Abstract
There are indications that lighting schedules applied during incubation can affect leg health at hatching and during rearing. The current experiment studied effects of lighting schedule: continuous light (24L), 12 hours of light, followed by 12 hours of darkness (12L:12D), or continuous darkness (24D) throughout incubation of broiler chicken eggs on the development and strength of leg bones, and the role of selected hormones in bone development. In the tibiatarsus and femur, growth and ossification during incubation and size and microstructure at day (D)0, D21, and D35 post hatching were measured. Plasma melatonin, growth hormone, and IGF-I were determined perinatally. Incidence of tibial dyschondroplasia, a leg pathology resulting from poor ossification at the bone’s epiphyseal plates, was determined at slaughter on D35. 24L resulted in lower embryonic ossification at embryonic day (E)13 and E14, and lower femur length, and lower tibiatarsus weight, length, cortical area, second moment of area around the minor axis, and mean cortical thickness at hatching on D0 compared to 12L:12D especially. Results were long term, with lower femur weight and tibiatarsus length, cortical and medullary area of the tibiatarsus, and second moment of area around the minor axis, and a higher incidence of tibial dyschondroplasia for 24L. Growth hormone at D0 was higher for 24D than for 12L:12D, with 24L intermediate, but plasma melatonin and IGF-I did not differ between treatments, and the role of plasma melatonin, IGF-I, and growth hormone in this process was therefore not clear. To conclude, in the current experiment, 24L during incubation of chicken eggs had a detrimental effect on embryonic leg bone development and later life leg bone strength compared to 24D and 12L:12D, while the light-dark rhythm of 12L:12D may have a stimulating effect on leg health.
Highlights
In nature, chicken eggs will be exposed to short bouts of light when the hen leaves the nest to eat and drink [1]
One effect that has been observed in other studies is that lighted incubation may stimulate leg bone development and leg health in meat type chickens (“broilers”). [6] found 2.2% fewer chicks that were too weak to stand or suffered from leg abnormalities at the moment of hatching in embryos incubated under 12h of light, followed by 12h of darkness (12L:12D) compared to continuous darkness (24D) they found higher developmental instability, based on composite asymmetry of the leg bones, at day 14 post hatching for broilers incubated under 24D than under 12L:12D [6]
Chicken embryo bones are first formed as a cartilage model [7], which later becomes ossified through a ring of bone material at the mid-diaphysis of the bone [8,9]
Summary
Chicken eggs will be exposed to short bouts of light when the hen leaves the nest to eat and drink [1]. Melatonin is involved as a pathway to alter bone development, we expect that 12L:12D will result in better bone development and leg health than continuous light (24L), since melatonin release is darkness dependent. We expect that these effects are long term and can be observed till slaughter age in better developed, healthier bones. The current experiment aims to investigate effects of light-dark rhythms (24D, 24L, or 12L:12D) applied throughout incubation of broiler chicken embryos on embryonic plasma melatonin levels, GH and IGF-I at hatch, leg bone development during incubation, at hatch, and at slaughter age (5 week post hatch), and incidence of tibial dyschondroplasia at slaughter age. The role of embryonic plasma melatonin levels, GH and IGF-I at hatch are investigated as a potential pathway for altered bone development
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