Yes, that field has full label rate of atrazine on it. For some reason our straight atrazine did not work well this year. We put straight atrazine down preplant incorporated on all our RR corn intending to come back with glyphosate. The atrazine worked very inconsistently. The photos were actually from a pretty clean spot. 4 rows over the grass and weeds almost sodded over the middles. But we have a great mulch of dead grass and weeds on the corn ground now.

Dad had a field with Lexar on it that failed as well. And I mean totally failed. We discussed spraying it and when he asked with what I mentioned the corn was all Roundup Ready. He did not realize his supplier had sold him RR corn. So we squirted some glyphosate on it and it looks pretty good right now.

This regrowth problem is similar to one I had with 30" rowed beans in the pre-RR days. I have one field that would be a foot lower if the ragweed seed were removed. I could spray it with Basagran and kill every ragweed there. But in 2-3 weeks I'd have a new crop of ragweeds. This is one of the reasons I advocate 15" rows. On a lot of my soils in a dry year 30" rows never close and the weeds take off.

Oh, the absolute best way to control Johnsongrass ( and I did this in some nasty infestations) is wait until plant is flowering, often just as the beans are just turning yellow.  Go through the field and cut off the Johnsongrass seed heads, put them in a bag and carry them out of the field an burn them.  Then put the ashes in a trash bag and let the garbage man haul them off. I think the ashes will grow.

Anyway, then go back with a hand sprayer and thoroughly spray the plants.  Your timing is perfect for the chemical going down into the roots.  You'll have seedlings to fight, but it will control the rhizomes.

It is a lot of work. But something about removing the seed head throws the plant in a tizzy (technical term) and the glyphosate seems a lot more effective.