Earlier quoted context omitted.
Drones do not seem like a logical step for me. I'm not a laser expert, but seems like the energy storage needed to kill hundreds to thousands of something with light is heavier than flying would realistically allow. Wheels and tracks are what farming is built around, no need to literally reinvent the wheel here.
What about putting them on an electrified wire? String a grid of them over the fields for motion and for power. Power the grid with solar.
If the camera sees the correct spectra for a known weed it can drop down and get a closer inspection then mark the spot or just burn the weeds with an attached laser.
I assume the crawler is necessary because--in order for the laser to work--the weeds need to be identified very early as they emerge from the ground. You might not be able to spot them at such an early stage from above without expensive optics.