I wish the Stiftung Warentest had published the full article free online. Since they haven't (at least as far as I can find) I can't comment on their conclusions.
However, there is apparently a some criticism of their test assumptions, for instance here:
http://petspremium.tv/hundefutter/hu...tement-dr-berg
(Just found this via my friend Google as an example - but it makes me wish that the Stiftungs Warentest full article were available as I'd like to compare and contrast.)
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The important thing is that every dog owner should educate himself/herself. Know how regulations work, which will differ slightly by EU/US/UK, etc. Know how to read a label, know what is important vs what is marketing, and most important of all - read all you can about veterinary nutrition, from various sources, speak with your vet, and evaluate the food you choose with your individual dog's needs in mind.
It's a jungle out there.