She's on a regular diet of layer pellets, and I mix in some crushed oyster shell.  
We give all 3 of our hens (the other two are production reds) leftover bread crusts,  fruit, and other appropriate scraps (cornbread, cantaloupe, cucumber).  We also let the hens roam our yard for a couple of hours each day during which time they will eat earwigs and nibble on green grass.
Our barred rock is around a year old...she started laying on January 6, 2009, and has been a very good layer, except for the shell-less eggs.
I should also note that as of late (the last month) our barred rock has had watery poop.  Not all of the time, but fairly often.
Thanks for your input.