I have loads in my hay every year and it grows all through my fields. I've not had a problem. It's all part of a mixed diet and hasn't done them any harm.
I know... i have an unhealthy obsession with weeds at the min. I moved horses into new place a few months ago before the filed started to blossom! now im wrecked looking all the diffrent weeds up to make sure they are safe, Not as worried about knapweed as much as the silver weed tho
I am pretty sure knapweed is a sought after plant in traditional hay meadows, although I could be wrong. I don't think silverweed is an issue, and generally avoided anyway ime.