that is very cool way to give them grass .I get a small bag of grass seed at a hardware store here. It is mostly winter rye grass like some people here plant so they have a green yard in winter. Golf courses use similar.
This grass is hardy all winter here but dies in the heat. I have a couple frames I plant in my birds run. They can't scratch up the roots but can nip at the green grass. Most will go to these first before eating their feed.
When I buy the seed I make sure it's not treated with any fungicides or pesticides.
I notice that with my chooks the first thing they do when I let them out is going to green grass and start nipping the fresh shoot .
chooks man