deer can jump a fence about as easily as you can step over a curb. in order to save your labor, time and material costs i would suggest fencing off an area just about twice as large as the area that the corn is distributed by the feeder. roughly about the size of one side of a tennis court.

make sure your feeder is near some woods or a treeline. deer like to have an easy escape route. you can even place two t-posts at the start and end of your fence, but leave a gap about 20 inches wide for the deer to walk through but not wide enough for the cows. just an idear.