Sentence:  I know that I am dilating upon a subject that I have already made mention of, but it cannot be too forcibly brought to the attention of planters that the selection of Mother Seed of the very best quality is of vital importance and that no rice ought to be planted that has not gone through a screening and <WSD>refanning</WSD> process so as to eliminate the light, chaffy, chalky and imperfect grains, all of which reproduce themselves in a plant that is weak and sickly and tends to deterioration. 
Meaning of the word marked with <WSD> tags:  To winnow again.