most commercial grass seed is chemically treated to prevent fungus and pest in the sprouting stages. I had good luck at my feed store with a bit of feed grade wheat milo and millet. It sprouted well and I scattered enough in there so that the hens scratching it and eating it covered enough to allow for a good cover of dirt. I did it just before a light rain and it worked wonders
Agreed. You have to be careful with birds eating commercial seeds. And if are trying to grow grass by seeding they will just dig up and eat all the seed. They will completely destroy anything you plant pretty rapidly. If you really want grass and chickens somewhere you can use wire screen to keep them from digging but I would NOT do this in their run as it is not good for their feet to be standing on all the time.
If you want greens for them I would suggest sprouting grains or growing fodder.
I used scrap 2x4s and built rectangles of whatever size I could and covered them with hardware cloth. THen, I put them in the run (like little raised bed gardens) and sprinkled wheat seeds, marigold seeds, lettuce, whatever. The plants grow and the chickens can nibble through the hardware cloth, but can't scratch out the roots. After a few weeks, i'd move them, and start over--letting the hens have some fun scratching out what was planted (which usually only takes a few minutes)