By the way, don't you know why in some sources it's not arvo, but ervo (vetch)?
I, for my part, don't, but most certainly whether
arvo and
ervo are variants in the manuscript tradition, or
arvo, which indeed seems to make a whole lot more of sense, is a conjectural correction dismissing
ervo as a scribal misreading.
Scholiast's fourth and main reason, quite well explained by him, is further magnified by the virtuosic placement of the adverb
quam, which creates an impressive ambiguity: while the easier, and of course correct, reading of the verse is "Oh alas!
how skinny is my bull in a fertile land!", the word order, by placing
pingui next to
quam, suggests the syntactic link between them and thus plays with the focusing on
pingui arvo: "Oh alas! my bull is skinny in
how fertile a land!", or "Oh alas! I have a skinny bull in
how fertile a land!" ("est mihi" = "I have") — and all the more so because the caesura falls after
pingui, helping to throw
quam and
pingui together even more tighly —, in such a way that the two contrasting focuses (or, since the bull is an extention and image of the shepherd, the subjective and the objective focus) crisscross with one another. Quite a magnificent verse.