I understand that Vergil often uses the -um genitive plural ending for some second declension nouns, instead of -ōrum. For example:
huc delecta virum sortiti corpora furtim (Aeneid, Book II, line 18)
Allen & Greenough, §49d, provide a bit of usage guidance, saying:
The genitive plural often has -um or (after v) -om [...] especially in the poets
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?