One method to adjust cows to a corn diet is to use strip grazing for the first 20 days when there is more than 8-10 bu/acre on the ground. This calculator will show you how much to allocate each day for 20 days before turning the cattle out free choice on an entire field.
Download the spreadsheet, input the bu/ac of downed corn in the field as well as the number of animals in the group to calculate the amount of acres to allocate for various corn intake amounts.
Download the calculator (Excel file)
Down Corn: Problem or Opportunity for Cattle Producers? (pdf)