Too much zinc can be a very bad thing, causes hemolysis (break down) of red blood cells. But for a 50-60 pound dog, 50 mg per day of zinc gluconate is typically very safe. Anecdotally, I think it is great for keeping the feet of working dogs in good shape. I use it in my dogs.
There are many forms of zinc, obviously, that each convert differently to elemental zinc which is the active form. I just stick with zinc gluconate.