Cat food is much higher in protein than dog food because cats are entirely predators, and have a less efficient gut. What comes out of a cat still has a fair amount of protein in it. Dogs, being scavengers as much as hunters, are not very picky about the condition of their food. (As anyone who has ever smelled dog food knows.) Their guts are built to tolerate partially decayed food. As a result, they really can extract useful nutrition from... ahem... used catfood. And they know it. Of course there's some disease-transmission risk there, not to mention the fact that it just plain disgusts humans (especially those of us who let dogs lick us). So it should be discouraged. But remember, it's a matter of asking them not to eat something they consider perfectly edible, which is not an easy training exercise. The simpler solution is to arrange things so the dog can't get to the litter box in the first place. Cat doors that are too small for the dogs, or barriers that are awkwardly high for the dogs but easy for the cats, might be your best bet. I've seen one design for a cat box that required the cats negotiate two jumps and a a restricted tunnel; that would probably defeat even a cat-sized dog... but then you may have to put in a bit of work to teach the cats how to use it. The other simple solution: Clean the litter box more often. Or get a self-cleaning box. Remove the temptation before Pooch can get to it.