Someone recently asked a similar question in a different section of this website.
If you live in a warm climate and your chickens will have a large area for free ranging, it might be possible for chickens to find enough food on their own. I would not expect them to be top egg-layers if all the food they were getting was from free ranging.
If you live in an area such as most of the United States and Canada where you have cold months in the winter, then you are going to have to feed your chickens.
My suggestion, either way, would be to supplement your chickens' diet with some laying feed so that you have good layers.
Leghorns have the best feed-to-egg ratio, and Leghorns are good at foraging. The Brown Leghorn varieties have better color for hiding from predators.