You're going to want to figure out how to keep it from ever coming out of the ground in the first place, but scorching the earth with a full rate of cobra, a bunch of dry ams, and crop oil has been my only rescue for several years. You won't want to look at the field for awhile.  

I'm behind on the Liberty train, but the locals are already using it at cut rates and on a foot or more tall weeds like they did Roundup, so I'm not sure how long switching seed will be the answer.

Where are you at that resistant water hemp is just now an issue?