In theory the heat will kill them. In actaul practice, it seems like there are alwasy a few that stay viable. It seems like it's usually the more undsirable seeds that don't get killed.
We had 40 some odd bales of sudan baleage sitting in a hard to get to corner of a fescue hay field when we bought this place. He gave them away but several broke open as they were being loaded and since they were in a waste area I just left them. These had been put up two years before. Each of the places where we left the broken bales sprouted with sudan plants, this year a few more came up in the same places. At least it gives the birds some seed to eat.