I tried to reply to this yesterday but it vanished

Are these creeping thistle? We have tried grazing out creeping thistle but found the only successful way to get rid of them without weedkiller is to top them frequently throughout the growing season. Before we had a topper we used the lawn mower

which had the advantage that we could remove the cut thistles but took ages.
A potential problem with grazing thistles is that the
orf virus can get in through all the little wounds around the mouth left by thistle spikes.
If they are spear thistle then you have to dig them out by hand at the point when the flower spike has grown but the flower hasn't opened. If you top spear thistle you will make it develop lots of small heads which seed just as much as a single flower, so next year your whole pasture will be full of the things. Sheep seem to prefer spear thistles to the creeping type - it makes me cringe to see them munching contentedly on the huge long spikes, but they seem to get them all arranged to go down spike last and of course they get lots of minerals from them.