The best food you could supply your deer with, other than commercial mixes, would be sugar beets.
This is according to nationally know food plotter Ed Spin. Per Ed: "There is nothing better to carry deer through the winter season than sugar beets. At 95%+ digestability and sugar content at 20%, dry matter, not to mention a high content of minerals and vitamins you will, 'I promise' see a major improvement in their health and size."