Please use Big imagery, fix the roads fisrt. THEN map the farmland polygons. 

There is a lot of bad farmland mapping in Japan, mostly people using the map as a coloing book. I see you are trying to map it nicely, but it needs to be a bit neater, and cleaning up the roads first will help.  Trying to clean up the roads and farmland polygons after it has been mapped is not a good use of other mapper's time. 