels and carcass yield when compared to normal white light (Yang et al., Reference YANG, YU, PAN, YING and ZHOU2016). This improvement could be explained by the important role of insulin-like growth factor-I (IGF-I) in promoting broiler satellite cell myogenic processes, as its circulation levels are enhanced by green and blue, compared to white and red light (Liu et al., Reference LIU, WANG and CHEN2010). Despite several reports of significant improvements in performance of broilers raised under green and blue lights, Senaratna et al. (Reference SENARATNA, SAMARAKONE, MADUSANKA and GUNAWARDANE2012), concluded that broiler chicken preferred red and white light colours to blue and green after their sensitivity to light colours was observed after 21 days of age. In contrast, Prayitno et al. (Reference PRAYITNO, PHILLIPS and OMED1997) reported that birds preferred blue light over other colours, after been raised under blue, green, red and white light colours for five weeks, except those initially raised under blue light. Furthermore, Rozenboim et al. (Reference ROZENBOIM, BIRAN, CHAISEHA, YAHAV, ROSENSTRAUCH, SKLAN and HALEVY2004) observed significant increases in bird growth rates when colour was changed from green to blue at 10 d of age and from blue light to green light at 20 d when compared with white light, suggesting that green light stimulated growth of birds at an early age, and shifting birds to a different light environment at 10 or 20 d of age might further stimulate their growth. At the same time, Mendes et al. (Reference MENDES, PAIXÃO, RESTELATTO, MORELLO, DE MOURA and POSSENTI2013), reported a significant increase in feed intake in birds reared under white LED light (500 nm) compared to the yellow LED light (635 nm) between 21 to 35 days of age. The authors suggested that this could be as a result of the birds’ special spectral sensitivity which could have made them perceive more brightness from the white LED bulbs. However, Pan et al. (Reference PAN, YANG, YANG, DAI and YU2015) conducted further research on more human-friendly light colours on broiler's performance as short wavelength lights (i.e., green light with 514 nm and blue light, 455 nm) that have been observed to stimulate bird's growth and reported that this treatment could have negative effects on broiler house workers. The researchers discovered that broilers reared under yellow light (580 nm; which is more human-friendly) had similar growth parameters with those on green and blue lights treatments. This is consistent with the findings of Sultana et al. (Reference SULTANA, HASSAN, CHOE and RYU2013), who reported increased feeding behaviour in broilers reared under yellow-red coloured light, which was similar to results from green light, but more than that observed under blue light. However, more studies are needed to ascertain these claims.

health and welfare

Aside from the effects of light wavelength on growth performance of broilers, various studies have shown that light colour could influence immune and stress responses which in turn, affect the health and welfare of broiler chickens. Xie et al. (Reference XIE, WANG, DONG, CAO, WANG, CHEN and CHEN2008), observed an increased proliferation of blood T lymphocytes and elevated level of immune responses to Newcastle disease virus in the serum of birds reared under green light compared to blue and red light treatments at 21 and 28 d of age. However, at 49 d, these parameters were higher in the blue light group, with significantly decreased level of serum interleukin-β when compared to green and red light treatments. The age-related influence of the blue light on the improved immune and reduced stress responses of the birds could be explained by calmer behaviour under blue light in mature broilers (Hesham et al., Reference HESHAM, EL SHEREEN and ENAS2018; Rozenboim et al., Reference ROZENBOIM, BIRAN, CHAISEHA, YAHAV, ROSENSTRAUCH, SKLAN and HALEVY2004). Sultana et al. (Reference SULTANA, HASSAN, CHOE and RYU2013) stated that long wavelength light (i.e., red and red-yellow mixed lights), when compared to the short wavelength lights (blue and green), induced fear responses in broiler. This might pose a negative impact on the birds’ welfare as a result of stress. Contrarily, from another perspective, Senaratna et al. (Reference SENARATNA, SAMARAKONE and GUNAWARDENA2016) opined that increased activity, such as intense movement which is facilitated by the red light treatment, could improve welfare through the decreased incidence of leg disorders, instead of the lengthened sitting behaviour observed in blue light treated birds. Reiter and Bessei (Reference REITER and BESSEI1998) stated that increased birds’ locomotion could stimulate bone development. In addition, Sørensen et al. (Reference SØRENSEN, SU and KESTIN2000) mentioned that restricted activity may lead to increased incidence of leg problems in broilers. Moreover, prolonged sitting may result in lesions on the skin, hocks, feet, breast, lameness and sometimes, paralysis especially if on poor quality litter.

carcass characteristics

To the authors’ knowledge, few works have been conducted to show the influence of light wavelength/ light colour on broiler carcass characteristics. However, the studies available reported significant effects that were, in most cases, as a result of increased body weight gain of the birds under certain light wavelength conditions, which is indirectly related to carcass quality or weight. For example, Liu et al. (Reference LIU, CHEN, WANG, DONG, CAO, XIE and JIA2008), reported increased breast and thigh muscle weight in the range of 16-25% in birds exposed to blue and green light when compared to other light colours (red and white) at 49 d of age. This is similar to what was seen by Li et al. (Reference LI, SUN, HUA, WANG, WANG, BAI, LIU, LI, LUO, CHEN and CHEN2015), where breast muscle weight was higher in birds reared under green light when compared to those raised under red light. Previous studies have shown positive effects of blue and green lights on broilers regarding final body weight as a result of stimulation of myofiber growth compared to other light colour treatments (Velo and Ceular, Reference VELO and CEULAR2016). Narinc et al. (2015) stated that, as broiler chicken have been selected for fast growth which yields more breast muscle, therefore, carcass parts weight, such as breast and thigh muscles and fat content might be influenced by factors that promote increased final body weight gain in broilers (Aksoy et al., Reference AKSOY, ÖNENÇ, NARINÇ, ÇÜREK, YAPIC and BAYLAN2010; Grashorn, Reference GRASHORN2006).

growth performance

Light intensity, as stated by Olanrewaju et al. (Reference OLANREWAJU, MILLER, MASLIN, COLLIER, PURSWELL and BRANTON2016) aids steady physiological growth and developments in broilers before accruing maximum muscle mass. Likewise, Prescott et al. (Reference PRESCOTT, WATHES and JARVIS2004) indicated that intensity of light could affect broiler performance. However, in literature, on average, no substantial effect of variation of light intensity on broiler growth performance has been reported, except in extremely low light intensity situations (e.g. below 1 lux) which caused ill effects on the birds (Olanrewaju et al., Reference OLANREWAJU, PURSWELL, COLLIER and BRANTON2011; Deep et al., Reference DEEP, SCHWEAN-LARDNER, CROWE, FANCHER and CLASSEN2010; Reference DEEP, RAGINSKI, SCHWEAN-LARDNER, FANCHER and CLASSEN2013). However, Deep et al. (Reference DEEP, RAGINSKI, SCHWEAN-LARDNER, FANCHER and CLASSEN2013), conducted a study to set a minimum standard of light intensity for the optimum performance of birds reared intensively. The authors used a range of light intensity level from 0.1, 1, 5 and 10 lux, and discovered that birds with 0.1 lux intensity had poor growth rate (77 g against 267 g, 268 g, and 263 g for 1, 5, 10 lux treatments respectively) and higher mortality (3.32%) between 7 d and 14 d of age. In another experiment by the same authors, feed intake (FI; 3.468, 3.489 or 3.477 kg), body weight gain (BW: 2.214, 2.237 or 2.202 kg) and FCR of broilers were slightly different when subjected to light intensities of 1, 5 or 10 lux, which were significantly higher compared to results obtained in the group under 0.5 lux (FI 3.38 kg; BW 2.153 kg). Based on these results, the researchers recommended at least 5 lux light intensity should be maintained after the initial brooding period. In addition, a study conducted and reported by Blatchford et al. (Reference BLATCHFORD, KLASING, SHIVAPRASAD, WAKENELL, ARCHERAND and MENCH2009), showed that high light intensity treatments such as 50 lux and 200 lux had no significant effect on broiler body weight gain when compared to 5 lux intensity. Further, these authors indicated increased activities of birds reared under high light intensities. However, FI and FCR were not shown in the report, but it was likely that the birds reared under high light intensity (50 and 200 lux) would have higher feed intake in order to compensate for their increased activities in order to achieve the same weight gain as those under low light intensity (5 lux). Kristensen et al. (Reference KRISTENSEN, AERTS, LEROY, WATHES and BERCKMANS2006), opined that varying intensity of light between 5 lux and 100 lux might have no or little effect on broiler growth. Although, the authors pointed out certain factors that could be responsible for their observations, one of which was low replications.

health and welfare

Eyes are one of the important organs of the body that are vulnerable to a certain number of external factors, one of which is light intensity. Extreme light intensities (too low or high) have been observed to cause poor eye health, negatively affecting the welfare of animals. However, in broiler chickens, little or no health effects of different artificial light intensities in a closed housing system have been reported (Olanrewaju et al., Reference OLANREWAJU, MILLER, MASLIN, THAXTON, DOZIER, PURSWELL and BRANTON2007; Blatchford et al., Reference BLATCHFORD, KLASING, SHIVAPRASAD, WAKENELL, ARCHERAND and MENCH2009). Although, a few reports have indicated that exposure to continuous high-intensity light in the early life of chicken may result into severe hyperopia, corneal flattening and avian glaucoma (Cohen et al., Reference COHEN, BELKIN, YEHEZKEL, AVNI and POLAT2008; Lauber and McGinnis, Reference LAUBER and MCGINNIS1966). In addition, complete dark rearing may result in mild hyperopia and minor changes in corneal curvature (Cohen et al., Reference COHEN, BELKIN, YEHEZKEL, AVNI and POLAT2008). Increased light intensity (ranging from 100 lux ≤ 500 lux) has been shown to improve the activeness of broiler chickens, such as intense birds’ movement, compared to low light intensity (5-6 lux) (Newberry et al., Reference NEWBERRY, HUNT and GARDINER1988; Blatchford et al., Reference BLATCHFORD, KLASING, SHIVAPRASAD, WAKENELL, ARCHERAND and MENCH2009). This enhanced activity may improve broiler health and welfare through prevention or reduced incidence of paralysis, skin bruises or lesions and heart problems which would have occurred as a result of prolonged sitting and greater body mass accretion in a low light intensity environment, but consequently, increase the energy expended for additional activities instead for growth. Moreover, ensuring adequate and improved welfare in broilers may be at the expense of faster growth rate, which remains the focus for a poultry producer.

carcass characteristics

According to Fidan et al. (Reference FIDAN, NAZLIGÜ, TÜRKYILMAZ, AYPAK, KILIMCI, KARAARSLAN and KAYA2017a), light intensity had significant influence on the yield of cold carcass (BLI 76.59% vs. DLI 75.97%), whole breast (BLI 31.96% vs. DLI 31.13%) and whole leg (BLI 37.24% vs. DLI 38.09%) after exposing the birds to either bright light, BLI (20 lux for 42 d) or dim light, DLI (5 lux from 1 d to 8 d, 2.5 lux from 9 d to 15 d and 1.25 lux from 16 d to 42 d). This report was in agreement with Deep et al. (Reference DEEP, RAGINSKI, SCHWEAN-LARDNER, FANCHER and CLASSEN2013) and Downs et al. (Reference DOWNS, LIEN, HESS, BILGILI and DOZIER2006) that total carcass weight and breast meat yield increased in line with light intensity. However, the study of Fidan et al. (Reference FIDAN, NAZLIGÜL, TÜRKYILMAZ, KARAARSLAN and KAYA2017b), showed that thigh, drum, and whole leg weights were not influenced by light intensity, which contradicted what was reported by Fidan et al. (Reference FIDAN, NAZLIGÜ, TÜRKYILMAZ, AYPAK, KILIMCI, KARAARSLAN and KAYA2017a), where DLI significantly increased the proportion of the whole leg as a percentage of live-weight than BLI. This is in agreement with the fact that either individual or combination of genotype, gender, light intensity, photoperiod length, and other environmental factors could be responsible for the differences in the responses observed in carcass characteristics in many studies (Fidan et al., Reference FIDAN, NAZLIGÜL, TÜRKYILMAZ, KARAARSLAN and KAYA2017b).