Ok.
Firstly, thistles in hay are completely edible. The prickles may hurt you when you handle the hay, but they won't bother the sheep, goats, cattle or ponies.
Secondly, topping works, is a lot more environmentally friendly than spraying, and doesn't waste good fodder - the ruminants will very happily munch up the felled and wilted plants. For best results, top when in flower and before seeding. If they regrow (many will) and you can top again the same year, you will see an even bigger difference next year. It could well be an annual chore, however.
Our pigs have dug up and eaten all the established thistles - but yes, disturbed ground and seeds...
Some native ponies will eat thistle flowers - my Fell mare particularly loves them. Our ponies are clearing a lot of pasture for us, one field each year...