Johnson grass re-emergence post herbicide. What else to try?

[Land_Owner]

I work hard to bush hog, Roundup, disc, fertilize, and seed a previously fallow field.  It used to be choked with air potato http://plants.ifas.ufl.edu/diobul.html and Johnson grass http://plants.usda.gov/java/profile?symbol=SOHA.

I have almost tamed the air potato by copius use of the herbicides Roundup (sparingly due to high cost) and Credit Extra (500-gallons of mixed formula), both with active ingredient Glyphosate.  However, the dormant Johnson grass seed in the ground is just too aggressive.

I planted iron clay peas, clover and sorghum in that field.  Within a month the Johnson grass had returned.  Now it is back with a vengence and I had killed it and rekilled it and re-rekilled it before planting.  I thought, stupid of me wasn't it, that I had the Johnson grass in check.

I don't want to use a soil sterilant if I don't have to.  Any other ideas?

[montveil]

Johnson grass has underground rhizomes from which the plant will resprout. Also it is a proflicic seed producer.
If you are trying to establish a broad leafed plant such as cl;over see the attached site

ttp://cropsoil.psu.edu/extension/facts/agfact4.pdf

There are several selective herbicides that can be used to kill Johnson grass with no effect on broad leaved plants

You can also check with you local university and county extension agent

[Land_Owner]

No "killing temperatures" here.  Growing season limited only by position of the sun and moisture content of the soil.  Guess I will need the Florida Ag Agent's site for equivalent recommendations. 

Used copius amounts of Credit Extra (glyphosate) to kill exposed plants and supposedly rhizome roots.  Glyphosate is foliar so roots and rhizomes should have been killed.  Everytime it stuck its head above ground, I killed it, at least twice.  I also killed them twice more following discing before I planted.  That's 5X times!

I believe seed regermination is my worst enemy.   Now they are back spreading seed again.  Spring tillage is nearing and I intend to kill emerging folage again.  Seems I need a better regime this time.

[deerslayer79]

The real problem with johnsongrass is the roots and rhizomes,really deep and spread like wildfire,I work for a weed control company and have found out that the only way to put a hurting on J grass is to wait until it is in full seedhead stage then cut loose with a 1.5%-2% solution of glyphosate and sticker,the sticker is important,then DO NOT touch until the plant is completely dead.we usually get 80-90% control within the first year of treatment sometimes it takes a couple of years to get it totally.

[Wynn]

I have no final solution to Johnson grass. I have tried everything in the groves I caretake. Now I just clean the radiator of my truck regularly during the summer and send out the bush hog   Same with Lantana. Dose it with Remedy and sticker but it is back in the spring.