Lets step back and say: could a bunch of humans with hand tools do this?
Yes, they could. (As opposed to, say, eradicating horsetail: if we can't even do it ourselves, then no, we definitely aren't getting a robot to do it).
One way could be the lasers-on-trailer approach, burn the undesirable plants so they're sufficiently uncompetitive. Another could have arms and cutters to reach down and sever the plant below the surface.
Either of the above could instead be a smaller autonomous robot working as part of a swarm day and night.
I don't agree, and I note that you also answered your question differently later in your post with the note about 'mixed farming' (grazing it off).
There are, of course, other answers than herbicides. Seasonal crops, harvesting and then seed-sowing amongst the stubble (provides some mulch & eroson protection), intensive strip-grazing (bovine, ovine, caprine, or fowl, all effective options), or even a cycle or two of fallow.