Towards the genetic improvement of bone quality in laying hens
The chances of bone fractures occurring in modern hybrids of laying hens are high during their lifetime and extremely high at depopulation.
This skeletal fragility is due largely to loss of structural bone mass throughout the life of the hen, leading to osteoporotic fractures.
Laying hen bones, however, are not like those of mammals; hen long bone marrow cavities contain a woven bone type known as medullary bone (MB). At sexual maturity, hens deposit this bone type as a temporary store of calcium for eggshell formation.
At night, when the bird is not taking in dietary calcium, this MB can be utilised as a calcium source. Unfortunately, the increased number of large bone-resorbing cells (osteoclasts) responsible for releasing calcium from the non-structural MB will not discriminate between bone types and structural bone is also lost.
During the laying period, only MB is formed and the missing structural bone is not replaced, leading to increased risk of bone breakage.
Investigations of osteoporosis in laying hens at Roslin Institute and elsewhere have revealed that good nutrition has a role to play and that adequate levels of Ca, P and Vitamin D are important.
However, whilst poor nutrition can exacerbate bone problems, there are very few dietary solutions aside from slight "tweaking" of diets with vitamin K and changing the form of dietary calcium. The latter is effective because calcium given as either particulate limestone or oystershell can be slowly released overnight, preventing excessive resorption from bone surfaces.
Human bone biology research shows that activity can prevent bone loss through functional loading of the skeleton. When this loading element is removed (as in space flight or prolonged bed rest) bone loss inevitably follows through increased resorption.
In hens the introduction of the battery cage and the first reports of severe osteoporosis in the same period were unlikely coincidences. Many researchers have shown that husbandry systems allowing increased activity are beneficial for bone strength However, there are also disadvantages to the introduction of these systems; apart from the increased chances of pecking and parasitism, the birds often have the chance to fly around, collide and crash-land.
Paradoxically the risk of bone fracture is increased in these systems and we should be careful to choose the appropriate hen to stock them with.
We had to ask ourselves, is there an appropriate hen of sufficient productivity to stock alternative husbandry systems with?
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Article made possible through the contribution of Lohmann Information