Ross said:

There are also specific grass killers that don't harm broad leaf plants.I know that glyphosate does not inhibit the regrowth of grass. It is specifically called for in rehabilitating an existing lawn. Besides that corn is a grass.

I'm confused as my impression of Roundup was that as a broad spectrum herbicide it is intended for broad leaf weeds plus unwanted grass.  There's certainly enough warnings all over the consumer version.  However, it must hurt corn if a sufficient amount is applied …  as there is a GMO corn variety that is Roundup-resistant.  DH armed with a Roundup spray is part of the reason (gads, don't ask) why chemical killer for anything is not used in our yard anymore.  Such shortcuts are seldom less costly shortcuts in the long run due to the need to apply again and again and again which harks to more chemical, allegedly inert, accumulating.  Happily DH's being very reasonable with digging some areas to rid the rhizome weeds (they laugh at herbicides!) and laying down layers of cardboard under rock or shredded bark.  And I actually enjoy the whole Zen thing of uprooting pesky weeds in the raised beds (which DH hates!).