Timing is everything, especially with thistle control in pastures. This fall from October to November is the most effective time to control these weeds to avoid even larger problems next year.
Did you have thistles this year? If so, walk out in those infected areas and look closely for the thistle seedlings. Most will be small—in a flat, rosette growth form. At this growth stage they are very sensitive to certain herbicides.
Figure 1. Large thistle rosette.
Several herbicides are effective and recommended for thistle control. Several of the newer herbicides like ForeFront, Milestone, and Chaparral work very well and two other herbicides, Tordon 22K and Grazon, can be very effective. Be careful with all these herbicides, but especially Tordon and Grazon, since they also can kill woody plants, including trees you might want to keep. While it's warm, 2,4-D also works well; however, you will get better thistle control by using a little less 2,4-D and adding a small amount of Banvel or dicamba to the mix.
Other herbicides, including Redeem, Cimarron, and Curtail, also can control thistles in pastures. No matter which weed killer you use, though, be sure to read and follow label instructions, and be sure to spray on time.
Next year, avoid overgrazing your pastures so your grass stands get thicker and compete with any new weeds or thistle seedlings.