I usually just take a machete to any thistles in my pastures, sometime in mid June/early July when they are starting to bolt, but pre-bloom. At that stage, if you wack em down, they do not come back unless you leave too much of the stem. The rosettes are easier to control (but much harder to find without walking every inch of pasture in late fall/early spring) with a backpack sprayer and some cheap 2,4-D with surfactant. Once they bolt, it takes a little more $$ herbicide to kill them. It appears to me that spraying in late winter/early spring with 2,4-D, I do not damage my clover. Sure, I kill some, but it's not complete eradication. By early to mid-March, I can frost seed clover (if needed) and control thistles AND have a good clover crop. Some pastures seem to be worse than others. I've never had an issue in my hayfields though. Seems that typically mowing and timing keeps them under wrap. Its that dang horsenettle that I battle in hayfields. Timing of chemical application is key to knocking that stuff out. You've got to get it at bloom, but pre-fruit set to get the best control...or atleast, that's been my observation.