Nancypo, what are you wanting in a supplement? Do you just want your chickens to be as healthy as possible? Do you have any particular health concerns or nutritional objectives?
Kelp meal might be a good idea. 50 lbs. would be overkill though. Objective wise- I know there's some nutrients they won't be getting when they can't be free ranging because the ground's frozen. So just some extra nutients for overall health...
In the winter, I like to supplement with sprouts and wheat grass, to compensate for the lack of fresh green forage. Plus, I like to give a bit of non-soy protein.
I'm planning on continuing with fruits, veggies, etc. I'm just looking for something that's balanced to add to theri regualr feed. I'm thinking of making my own feed too, with oatmeal, barley, and some other grains, but no corn, maybe...