Post Emergence Thoughts

There are a bunch of corn acres out there that were planted 3 weeks ago without herbicide.  Now a window has opened to get this job done.  What are the options?

This is an example of how to count leaf stages for herbicide application.

Primextra is extremely popular in this area either as a set up program in RR corn or in combination with Calisto as a one shot early post treatment in conventional corn.

Primextra can be applied post emergent. It is labelled up to 5 leaf corn, but the more important criteria is the stage of the weed. Grasses must be no more than 2-3 leaf at most. If you have 5 leaf corn, I guarantee the grasses are too big for Primextra. 
In RR corn, add Touchdown Total to catch the bigger weeds.  DO NOT add any other form of glyphosate to the tank because to use Steve Johns' of Novartis words, "you may get a tank of snot".
In conventional corn you may have to switch to an Ultim based program, eg. Ultim plus Marksman, to get the bigger weeds.