Malnutrition is why they are picking and attacking. Besides being very low in protien (about 9%), a straight corn diet will be lacking in LOTS Of nutrients. Deficiencies, breeding problems, and growth issues are likely. Getting the mix made with a mineral mix, and/or supplementing with a vitamin/mineral/electrolyte mix such as Vi-Tal is VERY important for all diets but pre-bagged 'name brand' balanced diets - especially if you have limited or no free range time. Corn is the worst feed to straight feed poultry, or any animal IMO. It is only used sparingly in any animal's diets around here. I usually up the corn content of diets in winter SLIGHTLY, when the quick calorie souce helps keep the animals warm.
If the price of feed is making you switch to cheaper options, try getting a custom mix made. Our custom mix is an 18+% (guaranteed at least 18%, feed mill estimated more like 20%), and is about 7.40 per bag, depending on current grain prices. At feed mills that do custom mixes, you must buy it by the quarter, half, 3/4, or full ton. I feed the same 18% mix to all birds, start to finish - except broiler chickens and turkeys. We get our mix ground completely. It is a mix of soybeans, oats, corn, alfalfa meal, a mineral mix. All ground finely so they can't pick any certain parts out. It's also mixed with a little Cocosoya to keep it from being too dry (you could also use a little molasses or corn oil, but watch the fat content!). Before when I had whole oats and corn mixed into the feed mix instead of grinding it all, they DID pick out the corn and oats, leaving most of the powder behind. Since I've switched, they can't be picky and are forced to eat the whole mix impartially.
You will still need oyster shell on the side, as it is a calcium source for the ducks/chickens. I don't put calcium into the feed mix because it's bad for the male animals, but females can still get as much as they want as needed.
If you would like, I can give you my exact feed mix so you have something to start with at least.
