From what I've read in the last day or so about hay density, it seems that 14 pcf might be a bit high. But let's assume that is a reasonable number. If the stack could be limited to a 4 ft. knee wall height, then a number like 60 psf is appropriate. However, I would not feel comfortable with that value. Hay stacks routinely reach 8-10 ft in height. 300 psf would require a 20 ft stack -- seems a bit unreasonable. Nevertheless, I would be inclined to design for a relatively full loft, assuming that the farmer has mechanical means to fill it.

Interesting question! Thanks Ken. I'll pose it to my structural analysis class next semester and see what they do with it.