I'm going to say that also but I have a hard time believing that a dog will eat yarrow (Achillea). Most of that stuff smells like some old-fashioned liniment.
Are you sure that he or she wasn't eating what looks like quackgrass growing amongst the yarrow? That would be what I'd expect a dog to go for.
Edited to say, I don't think that a dog would have much interest in anything except the grass in your photo's. Puppies will chew on anything, I suppose.