What speckledhen said. It's the genetics that make the birds lay a lot for a short period of time. However... my production leghorns haven't missed a beat though their second year of laying, and they were from the group that came down with cocci, which usually decreases their productivity as layers. I guess in a few years I'll know how well they continue to lay. They lay their weight in eggs every 2-3 weeks.
The layer feed gives them all the nutrients they need to produce eggs. Even with a good natural diet they can be lacking small trace minerals. The commercial laying feeds gives them a complete diet of proper nutrition. Everything else we do is extra.