The affix ṇamul comes after the verb puṣ 'to feed' when a word in the Instrumental -3rd case having the sense of sva is in composition.,
The word स्व means 'self', 'kinsmen' and 'property'. Thus स्वपोषं पुष्णाति 'he feeds himself'; आत्मपोषं, गोपोषं, पितृपोषं, मातृपोषं, धनपोषं, रैपोषं &c. See 1.1.68. Rule 46 applies here also.,