Light is an essential part of the physical environment and a powerful exogenous factor, which affects physiological development, behavioural processes and welfare of domestic fowls (Onbasilar et al., 2007; Parvin et al., 2014). Thus, artificial light has been widely used to modulate avian reproduction and growth rate. Over the past several decades, the light sources used in poultry husbandry are mainly incandescent or fluorescent lamps. Recently, light-emitting diodes (LED) are gradually replacing the incandescent or fluorescent lamps because they are much more energy efficient, provide adequate illumination, and improve growth performance of poultry (Rozenboim et al., 2004; Hassan et al., 2013; Kim et al., 2014; Riber et al., 2015).

Chickens have a superior nature of avian vision, and they are sensitive to different wavelengths (colors) of light because of the special extra-retinal photoreceptors in the hypothalamus or in other sites of the brain (Foster and Follett, 1985; Lewis and Morris, 2000). Some previous studies showed that light stimuli can be recognized by chick embryos early in their embryonic stage and has potentially positive effects on their behavioral and physiological development (Rozenboim et al., 2004; Zhang et al., 2012; 2014a). These studies suggested that green LED light stimuli during embryogenesis enhances both body and breast muscle weight at both the embryonic stage and posthatch. Stimulation with green LED light during incubation also increases the chick embryo liver weight accompanied by a high level of proliferating cell nuclear antigen (PCNA) expression and an enhanced antioxidant activity of the liver, which is mediated by melatonin levels and melatonin receptor 1c (Mel1c) expression (Wang et al., 2014).

The light stimuli have also been shown to affect eye health in chickens. Continuous lighting regimens were reported to disrupt the functional development of avian eyes during the incubation period (Aige-Gil and Murillo-Ferrol, 1992; Archer et al., 2009) or raising stage (Whitley et al., 1984; Oishi and Murakami, 1985; Li et al., 1995). Previously, we have demonstrated that stimulation with monochromatic green LED during incubation accelerates body weight (BW) and posthatch pectoral muscle growth of broilers (Zhang et al., 2012), but limited data are available for evaluating the green LED light effects on the development of eyes and other primary organs. Moreover, light-induced overheating is recognized as another potential factor that accelerates embryonic development (Rozenboim et al., 2003; 2004), but whether the growth-promoting effect of green LED light stimuli is closely related to an overheating effect is also still unknown. Therefore, the present study was undertaken with the following objectives: i) to examine whether the green LED light stimuli induces an overheating effect by determining weight loss rate of fertile eggs during incubation period; ii) to observe the development of eyes and other primary organs, including heart, liver and pectorals muscle, of embryos and newly hatched chicks.

Shutze et al. (1962) first reported that incandescent lighting accelerated chick embryo development and shorted incubation time compared to commercial dark incubation. Similar results were also found in White Rock chickens (Siegel et al., 1969), Rhode Island Red domestic fowls (Adam and Dimond, 1971), quails and broilers (Walter and Voitle, 1973). Overheating effect caused by incandescent lamps is recognized as a potential factor that could accelerate embryonic development (Rozenboim et al., 2003; 2004). In this study, we thus determined the weight loss rate of fertile eggs during incubation period to judge whether there was an overheating effect. The results showed that regardless of the experimental treatment effect, the weight loss rate of fertile eggs increased with the extension of incubation time from E5 (3.83%) to E19 (11.67%) (Figure 1). However, no significant difference was observed in weight loss rate of fertile chick eggs from E5 to E19 days of age amongst 3 experimental groups (p>0.05; Figure 1), implying that an overheating effect caused by 30 lx LED light was unlikely.

Shafey and Al-mohsen (2002) reported that 20-watt green fluorescent light stimulation of meat breeder eggs during the first 18 days of incubation reduced incubation period and increased embryonic growth and hatchability. On the contrary, our present study showed that both green and white LED light did not affect hatching time (p>0.05; Figure 2) or hatchability (p>0.05; Table 1), which is in accordance with previous studies on broilers (Rozenboim et al., 2004; Zhang et al., 2014a). The difference in results observed between these studies probably was probably due to an overheating effect caused by 20-watt green fluorescent lamps used by Shafey and Al-mohsen (2002). The first week posthatch is critical for chicks in terms of evaluation of growth potential, including muscle satellite cell mitotic activity, digestive organ development, and BW gain (Moore et al., 2005; Zhang et al., 2014a; Lilburn and Loeffler, 2015). In our study, compared with the control group, green LED light stimuli during incubation tended to increase feed intake (p = 0.080), and improve the BW gain of chicks from o to 6 days posthatch (p<0.05; Table 1), which once again demonstrated that green LED light has positive effects on chick growth during the early posthatch stages. More importantly, the power consumption of green LED lamps was approximately 2 kWh (4 watts×24 h/d×19 d), which only marginally increased the cost compared to dark incubation. According to some published reports, the possible mechanisms of the growth-promoting effect in broilers of green LED light stimuli during incubation can be summarized as follows i) green light stimuli enhances the proliferation and differentiation of satellite cells in late embryonic stages, and subsequent muscle hypertrophy (Halevy et al., 2006; Zhang et al., 2014a); ii) green light increases melatonin secretion by influencing circadian rhythm of melatonin biosynthesis in the embryo pineal gland, which increased the proliferative activity and the antioxidant status of hepatocytes (Piesiewicz et al., 2012; Wang et al., 2014); and iii) the higher secretion of somatotropic axis hormones (growth hormone [GH] and insulin-like growth factor-I [IGF-I]) may be another important contributor to chicken growth promoted by green light stimuli during embryogenesis (Zhang et al., 2014b).

The eyes of chickens are beginning to develop after 48 hour incubation, and the most useful criteria to determine their development can be obtained after 12th incubation days (Hamburger and Hamilton, 1951). Exogenous light can penetrate the eggshell and be recognized by eyes of chick embryos, and then affects the function of their lightsensitive pineal gland in the late incubation stage (Aige-Gil and Murillo-Ferrol, 1992; Zeman et al., 1992). However, light stimuli is also a source of corresponding risks to the development of avian eyes. Whitley et al. (1984) found that chickens raised from 2 weeks of age in continuous fluorescent light (1,044 lx) displayed abnormal buphthalmic eyes (in which corneal diameters and axial globe lengths the eyes become enlarged) and even blindness. Direct exposed to a source of white light during incubation also may cause a number of teratological effects in chickens such as higher mortality, lower eye weight, and the increase in number and size of the intracytoplasmic lipid droplets of the follicular pinealocytes (Aige-Gil and Murillo-Ferrol, 1992; Archer et al., 2009). However, the present study showed that providing 30 lx green or white LED light during incubation did not affect any of measured eye parameters, including weight, side-to-side, back-to-front or corneal diameter (p>0.05; Figure 3). In addition, no significant difference in embryo weight, chick BW, the weight percentage of eye, heart and liver to embryo weight or chick BW from days E15 to H6 (p>0.05; Figure 4) was observed among all experimental groups. However, 30 lx green LED light stimuli improved the weight percentage of pectoral muscle on day H3 and H6, which is a little sooner than the results of Zhang et al. (2014a), who reported that 15 lx green light stimulation during incubation accelerated BW and pectoral muscle growth starting from 7 d of age posthatch. This difference was observed probably because the light intensity in the present study (30 lx) was higher than that used by Zhang et al. (2014a). In addition, embryos or chicks in green light group had lower weight percentage of yolk retention on days E19 and H1 in comparison to those in dark or white groups (Figure 4), mainly because the embryos in green light group developed faster, resulting in higher nutrient consumption from yolk.

In conclusion, providing 30 lx green LED light stimuli during incubation has no detrimental effect on the development of eyes, heart and liver of embryos and hatchlings, but does have potential benefits in terms of enhancement of the chick growth during the early posthatch stages. In addition, 30 lx green LED light stimulation during incubation does not cause an overheating effect on fertile broiler eggs.