
The reason it is cheaper, is that vat is not payable on food for working dogs. Very often, when I've found a decent offer on working dog food, I've mixed it in with my normal feed to reduce costs.
The details you've given wouldn't make me overly concerned. Blimey, not just my Collies, but my Chi's have eaten working dog food, and although the Chi's haven't worked any harder on it, they have been fine. :-)