Here is a comment copied from a similar discussion, a couple of years ago. You may find it interesting.
Having a manifestation of marestail on a third of my new plot, I set about erradicating it.
I bought - or so I thought - a compost activator (ammonium sulphamate), which simply forces the plant to destroy itself and is the closest thing that you can get to a 100% solution to marestail. It is a bit brutal, though, killing everything around it and remaining active in the soil for about three months before breaking down into fertiliser. As such, it can't be used everywhere.
I began to get bizarre results. I sprayed, the marestail died, but everything else around it remained alive. I tried spraying other weeds, but mostly there was only scorching to the leaves; they survived. I checked the box: it wasn't ammonium sulphamate that I had bought, but rather ammonium sulphate, a fertiliser used on alkaline soils to make them more acidic!
As many of you will know, marestail/horsestail prefers marginal ground where other weeds don't often grow and hates very fertile soil. By applying the fertilser, I had reduced the PH of the soil and the marestail died. It has acted as a selective herbicide. Weird, eh?