The best way to control this plant is to make sure that it does not set seed. If you have sprayed it and this does not work, I would either mow it, or spend some time cutting the heads off of the plant. Again, this is something that has become quite timely, so if you notice this plant in your fields, you should think about controlling it in the next few weeks.
Again this is another plant that should definitely be controlled! The good news is that this plant will be easier to control than it's very closely related cousin, Canada Thistle.
Take care to make sure you are feeding bird seed that does not contain thistle that may germinate. STERILE THISTLE SEED is important to use at your feeders. You can also purchase Nyjer seed which has been heat treated and sterilized to remove any noxious weeds from the seed mix.
Personally, I feed my birds some sunflower seeds in the feeder and then plant plants throughout my property that will attract them to my house without the dangers of perhaps using seed that may actually not have been sterilized.