Grazon would probably be your best bet although the most costly of the chemical controls. However long term costs of leaving the thistles there would be very high, thus making Grazon quite cost effective when factoring in the increase in grass production that it will cause. 2,4-D will have some effect on the thistles but mostly in a suppression capacity, not controlling them. The best control of thistles will be with a chlopyralid product but most of these will have a grazing restriction. I do not know where you are from so I can not be throwing out trade names of products as they may be different across the 49th. I do work as an Agronomist and recommended the use of many of these products throughout my years.
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