The affix ghañ comes after the verb vṛ preceded by ni when the sense is that of grain.,
The वृ here means both वृङ् and वृञ्. This debars the affix अप्. As नीवाराः 'rice growing wild or without cultivation'.
Why do we say 'meaning grain'? Observe निवरा कन्या 'a virgin girl'.,