You just stumbled into my wheel house. I was the national champion horsetail/thistle grower in North America for a couple years running not long ago.
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You've got blank space between yours. Yer not so bad off. Quit trying to kill it, switch to mixed perennials, and broadcast a hot rate of forage barley in there as soon as daytime lows stay above 20 degrees in spring. Every time you spray it, you're just taking out it's competition. Once you get a mix going, you'll still have some, but it won't be near as prevalent. I quit spraying and switched to clover/chicory/cereals and a stiff shot of lime, and now I've got to search to find it in my plot.