Corn is not a complete diet - for best health and production, chickens need a diet for each stage of their life; ie, starter, grower, finisher or layer feed.
Most chickens eat only as much as they need. They should get a balanced diet, corn by itself is not a balanced diet for chickens. If they are a month and a half old they should be on a 18-20 percent protein grower. Corn is only round 10 percent protein.