Complex sentence: At the time of harvest, seeds show high nitrogen contents, but some remains in leaves, providing a nitrogen-rich straw which can be used for animal feeding or as fertilizer, if the plant material is incorporated into the soil.
Split into simpler sentences: At the time of harvest, seeds show high nitrogen (N) contents. Anyway, a lot of N remains in leaves, providing a N-rich straw which can be used for animal feeding or as fertilizer, given that the plant material is incorporated into the soil.