What I say might seem counter-intuitive at first, since a lot of the things I'm going to recommend adding to their diet don't necessarily have much in the way of carbohydrates. Layer pellets can be a base for their diet, but chickens are Omnivores and to be healthy they need a broader spectrum of nutrition than any one food source (be it natural or processed products) is even going to be able to impart.
Food scraps or leftovers to share that I'd highly recommend:
-Cabbage, Lettuce, Collard greens, and other leafy greens. These provide important vitamins and minerals you're not going to adequately find in the form of processed products, no matter what they claim.
-Fruits, such as Bananas, Pears, Cherries, Melons of all kinds, Mangos, Papaya, Apples (I take Apple seeds out because of their minor cyanide content, just to be safe), Black- and Raspberries, Kiwis, etc. I'm guessing you're living in a Temperate climate, so anything you might be growing or purchasing there would be ok to feed them. I've had the oddball birds that enjoy Ruby Grapefruits and Oranges, but in general they seem to find Citrus offensive.
-Fully cooked Rice (I'd recommend Brown and not White, because White is reduced down to its carbs by having all the nutritious bran on its outside removed), soaked or cooked Oats. Grains have a tendency to swell up a LOT and if you feed your bird dry grains it can potentially kill them, as is the case with raw rice swelling to several times its dry size. You don't want that happening in their body. Corn is fine sometimes, but go sparingly with it (10% or less of their diet) because too much will cause them not to lay. I'm not sure if this is hormonal or what, but keep it to a smaller portion of their diet, and fresh is best.
-Vegetables such as Squash (some only like it cooked), either ground/chewed up or cooked carrots (some of mine LOVE carrots over everything else), Tomatos, cooked root vegetables such as Sweet Potato, Yams, Potato, Taro.
-Share your meat scraps with them! Even the chewy, gristly cartilage is good for them. Fats, muscle meat, organ meats, and cartilage are all fed to my birds.
One of my earliest memories is coming across a beautiful Rooster standing on something it was eating while the sun was nearing setting. I walked closer to see what it was, and it turned out to be another Rooster - dead. They are not vegetarians, but Opportunistic Omnivorous Scavengers, and they really don't care where their meat comes from. Chicken, Pork, Fish, Rabbit, Beef, Squirrel, Racoon, Duck & ofther Waterfowl, Venison, Rats and Mice and Weasly things are all a-ok in their books.
If you have grassy (even dead grass) or weedy areas of your property that you could let them out into (even with temporary fencing to keep them out of your garden or from wandering off) this would be a good way to basically passively allow them to forage and get nutrition from this as well. They'd be getting roughage in the form of grass/leaves/roots as well as invertebrates which provide important animal proteins (and their amino acids) from this situation.
So in summary, they can have access to ten tons of "feed" all day but you'll often see much of it not being consumed. That's because while they need more in their bellies, they need variety. A lot of vitamins and minerals work together to allow them to more efficiently process all of what they eat, resulting in better fleshed out birds.
The last thing I'll say is to be diligent in making sure you do not ever feed them anything that has Chocolate in it. Theobromine, a chemical found in the Cacao, is toxic to a great many species of animals because their Livers, unlike ours, cannot metabolise it, and it can potentially kill them.
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