Sure, but a) that's still spraying, albeit more targeted (still a win) b) I'd have to give money to John Deere.

There are a few variants on that. One kind just overdoses the weed with fertilizer. One zaps weeds with a laser. One just stomps on them. A small one from WalMart takes out weeds with a string trimmer.

Most of the successful systems are wide implements to be towed behind a tractor, not autonomous robots. Farmers already have tractors.