Since we've had a baler which will roll a tight bale and netwrap it, the loss from proper outdoor storage is negligible.  Bales need to be in a solid, well drained location and lined end-to-end with the rows packed tightly so that the ends of the bales do not take water, and with sufficient space between rows so that the outside of the bales does not stay wet.  With that done, even when fed the second winter after baling there is only a couple inches of discolored hay on the outside of the bale.

Today's bales are a whole different world from an old Hesston Rounder "hay blob" with some twine wrapped around it.   Consider that both have the same nominal dimensions but the Rounder's "blob" would do good to weigh 900 lbs whereas the bales from our 664 can easily go beyond 1800 lbs.