Eggs are a source of highly digestible protein that have done much to improve world nutrition. To enhance the
sustainability of production there are persistent demands to increase egg quality and the length of time in lay.
However, a longer period of egg production can come with correlated problems of bone quality
Bones come in a variety of shapes and sizes and have a
complex internal and external structure. The hard outer
layer of bones is composed of cortical bone, also called
compact bone. Filling the interior of the bone is the cancellous
bone, also known as trabecular or spongy bone tissue. It has a
more open structure and its spaces are filled with bone marrow
and, in the case of laying hens, a form of bone called medullary
bone. The cortical and cancellous bone types are the key components that give the skeleton its strength.
Osteoporosis
The medullary bone acts as a special form of calcium storage.
This special bone is absorbed and laid down rapidly with the daily
cycle of shell formation. The cells which form bone are known as
osteoblasts. when hens are laying eggs, these cells are almost exclusively active in forming medullary bone.
There are also bone absorbing cells which are known as osteoclasts. They reabsorb the medullary bone to form the shell and
they reabsorb the cortical and cancellous bone. This results in a
gradual weakening of the skeleton over the lifetime of the hen. As
in humans, this is known as osteoporosis and it can increase the
chances of bone breakage.
Keel damage
Alternative housing systems can make the problem worse with
increases in breakage and keel bone deformities. This is caused by
the increased opportunity for damage due to collisions, etcetera. Keel damage has been identified as a particular problem, with
incidences of keel damage in commercial flocks varying from 20
to 80%. As the demand for economical food is unlikely to reduce,
solutions to improve bone health are required if the welfare of the
laying hen is not to be compromised.
Improving bone strength
“Our research with
LOHMANN TIERZUCHT has focused on developing approaches to improve bone strength”, says Heather McCormack. “As a consequence the welfare of the laying hen will also
improve whilst maintaining egg production.” The research team
used a programme of retrospective selection over nine generations. “we produced hens with a two-fold difference in tibiotarsal
breaking strength. we demonstrated that bone characteristics
can be improved by selection and that there is the genetic potential within commercial pure lines to both lay a high number of
eggs and have good bone quality at the end of lay.” Most importantly, this improvement in bone strength was accompanied by a
reduction in the incidence of fractures.
Genes
The next task was to identify the Quantitative Trait Loci (QTL).
These are regions of DNA which contain or are linked to genes
that underlie the bone quality traits. A large QTL was identified on chromosome 1 for component traits. Tibiotarsal breaking strength
for instance. “we were confident that selection with markers in
this region would improve bone strength”, McCormack says. “As
the revolution in genetics advanced, we were able to complete
high density genotyping of this region of the genome in a recent
generation of a white leghorn line. This produced DNA markers
which were highly associated with tibiotarsal breaking strength.
They linked to about a ten percent increase in strength and density of the structural bone.” so the application of markers has the
ability to improve bone strength and improve the welfare of laying
hens when applied in a breeding programme. This investment in
research over the last twenty years has now put us in a position to
make an impact on bone strength and hen welfare.”
New project
The team is about to start a new project funded through the
ANIHwA programme of the european Union. McCormack: “Over
the next three years we will develop and validate a number of
complimentary approaches to improve bone quality.”
1) A study will be carried out, using large genotyping arrays which
are now available. This will be done to identify further genetic
markers for bone quality. All these markers will be validated in
other genetic lines of laying hens for use in selection.
2) Using another tool of the revolution in genetics called ‘nextgeneration sequencing’ the research team has identified a novel
gene. “we believe this gene may be responsible for some of the
differences in bone strength”, says McCormack. “This in turn has
led to the identification of metabolic marker. we believe this marker can also predict variance in bone quality. Measurement of this
metabolite early in life could predict bone quality at the end of lay.”
3) Finally, ultrasound has been successful in determining bone
quality in humans. The team’s initial studies with chicken’s toes demonstrated good genetic correlation with bone strength. Technology has advanced since these studies. It will now utilise so called
‘axial transmission’ in the surface of cortical bone. This will allow
the keel and the long bones to be analysed.
Constant remodeling
The ability to measure bone quality in living hens will create a big
advantage in improving bone health.
“we will discover and test new ways of selecting hens to improve
bone quality. Plus, we will learn more about the biology of bone
formation of avian bone and the factors which influence it. Bone
is a living tissue that is constantly being replaced and reshaped,
following injuries such as fractures and micro-damage (which occurs during normal activity)”, McCormack explains. “Bone tissue
is constantly being remodeled. This is a lifelong process where
mature bone is removed from the skeleton and new bone is
formed. ” These processes produce characteristic changes in the
composition of the bone. The team will use sophisticated analytical techniques such as Infrared spectrometry, Optical emission
spectroscopy and 2D X-ray Diffraction, to further inform us about
the ultrastructure and composition of the bone.
Practical solution
“By using several approaches we want to deliver a practical solution to breeders that will realise the genetic potential of laying
hens with better bone”, McCormack states. “we expect to demonstrate that combinations of genetic markers can explain suf
Computed tomography of bone showing medullary bone in green
ficient variance in bone quality to be used in selection. And that
the markers work in several lines of chicken. we also expect that
the new non-destructive measurement of bone property phenotypes will have sufficient correlation with traditional but destructive measures of bone quality for it to be used predictively in selection. Finally, we will also be able to further our understanding
of avian osteoporosis and lay the foundation for future advances.”
Tibitorsal breaking strength (N)
Ian C. Dunn, Heather McCormack and Bob Fleming, The Roslin
Institute and RDS(V)S, University of Edinburgh, Scotland, U.K., Dirk
Jan DeKoning, Swedish University of Agricultural Sciences, Sweden
and Alejandro Rodriguez-Navarro, University of Granada, Spain.