The extremely high frequency and severity of keel bone fractures in laying hens represent one of the greatest welfare problems in the industry1. The problem mainly manifests as fractures at the tip of the keel with distinct fracture lines and a varying amount of new bone formation (callus) (Fig.1). Frequencies ranging from 20 to 96% have been reported from countries such as: Belgium, Canada, Denmark, The Netherlands, Switzerland and the UK. The problem has sometimes been confused with keel bone deviations which may be defined as unnatural bending of the keel bone from a theoretical two-dimensional plane. However, this latter condition probably has to do with perching and the types of perches.

In addition to welfare concerns, keel bone fractures caused the largest financial losses in laying hens, estimated at €3 per bird averaged by PROHEALTH in a literature study2. The study identified the dearth of literature on the costs associated with the condition because literature has primarily an epidemiological focus, with very few publications providing estimates of the financial impacts of diseases. However it seems obvious that keel bone fractures will have a negative impact on egg laying due to pain and reduced mobility, whilst affecting feed conversion efficiency.

Figure 1. Different manifestations of keel bone fractures. 1, 3 & 5 present quite productive fractures with new bone formation in the fracture lines. 2 & 4 present more discrete fracture lines which are difficult to palpate from the outside

The usual explanation for the cause of the fractures has been that the high calcium demand of modern hybrids for the increased egg production cannot be covered solely by the supplement in the feed and that this will result in resorption of structural bone to cover the need. This will leave the bone weak and brittle, and susceptible to fractures in case of collisions with housing objects. In addition to this, a long range of predisposing factors has been suggested. Factors such as bird genetics, lack of specific feed stuff components (such as omega-3 polyunsaturated fatty acids) and housing features (e.g. height and number of perches, absence of ramps connecting upper and lower aviary tiers) have all been shown to influence the frequency of the condition. There are some conflicting results regarding the involvement of the type of production system. Non cage systems would be expected to involve more bone loading and bone strength due to high activity in relation to movement between resources, but non cage systems actually have greater rates of fractures.

However, due to pathological characterization of the fractures and observations that fractures are almost as common in caged birds as in barn/free-range birds, new suggestions about the basis/cause of the condition have been put forward3. It is well known that the keel ossifies late compared to other bones; the caudal part (where most fractures occur) is not fully ossified before hens are 30-40 weeks of age4 (Fig. 2).

Figure 2. Arrow points to not yet ossified part of the keel bone (cartilage part)

The vast majority of the fractures observed occur in the area of the transition zone between bone and cartilage and such ossification zones are considered weaker than mature bone. It is a striking coincidence that the majority of the fractures are observed here. Fractures are not commonly observed during rearing, they start to appear when egg laying takes off. This is at the same time where the ossification zone have progressed to the area where most fractures can be observed and points to this area as a weak spot. In addition to this, it is not common to see signs of external trauma in this area, so something is happening here, but we do not know what3.  Further to this, experimentally it has been shown that beyond 49 weeks of age the rate of new fractures seems to level off or even decrease which is strange if weak bones depleted of calcium and phosphorus should be part of development of the condition. At 49 weeks of age the ossification is clearly complete which may reduce development of new fractures.

We do not currently have an explanation for the apparently increasing occurrence of keel bone fractures, but Age at First Egg (AFE) may be a parameter for closer investigation. It has also been suggested that earlier breeding goals, including breeding for a smaller bird and a higher egg production may have resulted in some unfortunate effects. Consequently, an increased egg production from a smaller metabolic mass has been speculated to result in depletion of the bird body reserves. Delaying sexual maturity may allow for greater skeletal maturity (including the keel bone) before egg formation and could be speculated to reduce the occurrence of fractures, but this has to be demonstrated.

Indeed progress has been made in the understanding of keel bone fractures and new methodologies in the form of radiography, CT (Fig. 3 & 4) and MR scanning and histopathological staining techniques will allow for further progress in the future. However, there is still a need for more information concerning the link between high egg production and fractures developing in the laying period, the role of Age to first Egg, detailed investigations of the ossification of the keel bone (currently only old investigations available) and the role of management and housing. In this respect it should be remembered that several factors may be involved at the same time and perhaps no single factor alone is responsible for the development of this condition.

Figure 3. CT scanning of laying hens

Figure 4. 3D modelling of keel with numerous fractures based on CT scans. Arrow points to major, old fracture, but more are seen at the tip.

Literature

1FAWC (2013). An open letter to Great Britain governments: keel bone fracture in laying hens. Farm Animal Welfare Council, London.

2Jones, P.J., Niemi, J., Christensen, J-P., Tranter, R.B. and Bennett, R.M (2018). A review of the financial impact of production diseases in poultry production systems. Animal Production Science. https://doi.org/10.1071/AN18281

3Thøfner, I., Hans Petter Hougen, Chiara Villa Niels Lynnerup, and Jens Peter Christensen (2020). Pathological characterization of keel bone fractures in laying hens does not support external trauma as the underlying cause. PLoS ONE 15(3): e0229735. https://doi.org/10.1371/journal.pone.0229735

4Buckner, G.D.; Insko Jr, W.; Henry, A.H.; Wachs, E.F. (1949). Rate of growth and calcification of the sternum of male and female new hampshire chickens having crooked keels. Poultry Science 1949, 28, 289-29