I refuse to put chemical in my garden, but we have SO many thistles in our pasture, and my gma says that she used to inject one or two thistles with it and the entire network would fall over dead. I think I need to try this.
Some farmers say that thistles grow where soil calcium levels are low, iron is high, and phosphorus is low or complexed.
Lime the heck out of the soil and add some phosphorus to the soil. I had to do this with my parents yard when they had a problem with thistle.
For a phosphorus fertilizer you could use Bone Meal or the pasture isnt going to be used for a wile you could use Triple Super Phosphorus (0-45-0).