Osteoporosis and associated bone fractures represent one of the most significant welfare problems in commercial laying hen production globally. The metabolic demands of intensive egg production, combined with housing systems that limit physical activity, create conditions for severe skeletal deterioration.
Laying hens produce approximately 300 eggs per year, each requiring about 2g of calcium for shell formation. While dietary calcium supplementation partially meets this demand, calcium is also mobilised from the skeleton. Over a laying cycle, this repeated calcium mobilisation causes significant reduction in cortical bone density (structural bone). Simultaneously, medullary bone (a metabolically active calcium store) provides a short-term calcium reservoir but cannot fully compensate for structural bone loss.
Keel bone (sternal) fractures are the most prevalent bone injury in laying hens, affecting 50-80% of commercial flocks in some studies—with prevalence depending on housing system and management. Keel bone fractures cause chronic pain (demonstrated by changes in feeding behaviour, pain postures, and response to analgesics), lameness, and reduced ability to compete for resources. Many fractures occur during low-impact collisions with housing furniture (perch ends, nesting box edges) or from dynamic loading during landing—suggesting both housing design and bone strength as intervention targets.
Bone health varies significantly between housing systems. Furnished cages provide perches that partially stimulate bone strength but limit exercise. Barn and free-range systems allow more movement but paradoxically show high keel fracture rates due to more opportunities for collisions and falls. Aviary systems with multiple tiers, while providing behavioural opportunity, show particularly high keel fracture prevalence due to birds landing on perches and equipment. The relationship between housing system and bone welfare is complex—more movement space does not automatically mean better bone health.
Egg production rate is negatively correlated with bone strength—breeds selected for maximum laying performance tend to have weaker bones. Breeding programmes incorporating bone strength traits (using dual-energy X-ray absorptiometry—DEXA—to measure bone density in selection candidates) can improve bone health without sacrificing production. This represents a long-term, sustainable approach to improving laying hen welfare across the industry.
Nutritional strategies to improve bone health include: optimising dietary calcium and phosphorus levels and ratios, vitamin D3 supplementation (essential for calcium absorption), and appropriate feeding systems (particle size of limestone grit affects calcium absorption rate). Exercise opportunity through the laying cycle, perch design (diameter, surface texture, positioning—avoiding abrupt perch ends) and housing furniture layout can reduce collision-related fractures.
Assessing keel bone health requires physical palpation or radiography—not possible routinely during the production period but feasible at depopulation. Post-mortem assessment of keel fracture prevalence is an important welfare indicator used in research and progressively in welfare monitoring programmes. Industry and certification body standards increasingly address keel bone welfare through requirements for perch design, space allowance, and welfare outcome monitoring.