I feed day old bread, too, but not much of it. One loaf for 20 ducks, minus 5 slices that go to the rabbits.
Let's see. The ducks eat 10 pounds of feed every day and about 1 pound of bread. Very roughly 10% of the diet. I sure wouldn't give them any more than that. Also, I feed a feed that is higher in protein than they need, so I am not worried about the bread reducing their protein intake too much.
My ducks won't eat white bread. Only the whole grain. The geese will eat white bread, but they complain about it.
Price of the day old bread just jumped 50%. It's still half the price of the feed pellets, but it's not a balanced feed, so I am thinking about dropping it from their diet. It's a lot less of a bargain at the new price.