Bone fragility in egg laying hens

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Bone is a common and ubiquitous tissue that forms the skeletons of most vertebrates, however nearly all of what is known concerning bone adaptation and fragility has been gleaned from studies in mammals. The egg laying hen is the most efficient calcium transporter among all vertebrates, transferring approximately 10% of its total body calcium volume daily to produce an egg shell, of which only half is estimated to originate from dietary sources. Estimates suggest that 20-35% of mortalities occurring in commercially caged egg-laying hens are caused by underlying disuse osteoporosis. Of concern is the prevalence of keel bone fractures that are reported to occur in 5.5% of hens at the onset of lay and affect up to 97% of hens by the end of lay. Canada has recently implemented a ban on new conventional cage construction and a 20-year phase-in period of alternative housing. Little is known about the effect of alternative housing on bone mass, structure and especially tissue level material properties in different strains chickens. Interestingly, studies have shown that hens in free run floor-housed or aviary systems have stronger bones, but a higher number of old healed fractures (occur prior to depopulation) compared to hens housed in conventional cages. The etiology of these fractures remains unclear, although some speculate that they are due to increased collisions. It remains unclear how bone adaptation to mechanical stimuli provided by different housing contributes to fracture risk. In this research program, I will examined hens housed in three commonly used commercially available systems which provide varying levels of mechanical stimuli. I will examine mechanical strains engendered in keel bones during various activities in two strains of hens (Lohmann Brown and Lohmann Select Leghorn-Lite) throughout the lay period (18, 32 and 54 weeks of age). I will also examine fractured and nonfractured keel bone mass, microstructure, and quality in hens to determine if outcome measures are altered in fractured bones. To determine if bone quality during fracture is also deteriorated at remote sites I will also perform these analyses in a long bone (tibiotarsus) of adult hens with and without keel bone fractures. The short-term goal of this NSERC DG is to elucidate the etiology of keel bone fractures in hens. The long-term goal of my program is to use bioengineering approaches to understand skeletal health of birds. This program will allow us to study in a highly-controlled animal model and environment, the detailed interactions between the two fundamental processes that play critical roles in bone regulation and adaptation - mechanical stimulation and mineral homeostasis. This research program is poised to provide Canadian poultry farmers information concerning the etiology of keel bone fractures and how housing environments may contribute to fracture risk, as they seek to adhere to new housing guidelines.