Take 10 acres of the 20 acre pasture and plant a mix of spruce, white cedar, hemlock, and maybe some white pines. Take the other 10 acres and split it between standing corn and brassicas. You'll never ask the question again of where your deer went. Deer need thermal cover and food in the winter. Sounds like them leaving is snow/winter anticipation related rather than pressure. You must be lacking one of the 2, maybe both.
How are the 100 acre corn fields harvested by you? Are they cut early for sileage? Are they still standing now? Are they harvested later for grain? Does the farmer chisel plow or otherwise work the field after harvest? These factors will determine how viable of a food source they are this time of year.