It used to be that the answer was no, never, ever. Silage (UK) or haylage (US) is hay that is wrapped and then fermented after harvest.
I grew up being told silage was okay for cows but would kill horses.
Silage made for cattle should not be fed to horses - it's simply too rich for them and will give them indigestion.
On top of that, silage always carries a risk of botulism - and horses are more sensitive to that toxin than cattle. Botulism is usually fatal in both horses and cattle.
Haylage that is intended to be fed to horses is made in smaller bales and designed for the equine digestive system. Also, we can now vaccinate animals against botulism. (Haylage should not be fed to horses that have not been vaccinated).
So the common "haylage kills horses" wisdom is much more complicated than it once was.