I throw my hens a little each day on my sandy soil. They then get some grit along with the high carb snack. I use oats, sunflower seed and song bird seed most often.
Hold back on treats to less than 10% of total food consumption...and make them the higher protein ones. You may should even knock of them for a while....as they may have been used to eating only a quality layer feed.
I still provide mine a separate container of oyster shells as they will only eat it if needed. I live on the northern coast of Florida and can get locally ground shells for $2.50 for 10 lbs.