Hello Yuji san. I assume that these are unused rice fields? They are probably 100's of years old, so it is important to maintain them or they will be lost? The grass could be ploughed back into the field, like a green manure, so the nitrogen etc will be put back into the soil? I am only guessing. Does this also mean that the water channels are unused too? That will be a lot harder to maintain! Rice is not grown in The UK, so I only know what I've seen on KZbin! Thank you as usual for your hard work and another great video.