As the etymology ἀγρός / ἀρόω, this one implies a formal manipulation, the insertion of a velar stop [g] (but that remains implicit). From the semantic point of view, the etymology is weird: what relates semantically to "field" is τεμνόμενος "cut", not the complement "by iron" which is given as the etymon, yet the etymology unites the lemma with the complement over the verb which is used only to link the two elements. For a similar type, see χείρων / χείρ. This etymology is probably in fact the result of a misunderstanding: the usual etymology derives ἀγρός from ἀρόω (at least since Herodian), and ἀρόω was derived by some from Arès, because to plough is to cut with iron (see ἀρόω / Ἄρης). The latter etymology is found in Orion. So that it is likely that the original formulation was παρὰ τὸ ἀρῶ τὸ ἀροτριῶ, ἀρὸς καὶ ἀγρὸς, παρὰ τὸ ἄρην, ὃ σημαίνει τὸν σίδηρον, ὁ τῷ σιδήρῳ τεμνόμενος: Ares was given as the etymon of ἀρόω, itself the etymon of ἀγρός (embedded etymology) but the compiler understood it as a different etymology for ἀγρός, which it was not, and inserted the ἤ.