Full grown boxers with diarrhea

I have two full grown boxers and they stay outside and the only thing I feed them is Purina Dog Chow well I have noticed that my dogs stools will be regular for a while then go to diarrhea  and it going back and forth with their stools staying hard most of the time. This has been going on since summer with the male and the female just started a few weeks ago. I have no clue if it could be a virus or what I have been giving them pedialite to keep them from getting dehydrated and I feed them dog vitamins every so often and they do get ribeye bones from the dog aisle at wal-mart. I am just looking for any suggestions as to why they could still keep getting diarrhea.

[Aleda M Cheng, D.V.M., C.V.A]

Do you have their stools checked often for parasites?  Since your dogs live outside and are of course barefoot they are constantly being bombarded by parasites.  Parasites have involved life cycles with active and dormant states.  The diarrhea may coincide with a parasite infestation.  It would be best for your dogs to be dewormed with Drontal Plus periodically, and also have their stools checked for the parasites that Drontal doesn't cover, such as Coccidia and Giardia.

[Circe123]

After deworming if the problem continues, you may want to ask your vet about eosinophilic colitis, which Boxers are predisposed to as a breed. A biopsy would be required for diagnosis. If diagnosed, this can be controlled with various antibiotics and steroids.