Q: Can I spread grass clippings in the chicken run?
A: No, grass clippings would be bad for your flock.
Most people will find this advice counter-intuitive, because chickens love to eat grasses, right? However, when chickens eat grass, they nip off of the tip of the blade; they eat tiny pieces, a little at a time. Putting a big pile of lawn clippings will give them longer pieces that they'll eat too much of, all at once.
What can happen is that the long, fibrous strands from the mower get bound up in the digestive system and cause a blockage. They can get tied in knots that won't pass, and the blockage can be fatal if not treated.
Don't offer grass clippings to your chickens unless they are very finely mulched, and even then offer just a scattered handful at a time.