If you spread the thistle seed on the pasture, you will get a lot of thistle plants in the pasture. The horses will not be able to eat the grass around the thistle plants. The thistle needles will be a danger to the horses and any other livestock in the pasture. The thistles will spread to neighboring fields and contaminate any hay fields they grow in. Thistle is considered a noxious weed in many if not most jurisdictions and you are required by law in those jurisdictions to control it. People have lost lawsuits in a court of law because they did not control thistle on their property and infected neighbor's fields.
I consider it an extremely bad idea to give the thistle any chance to either get established or to spread at all if you already have some thistle in your area. I spent way too many hours of my youth in the heat of an August cloudless, windless day patrolling pasturefields digging up thistle to try to keep it from taking over.
Personally I would burn the thistle seed, but I admit to being prejudiced against it. If you feel you must use it, I would suggest using it as bird feed this winter in an area of your yard that will be mowed regularly next summer to try to keep it from ever coming to a bloom and spreading and infecting other areas.
as dh calls them would eat it, have tried mixing in with their morning scratch but silly girls pick everything else out and leave the thistle, question is if it sprouts would it be dangerous for the girls to eat the plant or should i rake it out and and spread in pasture as the wild birds love to be with the horses and they would get a great treet.
