Last years hay which we cut on our friends land had thistles in. As Shep53 says, sheep don't like dry thistles, although they relish the spikiest growing ones. More to the point is that while the hay is drying, so are the thistle heads so that when you feed the hay you are also spreading thistle seeds all over your pasture. We hand weed spear thistle here and mow creeping thistle so we are gradually getting on top of the problem, so we definitely don't want to import more seeds. As we open each bale so we collect up all the thistle heads and spiky bits into a bucket for burning