Well Rob, here's my theory.
A dog is much more likely to take a knock running through woodland than anywhere else really. One knock to a toe can severely limit or end a dogs working life...yes? So the more you run your dog through woodland the greater the chance of it happening % wise.
My opinion is, if you don't NEED to run your dogs through woodland, if just out on a walk etc then don't do it, if you are working your dogs in it, then that is different. A dogs feet will get plenty of variation of surface/levels/terrain in any field, and road work to toughen the pads.
The theory that