What are keel bone fractures - and how big of a problem are they?

Most commercial laying hens have a fractured or deformed keel bone. It is one of the most common health conditions in modern egg production.

Studies using radiographic assessment (X-ray, the most accurate diagnostic method) put prevalence at 80–97% in adult commercial flocks. Yet outside veterinary and welfare-research circles, keel bone fractures (KBFs) remain almost entirely unknown.^[1][3]

That is beginning to change, and producers have good reason to get ahead of it. Keel bone damage is increasingly on the radar of the bodies that set standards for the egg sector (the European Food Safety Authority and the EU Platform on Animal Welfare have both flagged keel bone health as a welfare indicator to monitor), and the condition is linked to reduced mobility and to measurable effects on flock performance. This piece sets out what KBFs are, what we know about why they happen, and what the implications are for producers, welfare standards, and the industry as a whole.

What is a keel bone fracture?

The keel bone is a long, prominent ridge of bone that runs along the underside of a bird's sternum. In humans the sternum is a flat plate; in birds it has evolved into a deep keel-shaped structure that anchors the flight muscles, and since birds lack a muscular diaphragm, they depend on the keel bone and ribs to drive breathing. In laying hens it is the largest single bone in the body, and the most vulnerable.

A few terms are worth distinguishing:

• Fractures are breaks in the bone.
• Deviations are bends or curves in the bone that may or may not involve a distinct fracture line; they tend to accumulate gradually and are even more common than fractures.
• Lesions and damage are broader umbrella terms used to capture both.

Fractures have the clearest welfare significance, but deviations are increasingly recognised as relevant too.

How common are keel bone fractures?

So the exact answer depends heavily on how you measure. Keel bone injuries are assessed by palpation (feeling the bone by hand), radiography (X-ray), or post-mortem dissection, and these methods produce very different figures. Radiography captures fresh fractures and small breaks that leave no external sign, which is why it finds prevalence far higher than palpation does.^[3]

What can be said with confidence is that across virtually every commercial cage-free flock assessed with rigorous methods, fracture prevalence at end of lay is high, by radiographic assessment typically well above 80%, across different housing systems (including free-range and organic) and across the UK and Europe.^[1]

What causes them?

To understand why keel bone fractures are so prevalent, it helps to start with what is happening inside the bird.

Commercial laying hens are bred to produce a large number of eggs (a modern hen may lay more than 300 a year). Each eggshell requires significant calcium, and calcium is also essential for bone formation. The keel bone is unusual in that it does not fully harden ("ossify") until well into the laying period, typically not until 35–40 weeks of age, leaving it particularly vulnerable at exactly the moment production demands are highest.^[4]

As lay progresses, hens cannot absorb enough dietary calcium to meet both eggshell and bone requirements, and begin drawing on their own skeletal stores. The result is a keel that is chronically under-mineralised and increasingly fragile as the laying period advances.

That explains fragile keel bones. Several distinct mechanisms then commonly take a bone from "fragile" to "fractured":

Collisions. Hens in aviary systems navigate multiple tiers by jumping and flying, and collisions with perch edges are common. A badly executed landing that a well-mineralised bone might absorb without consequence can fracture a bone weakened by months of calcium deficit.^[4]

Perching stress. Even without a collision, prolonged resting on hard perch edges applies chronic pressure to the keel, which can cause stress fractures. Intervention studies have shown that softer, more compliant perch materials significantly reduce keel bone damage.^[5]

The internal load of egg laying. There is also a less obvious mechanical load: each egg passes through the body under internal pressure, bearing against surrounding structures including the keel. A hen laying an egg almost every day is subject to this repeated load throughout her productive life.^[6]

Catching and transport. A distinct category of injury arises at end of lay. Fresh fractures are regularly found at slaughter that were not present on-farm, attributable to physical handling during catching and transport.^[2]

What does it mean for the bird and the flock?

The most consistent practical effect is on mobility. Hens with keel bone fractures tend to move less, are less willing to access feeders and drinkers, and make less use of the full height of the system, patterns observed across multiple independent research groups.^[2] That reduced mobility is also where the production signal comes from: birds with fractures show modestly lower egg output and feed-related performance, a relationship increasingly noted by researchers and parts of the industry.

Welfare scientists regard fractures as a genuine source of discomfort, and the condition is treated as a meaningful welfare issue by EFSA and others, one good reason it is rising up the agenda. Recovery is also slow and often incomplete: the keel is under mechanical load during normal activity, and the metabolic environment of a laying hen does not favour rapid healing, so fractures sustained early in lay can persist for months while new injuries accumulate alongside them.^[2]

In short, a flock with healthier keels is likely to be a more active, more uniform, better-performing flock, and one better placed for the welfare expectations now taking shape.

The way forward

The encouraging news is that the problem is tractable. Housing design, perch materials, nutrition and genetics all point to real, practical levers for reducing fracture prevalence. The challenge is translating that research into accessible guidance, and that guidance into changed practice at scale.

For producers, the picture today is complicated by a lack of clear, widely available recommendations on what to change and what difference it will make. But the direction of travel is clear: keel bone health is increasingly visible to researchers, welfare organisations, and the standard-setting bodies of the egg industry, and is moving towards becoming a routinely monitored welfare indicator. Producers who engage with the evidence now will be better placed as standards evolve, and stand to gain on flock performance in the meantime.

Stronger Flocks recommends actively monitoring keel bone damage in your flock and weighing the levers available to improve bone health. For practical, tailored advice, get in touch at hello@strongerflocks.com.

For the industry more broadly, keel bone fractures are as much an opportunity as a challenge. Better welfare outcomes and better productivity outcomes are not in tension here, and demonstrating genuine progress on a significant, previously neglected issue would strengthen the egg sector as a whole.