I personally never heard of feed really causing egg bound birds, so I looked it up. I found that an unfit bird, which is overweight, might not have strong enough muscles to lay an egg. So make sure your birds have plenty of exercise time, free-range of course is the best.
There is also an expressed praise for calcium, because lack thereof can cause soft-shelled eggs to form and the bird may not be able to push them out.
So keeping your birds healthy and fit, making sure they have plenty of calcium, and arent snacking on to fatty of foods should help keep them from getting egg bound hopefully.
Also from what I hear, it is the more productive hens that tend to suffer more.
I wish you luck, and hopefully no more egg bound hens.