I hunt in SE Alabama. We have over 35 food plots ranging from about 1/2 acre to 2 1/2 acres. We plant our fall/winter food plots in mid to late September. We plant a typical "tri-mix" of wheat, forage oats and grain rye. We incoporate purple top turnips, planting is about 50 pounds of seed across 35 +/- acres of food plot area.
The deer seem to stick with the tri-mix , and more or less lay off the turnips until the turnip leaves become rather large and the roots about 1"-2" in diameter. Once the first frost hits and the roots "sweeten up", the deer start hammering the turnips. Not much above the ground is left come about January 15 or so.
One thing I really like about planting turnips .... I can cook a pot full for supper and have done so many, many times.