Question: I understand that Vergil often uses the *-um* genitive plural ending for some second declension nouns, instead of *-ōrum*. For example:

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> huc delecta **virum** sortiti corpora furtim ([*Aeneid*, Book II, line 18](http://www.thelatinlibrary.com/vergil/aen2.shtml))
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Allen & Greenough, [§49d](https://books.google.com/books?id=Q7cAAAAAYAAJ&pg=PA22), provide a bit of usage guidance, saying:

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> The genitive plural often has **-um** or (after **v**) **-om** [...] especially in the poets
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I'd like a better understanding, however, of "often" and "especially" here. Do we have any examples of *-um* being used in this way outside the constraints of meter? Was it especially associated with particular authors, or periods within Classical Latin? How common was it compared to the "standard" *-ōrum* ending?

Answer: 
@Cerberus is right that it does appear in prose in limited circumstances. In addition to *deum* and *virum*, genitive plurals of second declension nouns denoting money or measure often end in *-um*:

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> Nam cum fere constaret, curriculum stadii quod est Pisis apud Jovem Olympium Herculem pedibus suis metatum [esse] idque fecisse longum pedes sescentos, cetera quoque stadia in terra Græcia ab aliis postea instituta **pedum** quidem esse numero **sescentum**, sed tamen esse aliquantulum breviora, facile intellexit. . . .
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> For since it was quite well known that Hercules measured out the track of the stadium of Olympian Jupiter at Pisa and made it six hundred feet long, and that the *rest* of the stadia built later in Greece by *others* were **of six hundred** [feet], but a little shorter, he easily understood. . . .
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> Aulus Gellius, *Noctes Atticæ*, I.1
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I believe it is also used in prose with certain *words*, like *deum* and *virum*, although it is indeed less common. [Livius, *Ab Urbe Condita* V 14.4](http://www.perseus.tufts.edu/hopper/text?doc=Perseus:abo:phi,0914,0015:14:4):

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> ... pestilentiam agris urbique [esse] inlatam haud dubia ira **deum**, quos pestis eius arcendae causa placandos esse in libris fatalibus inuentum sit; ...
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"...by/through the undoubted ire of the gods, whom in order to placate...". The plural relative pronoun shows that *deum* must be plural, and accusative singular wouldn't fit the sentence anyway.