As mentioned, if your corn isn’t RR, there’s not much you can do to kill the grasses. At least not on a small scale, economically. You can use 2,4D to take care of the broadleaf weeds with no harm to the corn. Corn likes a dose of N after it’s a foot or two tall, so just get some urea rather than N P K fertilizer. Put it down right before a good rain, or most of the N will be lost to the atmosphere.