It is important to understand that a weed like thistle is very prolific. A single thistle plant can produce 4,000+ seeds. You can rapidly go from a situation where you only have a few plants to a very high populations in one or two years. The good news is that the herbicides available to control thistle will also control many of the winter annual weeds that are out there right now in Panhandle pastures and hayfields. Control measures include 2,4-D, Weedmaster (2,4-D+dicamba) or their generic alternatives, GrazonNext, Pasturegard, Remedy Ultra, or metsulfuron.
One thing to remember is that these herbicides will injure clovers. Some, such as Grazonnext and metsulfuron, have residual activity that may affect clovers that are over-seeded later.
Source:ufl.edu