im pretty skeptical of gluc/chon thats added to dog food. even many of the supplements have been shown to have very little of what they claim. plus, im not sure what the dog food processing does to whatever supplements they have added.
since grains are a known inflammatory, the first thing ive done for any dogs with joint issues is get them on a grain free diet and add a reliable supplement (i use cosequin DS, but there are others).