It's a bit odd that only the large fowl have been affected and not the bantams. It's hard to know, but if you just put out "fresh feed" and they are dying, then I would throw it out.
The date is usually stamped on the white tape portion of the bag on the end with the nutritional analysis tag.
Unfortunately, without testing there's no way to know the cause of death. If you still have a couple of bodies, it would be wise to send them for necropsy to your state lab - I'm not trying to be unfeeling or morbid, but this hopefully will give you answers. (Ask if you can send some samples of the feed too).
For the remaining that are still sickly - offer them some poultry vitamins in their water and see if they will eat some chopped egg.
Since you don't have your location in your profile, here's some lists of all the state/necropsy labs in the US - just look up your state.
http://www.metzerfarms.com/PoultryLabs.cfm
http://www.usaha.org/Portals/6/StateAnimalHealthOfficials.pdf
https://www.aphis.usda.gov/animal_health/nahln/downloads/all_nahln_lab_list.pdf