Hi,
I'm assuming that your question pertains to the "extra" treats that you feed the chickens. If not, the best thing to feed them is a good, well-balanced chicken feed. I feed Purina Flock Raiser and leave oyster shells and grit out extra all the time. It's 20% protein and is a good food. My neighbor, whose chickens are right up to the fence, feeds a generic layer he got at Ace Hardware. On other food posts, other member mentioned that Flock Raiser produces less smelly poop and I think I can agree with this. His birds are kind of smelly. I'll stick my head into my girls' house and take a deep breath and no stink. It's a good quality food and as I only have six birds, they don't go through it too fast.
As regards to keeping them fat - you don't really want to do that unless you are planning to eat them. Fat layers have problems. The fat presses against internal laying organs and can cause egg-binding and other difficulties. It's best to keep them fit and trim.
I use a homemade scratch for mine made up of 4 parts whole race horse oats, 4 parts whole wheat (feed stock, NOT seed stock), one part BOSS (Black Oil Sunflower Seeds) and, now that it's gotten cold, about 1 part cracked corn. In addition, they get yogurt and Cheerios a couple of times a week. They're still eating grass and greens from the garden but I plan to keep giving them alfalfa (soaked cubes or the leaves at the bottom of my friends' feed piles) and I can buy a bag of old lettuce at the grocery store for them. In addition, they get the occasional handful of raisisn and some mealworms from the pet store.
They are fit and healthy and have just started to lay beautifully formed, hard shelled little eggs at six months old.
Thanks,
Mary