Our allotment is riddled with Horsetail. In my experience, glyphosate has very little effect
We have come to the conclusion that it's best to just pull it up wherever/whenever possible, and not to worry about it the rest of the time. It actually takes very little from your plants - it casts little shade & has roots from here to the centre of the earth so isn't taking much immediate nutrition from your plants. Apparently it prefers unworked, badly drained/compacted ground so just by digging & improving your soil, it will be at a disadvantage. You may never fully get rid of it, but you will certainly weaken it. Oh, and I discovered last year, don't pull it up from your onion bed - it'll upset the onion roots & then they will bolt (go to seed)
