I'm sure it wouldn't be a favorite everyday snack for them, but I have seen dogs nibble on grass and some bushes leaves when they feel the need (digestion problems? chlorophyll enzymes like we will eat parseley). Recently, I have seen wheatgrass sold for cats to nibble on (probably similar to catnip). I would think that anything their bodies need at a time when they're feeling out of sorts is recognized by them as being necessary because of texture and color, smell,...similar to the way we will crave a certain food because we recognize it. Animals know not to drink bitter water because bitter means 'poison', in nature 'sweet' is safe.