I think better idea would be four tall legs (like a table legs) so it can move around the field. It doesn't need to move fast. You also need something to detect

I think better idea would be four tall legs (like a table legs) so it can move around the field. It doesn't need to move fast. You also need something to detect its position (laser beacons? radio beacons?) and some place to hide from rain. And a hand with a camera for harvesting crops into a basket.