Ken,

The load is quite high, probably because no one knows a priori the geometry of the structure that will support the load or the exact nature of the load. If you know the density of the hay, then you can account for the limited headroom along the sides and possibly reduce the load. The load standard has a footnote that might tickle you: "Use weight of actual ... stored material when greater." Yea, no kidding! Basically, I think that you are back to weighing and measuring a bale. You might want to soak it with water first!

Dick