DDG will actually enhance marbling, maybe keep it at 20% inclusion instead of 50%.  At 50% or more I start to see some pull back in adg, but the economies more than make up for a few more days on feed.  Keep at least 15 lbs. shelled corn in the the ration the last 60 days for grading.  Get rid of the self feeders.  They are convenient but costing you a lot of extra feed.