It would seem that huge combines would still be needed to harvest grain crops, even if solar electric powered robots did the planting, fertilizing, weed/pest control, and watering. These could be electric powered, but the batteries would be very heavy unless they were exchanged every few hours. That's a posasibility. But at harvest time, big expensive grain combines need to act quickly to get the crop in. Most farmers hire this job done now, so they don't need to invest in these very expensive machines. I'm assuming that would continue. Even if harvesting still takes fossil fuel driven behemoths for a couple more decades, replacing the rest of the tractoring needed for industrial scale ag with solar powered robots would still pay for the individual farm.