I know that there are some risks involved but we have been swath grazing some of our hay land for several years now.  We graze our hay land to set the timing back or bale early.  Then sometime in Sept swath it and leave it to graze in the winter.  Yes there is a risk of killing the crop underneath the swath that is why we cut in Sept when the growth is about over preferable after a few big frosts.  It give us the ability to graze in crusted snow and preserve good feed with minimum mechanical intervention and no tractor starting.