When I was at Auburn I remember reading a study they did to see which of the common three (wheat, oats, and rye) that pen raised deer preferred. It turned out they preferred all three but at different times throughout the growing season. They attributed it to the fact that the deer seemed to prefer which ever one was growing the fastest at that time. If I remember correctly when a plant was growing at its fastest it also contained the most nutrition and the deer were able to detect this.