1. Introduction
The rising public concern for poultry welfare and increasing labor costs have resulted in in stacked natural mating colony cages becoming a trend in housing systems for commercial layer breeders in China. Layer breeders are the parent-stock of laying hens and in colony cages are confined together with roosters. The ratio of roosters and hens is generally kept between 1:10 and 1:8 and the flock size is usually maintained between 40 and 100 per individual cage. Compared with the cage system using artificial insemination, the natural mating behavior of breeding hens can be expressed in the natural mating colony cage system, taking into account animal welfare, high efficiency, energy savings, and clean production characteristics [
1]. However, this housing system is still in the stage of exploration and optimization. Behavioral issues such as feather pecking (FP) and cannibalism are prominent in this system, contributing to economic losses and diminished health and welfare of hens. Currently, limited systematic research on FP and cannibalism in natural mating colony cages can be found. Available and efficient management measures are urgently required to ease the negative effects caused by FP and cannibalism in this colony cage system.
Feather pecking and cannibalism can occur as a result of numerous factors including genetic background [
2], hormones [
3], nutrition [
4], group size and stocking density [
5], and environmental enrichment [
4]. Indeed, light management is a crucial eliciting factor of the incidence and severity of FP and cannibalism of hens [
6]. Measures such as keeping the hens under a reduced light intensity or altered light color are usually adopted to alleviate FP and cannibalism when necessary [
6,
7]. The objective of dimming the light or altering the light color is to diminish the birds’ perception of colors and visual detection among them [
8].
Excessive light is a vital factor initiating and favoring FP and cannibalism [
9]. It was reported by Blokhuis and Arkes [
10] that higher light intensity strongly impacts the occurrence and severity of FP in hens, resulting in more pecking damage. Reduced feather pecking behaviors and incidence of aggressive behaviors were observed by lowering the light intensity according to the results of Braastaad [
6]. Hens confined close to light sources at an intensity level of 11–44 lux were more likely to perform FP than those further away where the light intensity ranged from 1 to 11 lux [
11]. However, Kjaer and Sørensen [
12] found that light intensity had no impact on the frequency of FP in any of the tested genotypes. Experimental results on the effects of light color on FP or aggression behavior are contradictory [
7,
13,
14]. Due to other environmental effects and the strain differences between hens, it is difficult to draw any firm conclusions from these experiments.
Light-emitting diodes (LEDs) are a special kind of semiconductor diode which can give monochromatic light. Compared to incandescent light and fluorescent light, LED light has a marked longer life, specific spectrum, lower thermal output, higher energy efficiency, and higher reliability and frequency, as well as lower maintenance costs [
7,
13,
15]. Knowledge about the influence of light condition on FP behavior is well documented for laying hens in other housing systems and strains, such as Oakham Blue [
8], White Leghorn [
12], and Brown Nick laying hens [
16] in free-range systems, ISA Brown [
9] and Lohmann Brown [
17] hens in deep litter systems, Dekalb white breed hens in aviary systems [
18], White Leghorns hens in battery cages [
19], and so on. However, effects of LED light wavelength and intensity on FP and cannibalism have rarely been investigated in natural mating colony cages. The results of both wavelength and light intensity on the behaviors of laying hens in other housing systems may not be applicable to this colony cage system. Therefore, it is crucial to explore the effects of LED light color and intensity on FP and cannibalism in order to provide a basis for the regulation of light environment for layer breeders in natural mating colony cages. The objectives of this study were to investigate the effects of four LED light colors (white, red, yellow-orange, blue-green) with two light intensities in each color on FP, plumage condition, mortality from cannibalism, fear, and stress hormones for layer breeders in natural mating colony cages.
4. Discussion
Behavior is a good indicator for the evaluation of laying hen welfare. In this experiment, hens under RL and BG tended to express more frequent GFP and ENP than birds exposed to the other two lighting colors, but a lower SFP frequency under RL (especially compared with WL and YO, with BG being intermediate). These results suggest that hens under RL and BG were more engaged in explorative behavior. We also noted that the pecking activities were promoted by high light intensity and hens under high light intensity were more vulnerable to suffering from SFP. Clearly, pecking behavior may be affected by the wavelength of light as well as by light intensity. Huber-Eicher et al. [
16] investigated the effects of colored LED illumination on behavior of laying hens. Hens under green light spent more time on pecking at objects and had more frequent pecking at conspecifics compared with red and white light. Hens under red lighting showed less often severe pecks or distress calls than hens under white light, with green light being intermediate. Mohammed et al. [
17] looked at the behavior of laying hens under four different light sources. Higher frequency of GFP and aggressive behavior were increased by blue light and high light intensity. This current study confirm these findings that red light alleviates SFP. The higher contribution of longer wavelengths contained in red light may have reduced SFP behavior, although this needs confirmation. This effect was due to the wavelength and should not be confused with eventual effects of intensity. There is now a general agreement that a particular causative factor that is positively correlated with FP is the inhibition of foraging or dust bating behaviors, such as ground pecking or ENP [
23]. It has been suggested that FP is a redirection of oral behavior toward conspecific under barren conditions [
24]. In our study, the hens under RL and BG spent more time in their explorative pecking behaviors (GFP and ENP) compared to hens of the other treatments; therefore, attention and severe pecks of the hens shifted from conspecifics towards the surroundings. In other studies, Sultana et al. [
7] studied the effect of various LED light color on the behavior of laying hens and indicated that hens in red light were more active and expressed more feather pecking than those of hens in blue light. Prayitno et al. [
13] suggested that broilers illuminated with red light showed more aggression and did more floor pecking than birds under white, green, or blue light. Similar increases in aggressive behaviors were recorded in a separate investigation of broilers maintained under red, compared with blue, lighting through to 8 weeks [
25]. These results are likely a consequence of the perceived increased intensity, as broilers are more sensitive to this range of the spectrum than that measured by lux [
26], and birds have greater visual acuity in red light, while higher light intensity increases aggression. Long wavelengths may alter the reflectance of both the plumage of hens and the appearance of the experimental houses [
20,
27]. This may well make plumage and objects within the environment more attractive for the birds to peck at and explore. However, Leighton et al. [
28] suggested that light sources do not affect these behaviors. Lewis and Morris [
29] also mentioned that light color appears to have minimal influence on FP, as red light would reach the hypothalamus more rapidly than blue light. In the above studies that differ from the results in the present experiment, only Sultana et al. used LED light. It is difficult to reach a consistent conclusion from previous studies about wavelength effects upon FP. The discrepancy between the results may be caused by the differences of spectral sensitivity of the fowl, the spectral output of the light sources, the adaptability of birds to particular light environment over time, the housing system, stocking density, group size, and so on. Those aspects complicate direct comparisons of the data. The reduction in SFP under red light needs further evaluation because it could be of interest in commercial production situations.
The results in the present experiment indicated that hens caged in RL had effectively reduced mortality from cannibalism and cannibalistic injuries, in accordance with the finding of Wells [
14] who found that the employment of red filters or red paint to light sources may be a simple and effective method in alleviating SFP and cannibalism. However, it may be surprising that in spite of the probable differences in the intensity perceived by hens, even where the light had been adjusted being equated for irradiance, wavelength generally did not significantly affect mortality rates in broilers [
30]. The parent-stock hens in colony cages were confined together with roosters. The frequent mounting behavior may generate inferior back and rump plumage conditions, which resulted in hens suffering from injuries or scratches on the back and rump. There is a risk of severe feather pecking and cannibalism, especially if there is hemorrhage, broken skin, and fresh wounds. Therefore, the explanation of the red light reducing mortality from cannibalism and cannibalistic injuries may be that the birds cannot easily see red blood or fresh wounds in red light [
31]. The elevated mortality and cannibalistic injuries under high light intensity noted in the present study was in accordance with Kjaer and Vestergaard [
9], who suggested that high light intensity in both rearing and laying periods tended to increase mortality during laying, especially due to cannibalism.
Light sources have influences on plumage condition of hens through the influences on FP, as described by Long et al. [
18], who showed that different light sources might affect plumage condition as judged by the incidence of feather pecking. In the current study, back, rump, belly, and overall plumage condition of the hens under RL and BG tended to be superior to those under WL and YO. Also, the increased plumage damage under high light intensity found in the current experiment confirms previous findings by Hughes and Duncan [
11], Hughes and Black [
32], and Allen and Perry [
33], who indicated that high light intensity strongly affects the occurrence and severity of FP in laying hens with higher light intensity resulting in more damage. According to Bilcík and Keeling [
34], GFP does not contribute to feather damage, while SFP is identified as the major cause of feather pulling, damaging, and plucking. Huber-Eicher and Sebö [
35] suggested that at an early-age GFP is prevalent, whereas more SFP can develop later, resulting in more deteriorated plumage in older hens, consistent with our observations. Therefore, it could be speculated that hens under RL and BG had a better plumage condition which may attribute to being engaged less in SFP.
Reactions to humans or a new environment are widely employed to estimate the fearfulness of hens [
36]. The ability to deal with this situation reflects the stability of the nervous system and the degree of individual excitability [
3]. In this experiment, it seems that the likelihood that hens under RL and BG and caged in low light intensity approaching the NO was higher, the duration of the TI test was shorter, and the distance of the AD test was closer than hens in WL and YO and high light intensity. In addition, WL and YO tended to cause longer freezing time, longer latencies, and more distress calls. This indicated that hens caged in WL and YO were more fearful and susceptive to fear tests. However, the results of the study were in disagreement with those of Scott and Siopes [
36], who found that no behavioral indications of stress were observed when mature turkey hens were exposed to blue, green, red, or white illumination of the same photon flux from commercial lamps between 30 and 53 weeks. One possibility for the discrepancy may be that the different breeds of hens may respond to light conditions differently. Studies have shown that the fearfulness of hens was associated with feather damage in commercial breeding [
37]. The results are in accordance with those of the study by Johnsen et al. [
38], which reported that severely feather-pecked birds tended to have an inferior feather coverage condition and were more fearful than birds with minor pecking damage. Hughes and Duncan [
11] also found that fearful behavior was associated with greater feather loss. Other studies suggested that on an individual and flock level, having high levels of fear at a young age can become a risk factor for developing feather pecking as adult [
39]. Therefore, the effects of light condition on behavioral response to fear tests of hens might through the effects on FP.
According to previous studies, thyroidal hormones are considered to be physiological indicators of various forms of stress in fowl [
40]. Triiodothyronine (T3) regulates the metabolic rate and T4 is considered to be inducing molting of laying hens [
41]. However, in this experiment, T3 and T4 concentrations were not affected by the light treatments. The hormones may be correlated with the quality of feather coverage. In addition, the CORT and 5-HT levels have been proven to be associated with fearfulness and feather pecking [
42]. Hens caged under RL and low light intensity tended to have a higher concentration of 5-HT, a lower CORT concentration, and a lower ratio of heterophils to lymphocytes than WL and YO, which suggested that hens treated with RL and low light intensity showed a lower stress response. The results of the study were in disagreement with those of Olanrewaju et al. [
43]. who found that there were no effects of light sources on plasma CORT concentrations. Scott and Siopes [
36] also indicated that blue, green, red, and white lights were not stressful to the birds. However, sampling data for the 45 and 53 week showed that the birds exposed to red light had the lowest proportion of heterophils and the narrowest H/L ratio [
36]. However, the effect of light color on the significant effect on CORT concentration and H/L ratio in this study was not clear; this effect might be caused by the effect of light condition on hens’ behavior. As previous studies regarding the effect of light color on fear response of layer hens are scarce, a direct comparison is difficult. However, these results showed a consistent tendency towards greater CORT concentration [
3], lower levels of whole blood 5-HT [
44], and a higher H/L ratio [
44] in highly fearful hens, which showed long tonic immobility durations, a far avoidance distance, and particular fearfulness of novel objects in this study. Cockrem [
3] found that corticosterone responses and fearfulness were linked and indicated that greater fearfulness was accompanied by larger corticosterone responses to potentially threatening stimuli. Bolhuis et al. [
42] suggested that hens from the generation of the low mortality line showed less fear-related behavior and displayed higher whole-blood 5-HT concentrations. José et al. [
44] indicated that hens suffering from cloacal cannibalism were more asymmetrical, stressed, and fearful than non-vent pecked birds, with increased heterophil to lymphocyte ratio and tonic immobility duration. However, in the present study, the differences of the level of fearfulness were not reflected precisely in the concentration of thyroidal hormones. Therefore, measuring the thyroid hormone may not be a particularly appropriate method for evaluating stress in hens, because some factors related to welfare appear to lead to a rise, whereas others result in a fall [
40]. Under closely controlled conditions, circulating stress hormones can be a measure of the hen’s reaction to its environment. The condition is apparently not so straightforward in actual operations. The only safe conclusion seems to be that for stress hormones too many uncontrolled factors exert an effect to permit these indicators to be employed as simple and practical assessment of welfare.