This is the comment I came here for.

I had a professor in college who was building self-driving tractors and would come in every other week complaining about John Deere this, or Case that, trying to steal his business with more expensive solutions. It turns out you can use GPS for a rough location and a fancy $200 gyroscope for millimeter precision. Then just plant the seeds on an exact grid and you know that anything not on the grid is a weed.

And actually, his suggestion was to use high-pressure water jets to cut the weeds instead of lasers. It would/could be less energy-intensive.