Warning, please. be careful ,,,they could also be getting impacted crops. esp if they don't pick off the grass and weed TOPS themselves. and don't have access to grit and or natural small stones themselves. 
One hen, I  observed years ago got an impacted crop when  she was in chicken "hospital in the owner's house" for quite a while,,,,  Was in "chicken hospital,.....That owner gave  fresh. just mowed"pieces, bigger  grass clipping and pulled up whole weeds.   It didn't occur to  that person that the hen gorged herself, because the hen, had  not been outside herself, for quite a  while. The flock  were all used to" semi free ranging". not cooped Up or coop "run" UP.ED  
   Now if a person observes a chicken, the. (  little precious  darlings),,,,pick or peck  small pieces of weeds and grass themselves, the tops actually
. Because that  grass and weeds are still in the ground....   Not whole  longer pieces or strands in a pile or piles .... That is my personal observation  
that is my two cents worth of suggestions...  Cause bagged feed is so BLAH. and not natural... Chickens need real stuff too,
Good luck and Bless  you for caring for some of Gods precious creations