I decided to remove the food when I let them out and return it to the coop in the late afternoon and they all went inside for food. Success. Or is it? This means they have to forage for 10 hours without grains all day.
If you are available in the middle of the day, you could take the food up at lunchtime instead of earlier, and still return it late in the afternoon.
Or you could leave the food, but offer a special treat in the late afternoon.
Wet chicken food often makes a good treat-- chickens usually seem to like it much better than dry chicken food, and there's no worries about messing up their nutrition by giving "too much," because it's the same stuff they would eat anyway.