Maybe graze sheep on it for a couple of years?
Long grass tends to be sparse, while it thickens up if cut frequently. Frequent mowing would thicken it up, but is a waste of good grass, so use it as grazing for a period. In spite of the added fertiliser, I suspect that continuous removal of haylage is depleting the nutrients as well, so a rest would do it good. Grazing with sheep adds manure while also keeping the grass short.
John