In a year like we have been experiencing, where heavy rains have delayed planting and the preemergence herbicides have been under pressure, we will in all likelihood see a number of weeds emerge in both corn and soybean fields. Some of the more common weeds that emerge late when preemergence herbicides have been subjected to heavy rains and particularly the grass component of the program has been weakened are: 

Morningglories Waterhemp Burcucumber Fall panicum Common ragweed Giant ragweed Yellow nutsedge Shattercane Crabgrass Lambsquarteres Smartweeds Nightshades   Barnyardgrass

Several of these weeds not only emerge later in the season than our common weed populations, but they will also have multiple flushes throughout the first half of the growing season following each rain occurrence. A single application, or even a second application of glyphosate or Ignite may not be enough to control some of these weeds. Depending on the weed population mix in an individual field, additional herbicide choices may be needed to control these herbicides. There is a large choice of postemergence herbicides labeled for these weeds, but care is needed to make the right choice, and timing for weed height and corn growth stage in critical to make these herbicides work. This just may be the year to scout field for later emerging weeds and study the efficacy tables in the Weed Control Guide (http://www.btny.purdue.edu/Pubs/WS/WS-16/) to select the proper herbicide that can control these late weeds. Sometime the application can be mixed with a standard post application, but this may be the year that an additional trip across the field is needed.