Sunflowers in November

Beautiful weather, so Caspar and I walked 10km through the fields.
This time I spent more time on the sunflowers. Actually, more of them were flowering/blossoming. They are smaller than the ones flowering in July. So it must be another species.
At home I learned that some species indeed flower in autumn, even until mid November. Another page stated that they are perennial, but I have seen many of the fields, now with sunflowers, being used for other plants earlier this year. Perhaps some way of fertilising?