Random Thistle in good grass hay field

If you are going to cut it soon, chop them out and burn as they regrow. Weedmaster should work. If you are spot spraying a few clumps in a field you will be hard pressed to transport much residual and duracor smokes them

Thistle doesn't just spread with seeds. The root system is very prolific and spreads through the roots also.

I have had good results in pushing back thistle here in PA with dicamba mixed with 2-4,D. (I use a pre-mix product called Latigo, not a restricted use herbicide and no restrictions on taking the hay off farm.) Thistle eradication is tough and takes time to get rid of completely. In my opinion, your best route may be to cut out the plants now (as Hayman1 suggested), spot spray with Dicamba once you see it starting to regrow with the 2nd cutting (spraying in the summer will just burn back the top growth but not do much to the root system), and then spray again in September (this is when the plant is moving nutrients to the root system for the winter and takes the chemicals into the root system for more effective long term kill.) Be prepared to repeat this for next year or two, although it will be less pronounced each year.

I would cut that bale it and feed it to my cattle it’s high in protein and not toxic to them. Then I’d burn it down with 2,4D or dicamba. Just make sure it’s allowed for hay crops Most do.

This really only works with the right timing. I have had good luck in cattle pastures mowing thistle patches right before a rain followed by a hot stretch of weather. Best is to do it right before the plant sends out a seed head. The rain seems to be the critical part, the hot weather after a rain just seems to cook it off better. If you have the option to spray it I'd spray it