Recent rains and warmer temperatures will make the weeds grow as fast as the crop, and shorten your time for applying a post emergent herbicide if your pre-emergent herbicide has failed or was never applied.  University of Illinois Extension weed specialist Aaron Hager says you have several choices. Visit the University of Illinois Extension site.

1) Remove the weeds during the current critical period for corn and yield will be saved.
2) If the corn is beyond the labeled recommendations, the crop can be injured.
3) Corn under stress from water or other issues can be injured by herbicides.
4) Check labels since some herbicides now have restrictions against certain insecticides.