Our field has been becoming more and more heavily over run with thistles, and this spring the problem looked to be particularly bad. A few years ago we probably would have sprayed, but I have been hunting around for a better solution.
I found a few tit-bits of info that were helpful...
Thistles do well in pasture that has been overgrazed particularly in winter or early spring.
Our pasture has definitely been overgrazed. A neighbour runs his sheep on there and took them off a week ago. The grass is eaten right down a there are clear gaps between plants. Thistle seedlings are finding it east to get established. Not much use for this year but a lesson learned for next.
The best thing I found was a brilliantly designed weed puller that uses clever leverage to grip the plant at the root/crown and pull it up smoothly. We had a go with it yesterday and pulled well over 500 plants in a little over an hour. It copes with all sizes from massive to seedlings and generally gets a large chunk of the roots, or all of it in the case of the seedlings. I wouldn't want to try to do the whole field at once with it, but you could definitely get through the 6 acres or so doing half an hour at a time.
Weed Puller on Amazon