This question has troubled me for a while. I have looked at all the online dictionaries/forums/other resources for an explanation, in vein.
I have encountered several articles on animal rights describing battery cages as "barren".
For example, on Wikipedia's battery cage page,
Battery cages are the predominant form of housing for laying hens worldwide. They reduce aggression and cannibalism among hens, but are barren, restrict movement, prevent many natural behaviors, and increase rates of osteoporosis.
"It is clear that because of its small size and its barrenness, the battery cage as used at present has inherent severe disadvantages for the welfare of hens".
A recent review of welfare in battery cages made the point that such welfare issues are problems of management, unlike the issues of behavioral deprivation, which are inherent in a system that keeps hens in such cramped and barren conditions.
Another article is titled "Barren, Cramped Battery Cages"
This really confuses me. It can't mean infertile, fruitless, and unable to bear children, as in "barren land" and "barren person", because it would be pointless to have laying hens in such cages.
None of the online dictionaries I have consulted lists any definition that seems relevant in this context.
For example, here is Merriam Webster's entry.
I did find only one site that has a rare definition of this word, "lifeless". But battery cages can't be lifeless either. However, a whole bunch of animal rights writers have used this word to describe cage conditions.