Cage or free range? Don't put too much stress on housing system
22 March 2012
Free range hens are not necessarily less stressed than cage or barn housed hens, a new study from the Faculty of Veterinary Science at the University of Sydney has found.
Researcher Dr Jeff Downing, from the Faculty of Veterinary Science, said his study did not point to one particular housing system creating more stress than any other.
Dr Downing analysed levels of corticosterone, a stress hormone found in hens. The study was over 72 weeks and included 12 farms using free range, barn and cage production systems.
"When we combined the results of each housing system there were no statistical differences between corticosterone levels in cage, barn or free range hens," he said.
"We found the variation in corticosterone levels between different farms of the same type of housing system was greater than the variation between housing systems. Regardless of their housing system birds on some farms were showing comparatively low levels of the stress hormone, while birds on other farms were showing higher levels."
Dr Downing hypothesises that the difference in corticosterone levels on different farms had less to do with the housing system hens were living in than with how well a farm was managed and what challenges the hens have to cope with.
"In most of the farms concentrations of corticosterone were higher in the earlier part of the production cycle, suggesting there are greater challenges for hens and potential for poor welfare at this time. In the second half, birds appear to adapt to their housing system. This could mean that the way young hens are reared and moved to the production farms has implications for their welfare.
"On some farms, however, corticosterone levels were more constant. Within each particular housing system there were farms that appear to be doing a good job bringing them into the production system and limiting the challenges they face."
Environmental conditions could also be a factor, according to Dr Downing. "One barn farm where hens had particularly low corticosterone levels had very good environmental controls and limited temperature variations."
He added: "While the three housing systems remain in use, comparisons between systems are of less significance to the hen than what is happening within her housing environment".
Dr Downing discovered he could get a measure of corticosterone levels in hens' blood by analysing the hormone's levels in egg albumen. The albumen levels accurately correlate with the hormone levels in hens' blood over the four to six hour period of albumen deposition during egg formation.
The study is the first time in the world that hens' stress hormones have been tracked over time by measuring the concentrations in eggs. Previously corticosterone was measured by taking a blood sample from birds, but this process stressed the birds and others nearby, tainting future samples.
Dr Downing carried out his research independently, but received research funding from the Australian Egg Corporation Limited. His report was reviewed by independent, external referees.
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