rotational grazing of chickens

I want to know if I build chicken tractors and move them often to "mow" my yard do I have to give them additional feed or treats? or is an ample amount of grass and water all they need? I am talking about roosters and/or young hens not laying yet. I would more than likely add a can of grit so if there is not enough stones or of an appropriate size they could use that. If I do have young pullets I will have calcium a;so available, once laying they will likely need more food/source of protein and nourishment.