Been giving my arthritic hound the powdered equine cortaflex for years. With fantastic results i will add ;)
You are right Zarah the only difference is the copper sulphate and you are right again that Equine America will not give you a straight answer to your query and if you look at the price difference in the canine/equine products you will see why

I, as have many others I know, have been advised by very specialist people (inc vets) that the powdered equine product is as beneficial to dogs as the canine liquid/powder.
It costs me around £55 a year (at 1 teaspoon a day) instead of approx the same every 2 months!!
All I can say is that I have only had positive effects from it.
Sarah.