5 | Post hatch lighting schedules

5.1 |

So far, effects of incubation lighting schedule on leg health and bone development have been described. Another important phase in bone development is the early post hatch period, when a high rate of leg bone ossification and length growth takes place (Apple-gate and Lilburn, 2002; Kürtül et al., 2009). It was expected that light-dark rhythms in the first 4 days post hatch (the brooding phase) would be beneficial for leg bone develop-ment. In two experiments, described in Chapter 7, broiler chickens were incubated under 24D and then brooded in a HatchBrood system (Van der Pol et al., 2013) under 24L, 2h of light, followed by 1h of darkness (2L:1D), or 2h of light, followed by 6h of darkness

Chapter 8

| 184

(2L:6D). 2L:1D was chosen because practical experience showed that this lighting sche-dule stimulated activity and feed intake in chickens housed in HatchBrood compared to 24L. 24L was chosen as the most contrasting schedule, without dark periods, and 2L:6D was chosen as a lighting schedule with prolonged dark periods to possibly stimulate bone development. Leg bone dimensions were measured at D4 post hatch. Higher femur and tibia diameter was found for 24L than for the light-dark schedules in the first experi-ment. Within the light-dark schedules, longer femurs and tibias were found for 2L:1D than for 2L:6D. In the second experiment, relative asymmetry of femur and tibia length and femur diameter was higher for 24L than for applying a light-dark schedule. Relative asymmetry is a measure of developmental instability (Møller et al., 1999), and 24L as a post hatch lighting schedule has previously been found to result in higher developmental instability than applying a lighting schedule (Møller et al., 1999; Campo et al., 2007). It can be speculated that asymmetry of leg bones creates uneven weight load on the legs, which may predispose a chicken to leg pathologies in later life. Unfortunately, leg patho-logies at slaughter age were not measured in the experiments described in Chapter 7, so this speculation cannot be supported with later life data.

Chickens in the brooding phase, such as in Chapter 7, are beyond the window of op-portunity to affect brain lateralization through light. According to Rogers (2008), late-ralization of the brain occurs during late incubation, because then one eye is exposed to light, while the other is covered. Post hatch, both eyes are exposed to light. An effect of lighting schedule in the brooding phase on behaviour can still be observed. Bayram and Özkan (2010) housed 2-day-old broiler chickens under 24L or 16L:8D. They found more activity, more comfort behaviours, and less stress behaviour in the light period in 16L:8D housed broiler chickens than in 24L housed broiler chickens. Behaviour was not observed in this thesis, but it can be speculated that activity is stimulated in 2L:1D brooded chickens compared to 24L brooded chickens.

Lighting schedules in the brooding phase may also affect early life bone development through differences in metabolism, but the mechanism is different post hatch than in ovo. While in ovo, the chicken embryo is limited in its gas exchange through the eggs-hell, and fat oxidation may be limited because of low O2 availability (Maatjens et al., 2014). Post hatch, low O2 levels are rarely an issue, and chickens can use nutrients from exogenous feed. In Chapter 7, it was found that femur and tibia diameter were higher for chickens brooded under 24L compared to a dark schedule, and within the light-dark schedules, 2L:1D led to longer femurs and tibias than 2L:6D. This may simply be a result of the bones growing in proportion to body weight, as yolk free body mass was higher for 24L than for the lighting schedules, and higher for 2L:1D than for 2L:6D.

Based on literature and the fact that some differences in leg bone development and leg bone pathologies are not well understood from the pathways studied in the current the-sis, post hatch activity might be involved, and it would be interesting to investigate this

General discussion

185 | into more detail.

5.2 |

In Chapter 6, the interaction between incubation lighting schedule (Inc24L, Inc16L:8D, and Inc24D) and a matching or mismatching post hatch lighting schedule (PH24L or PH16L:8D) from D0 till slaughter at D35 was investigated. No effects of the incubation x post hatch lighting schedule interaction on bone development at slaughter age, gait sco-res on D21, D28, or D34, or leg bone pathologies were found. PH16L:8D has the same total number of light and dark hours per day as PH2L:1D, which was tested in Chapter 7 in the brooding phase, but no interaction with incubation lighting schedule was tested in that experiment (see paragraph 4.1). Post hatch lighting schedule did have a main effect on leg health in Chapter 6: at slaughter, PH16L:8D had poorer gait scores and a higher score (indicating a higher and/or more severe incidence) of tibial dyschondroplasia and epiphyseal plate abnormalities, but also higher femur and tibia mineral content and mineral density, than PH24L. PH16L:8D did not lead to worse leg health in the whole grow out period, as gait scores on D21 and D28 post hatch were better for PH16L:8D than for PH24L. Other leg health variables or bone dimensions were not measured be-fore slaughter age, so it cannot be said when differences in bone development arose, and how the brooding phase contributed to bone development in this experiment.

Even though no clear circadian rhythm in melatonin release was found for embryos incubated under a lighting schedule in the current thesis, chickens may still have been entrained to a certain post hatching environment. Özkan et al. (2012b) incubated broiler chickens under 24D or 16L:8D during the whole incubation period or in the last week of incubation, and hatched chickens were housed under 16L:8D or 24L. Plasma was collected at D6 post hatch. They propose that housing chickens under lighting conditi-ons that matched their incubation conditiconditi-ons reduced stress, because they found lower corticosterone levels at D6 post hatch in chickens incubated under 16L:8D and then housed under the same schedule. Very high corticosterone levels reduce differentiation and proliferation of the chondrocytes (Robson et al., 2002; Van der Eerden et al., 2003), thereby possibly impairing length growth of the leg bones. With these results in mind, leg bone development was hypothesized to be increased, and leg bone health hypothesi-zed to be better, for chickens that were housed under the same lighting schedule during incubation and post hatch (Chapter 6). Eggs were incubated under 24L, 16L:8D, or 24D, and housed under 24L or 16L:8D post hatch. No differences were found in plasma corticosterone at D35 post hatch, but this may have been too late to still observe stress from a mismatch between incubation and post hatch lighting schedule. Furthermore, no differences were found for leg pathology scores, gait scores, or leg bone morphology at slaughter age. Entraining embryos to a post hatch lighting environment did not prove to be effective in improving leg health in this thesis. Possibly, newly hatched chickens are

Chapter 8

| 186

able to adapt quickly when they are moved from a free running rhythm to a fixed rhythm (like in Inc24L or Inc24D to PH16L:8D); in nature, too, chickens are moved to a true circadian rhythm only after hatching as a broody hen will not leave her nest daily (Ar-cher and Mench, 2014b). Moving newly hatched chickens from a fixed rhythm to a free