Bubblegum -- in my part of the world, the biggest concern with haylage is the possibility of tetanus from rotting protein in the hay (mice, etc.) The vets here put haylage-fed horses on an injection routine to prevent infection. It is several shots over, I think, 3 months. Thereafter it is semi-annual to maintain.
Why not make a quick call to the local vet?
And Haylage vs. Silage? All haylage is silage, but not all silage is haylage. Silage is any fermented field crop (hay, oats, corn, barley, or mix thereof). Haylage is silage made from hay, Oatlage is made from oats, etc. etc. In my experience, silage is whatever the farmer had leftover that he could feed over the winter as a general purpose fibre feed. Generally, plain-jane silage is piled in a pit, compost area, silo, etc. Haylage, oatlage, etc are wrapped as they are more marketable that way for their specific feeding purposes.