Peter Corke and his huge team at QUT are doing great stuff in this area. They have spot-spraying robots, that zap only weeds and fertilize only crops, and also a great device that just mechanically smashes up weeds one-by-one.

Peter has an admirable ability to choose projects and deliver the goods on them. Robotics needs more like him.

Interview with Corke on this subject:

http://robohub.org/robots-zero-tillage-robotics/

Homepage with free stuff including books and MOOC:

http://petercorke.com/wordpress/

edit: I'm not affiliated with QUT and have no conflict of interest. I'm an academic that does related work but not in agriculture.