bailed feed hay
UVM Extension Professor of Agronomy, Sid Bosworth, provided this answer:
I assume you mean hay that is still in the field just before harvest? If it is dry and then gets wet, it will probably not have nutrients washed out per se (leached); however, there will likely be a lot of leaf shatter and a reduction in quality. When hay becomes "mulch" hay depends, but if harvest gets delayed due to multiple rain events, and if the forage in the windrow begins to get dark and moldy, that is certainly a candidate for mulch. Just be sure the hay is dry when harvested and stored. We often see more barn fires in wet years due to spontaneous combustion of wet hay.