I don't normally feed COB to my ducks. It certainly wouldn't hurt them, though.
They get a locally milled poultry pellet. I feed about 1 cup of cat food every day, and that gets mixed into their pellets. There are 50 ducks and 7 geese that are sharing that, so they don't get much. The very first thing they do is to pick out the cat food and eat that.
The cat food I use contains a lot of grain, which means it is pretty worthless for cats, but grain is fine for ducks. I buy the one that is made with fish, except for right before slaughter time, and then the ducks get the one made with meat meal. I don't know if the fish would taint the meat and I don't want to find out by ruining 20 ducks.
You can purchase cat food that is nearly all meat or fish, if you want to spend the money for it.
The dog food should be fine in small amounts, too. Again, the cheaper it is, the more grain it will contain. It tends to be lower protein than cat food, so that is better. Cat food comes in really small pieces, easy for ducks to swallow, but you can buy some brands of dog food in small pieces.
I think of it in this way: the poultry pellets I feed are a complete and balanced diet. If I feed a lot of anything else, I risk throwing the diet out of balance. So the additions they get are small. A couple of tablespoons of flax seed mixed into their dinner, for the omega oil it contains (that's for everybody, not for each one). They get the occasional slice of bread as a treat, like a piece of candy for a child, not like it is food. They get the greens they can pull, and I bring them all the weeds that I pull.
I am working on getting a system set up to sprout oats for greenery in the coming winter. I haven't gotten the kinks worked out to where I can provide a steady stream of fresh greens, but hope to have it figured out before snow flies.
