There's never a good, simple answer. Are we talking about a garden? Food plot? 10 acres? A hundred acres? I only ask because some herbicides for sweet corn only come, at minimum, in 2 1/2 gal containers - and you would need two different herbicides. One for grass. One for broadleaf.
Pre-emergence or post-emergence? Is the corn planted and up? Either one probably precludes a pre-emergent herbicide.
I've found that a kid on a hoe works OK.