I have that too, mainly with nursing does.
Could it be some kind of mineral deficiency ?
Is it possible they can somehow taste iron, or something else that they are lacking?
I've often wondered.
The last pen I built, the wire is green coated, and they won't chew on it. They go to the uncoated divider fence. So they must taste something good in the uncoated, galvanized fence. Zinc? hmmm