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Commercial layer hens have been genetically selected to lay a very high number of eggs, and are therefore highly susceptible to poor bone strength and osteoporosis. Osteoporosis is related to nutritional imbalance, level of egg production, and the birds’ inability to move and keep their muscles and bones healthy. The formation of egg shells requires the deposition of calcium. Since eggs are laid at a very high rate, this leads to a loss of bone calcium. Osteoporosis is an end result of this process and is a widespread problem in commercial hens. It can result in an increased susceptibility to bone fractures. Poor bone health and bone fractures cause pain. Additionally, fractures cause stress, and negatively affect activity levels, egg production, and egg quality.
The severe behavioural restriction in battery cages causes a condition called disuse osteoporosis. Hens in battery cages have the poorest bone and muscle health, and the highest number of fractures at the end of their lives of all housing systems. Hens in cage-free systems have the best musculoskeletal health of all systems, but a higher rate of fractures during the production period likely due to flying into objects such as perches and nest boxes. Although fractures heal, it is important that housing systems are designed to encourage birds to move around in a manner that avoids injury.
Furnished cages allow hens to perch, which contributes to improved bone and muscle strength. Hens housed in furnished cages experience the lowest number of total fractures compared with both cage-free and battery cage systems. This is probably due to the absence of poorly constructed and maintained perches and environmental complexity which can be present in cage-free systems, and the improved musculoskeletal health due to the use of perches when compared to battery systems.
Bone strength has been found to be heritable, and genetic selection is extremely effective in improving bone strength and resistance to osteoporosis, with bone strength improving over just one or two generations. A study by Fleming et al. (2006) found that hens selected for improved bone strength also had significantly higher egg production. The number of fractures sustained by birds in cage-free systems should therefore be addressed through a combination of selective breeding, optimised diets, plus improvements in the design, placement, and maintenance of structures in the shed, including perches.