The corn and sunflower thing is a great temporary solution. If this is a permenant field you will need to start looking for some trees and bushes. Often times the franchise nurserys have an after season sale on bushes and trees for next to nothing. The problem is the time it takes to establish the transplants. Autumn olives are prolific growers and cheap. If you have a 5 acre field you need to break the field up into sections. Start planting the choosen cover medium in the middle of the field and span out.