I understand that what you're getting at is that the feed corn may be roundup ready simply because most farmers now plant RR corn and the feed corn comes from that.
Not sure how that works, what I do know is that to buy RR corn seed you have to sign some type of statement saying you will not resell the seeds for planting from what grows. Or something to that effect.
My guess would be that the feed corn you buy isn't roundup ready, but would be an interesting experiment to spray it and see what happens.
My experience with corn is that we can't plant a big enough plot to let it get to the point where it's growing ears before the deer, coons and other animals strip it clean. We've planted plots up to two acres in size, and even those are stripped clean well before season starts. My advice, go with beans. They tend to grow back even after being browsed.
“There are two spiritual dangers in not owning a farm. One is the danger of supposing that breakfast comes from the grocery, and the other that heat comes from the furnace.”