The effect of early rearing environment on skeletal traits in laying hen pullets

Early life experiences of physical activity have been shown in humans, rodents, and chickens to have long-lasting effects on bone density, structure, and fracture susceptibility. Nevertheless, commercial egg-laying chickens reared with opportunities for exercise still exhibit high fracture incidence. Thus, this study compared two genetic strains of female egg-laying chickens raised in four housing systems differing in structural complexity and the amount and type of exercise possible. At 6, 11, and 16 weeks of age, long bones of the wings and legs were analyzed for length, diameter, breaking strength via 3-point bending, and DEXA-based bone mineral content. Cortical bone density and microstructure was assessed with micro-computed tomography images in tibiae at 16 weeks of age. We found that both rearing environment (i.e. loading history) and genetic strain influenced bone structural and mechanical properties. Compared to conventionally cage-housed chickens, aviary-housed birds consistently exhibited stronger, longer, and wider bones normalized to body weight, as well as tibial microstructural changes associated with increased bone strength. Surprisingly, most improvements compared to caged birds were observed in the moderate-complexity aviary group. The white-feathered genetic strain tended to have longer, stronger bones than the brown (normalized to body weight). Overall, restricted activity was detrimental to bone strength and structure, and bone structure and strength differed between genetic strains of chickens. These data allow for a broader, cross-species understanding of the relationship between bone form and function, and also inform egg farmers when selecting optimal housing environments to improve the welfare and bone health of their flocks.