Our dogs are outside 100% within their invisible fence. GSD is about 75 lbs. and mix is about 70 lbs. They chew whatever they want to. The mix isaround 6 yrs. old and stopped chewing on most things around 2 yrs. old. The GSD is around 3 and still chewing on the weirdest things I've ever seen a dog chew on. I've seen him chewing on aluminum beer cans, pieces of pottery, all kinds of crap out of the burn pile, and the strangest thing, glass bottles out of the junk pile in the woods. A few months age he had a 1 gallon glass bottle that was broken in half. Obviously I took it away from him and put it in a barrel out of reach. Somehow he manages to not cut himself. Our front yard becomes littered with all kinds of sticks, rocks, and the above mentioned items. I clean it up and it stays that way for a couple of weeks before I have to repeat the process. It hasn't mattered what I try to substitute for the GSD, he just likes a variety at this age I guess.
Kevin