Did you spray thistles this past spring and summer? If so, it would be a good idea to revisit those areas as there are likely some remaining plants or new growth that has occurred. October and early November is a key time to control thistles in pastures.
There are several biennial thistles, but musk, plumeless, Scotch and bull thistles are our most problematic. Biennials require portions of two growing seasons to flower/reproduce. They develop from seed the first season into a flat rosette. When trying to control biennial thistles, destruction of rosettes prior to flowering (bolting) is an effective means of preventing seed formation and subsequent spread.
Another thistle to look out for is Canada thistle. Canada thistle is a creeping perennial that can be controlled with fall spraying, in conjunction with other management options in the spring.
While in the rosette stage, thistles are more effectively controlled using herbicides. It is important to note that fall spraying of thistles is not a silver bullet and effective control often needs repeated applications. It will take several years of timely control before the soil seed bank is reduced. There are many herbicides labeled for thistle control. Note that some products traditionally recommended for spraying thistles have recently changed product names. Take care when purchasing products and always read/follow label directions before use.
GrazonNext® HL, Milestone®, Chaparral®, Graslan® L, Stinger®, Overdrive® and Tordon 22K® are all products that are labelled for use on biennial thistles, as well as Canada thistle. 2,4-D mixed with dicamba is also an effective option but should be sprayed when temperatures are warmer for the highest efficacy. When using Tordon 22K® or Graslan® L, both products are restricted use and contain picloram. Use extreme caution around other vegetation, especially trees, as both products will kill woody plants.