There is what we call the junkyard rule! Buy a $500 horse and put him behind one string of barbwire in a junkyard and he will live on a rd bale and a nasty pond until he dies of old age at 35yrs. Buy a $50,000 horse and put him in the same conditions and he will die or at least be at the vet hospital inside of 12 hours.
Feeding round bales to horses is summed up in this story! The more pampered your animals are, the more likely you will have severe problems feeding rd bales to them outside. The less pampered your animals are, the more likely you will never have problems feeding rd bales to horses outside, in the rain, in the mud, etc.