[danblick]

This video of punching the weeds looks promising, but just a thought: maybe you could focus sunlight to burn them? :)

[fchollet]

While it would be much more energy-efficient, such a process is bound to have a very low rate of weeds destroyed / minute.

[TeMPOraL]

It's a field, you have time. Also, maybe that means you need more of those robots? :).