If your chickens have always had hay and access to grass there are no worries. The problem arises when chickens that do not get free range time or lots of yummy greens all of a sudden get a box stuffed with hay and yipppeee, this stuff is yummy. They end up eating too much too quickly and it becomes a tangled mess in their crops and will often lead to a blocked/sour crop. Hay is typically fine. Hope this helped with your question
