I had one hen die last week from sour crop now I have a second hen with this condition. Both are bared rocks 15 months old approx age. Both had cleanish smelly. Discharge from beak when held at 60 degree angle and gently helped them vomit. Could corn cause problem? I only feed at night to get them into chicken tractor at night. They free range all day. The first hen had heavy breathing before she died the second one is now heavy breathing. Any comments from your experiance appreciated.