I have found that deer really like hay/alfalfa/clover right after it is freshly cut. About a week or two after all they new growth really attracts them.

Also, like wis said above, if it is allowed to grow really tall like switchgrass deer will use it for cover bedding...

I assume this hay your are talking about is gettin bailed so hopefully the farmer bails it a few weeks before the season and the deer should hit it pretty hard!

Good luck!