ISU has a good publication on Horseweed (marestail) resistance. Seems it didn't just become resistant last yr, or the yr before that. Also seems it's resistant to more than glyphosate, and in more countries than just US. Countries that have probably never used glyphosate. I realize some folks think that using minimal approved rates vs maximum rates can cause resistance. However, some also believe that a 500x rate will not kill a truly resistant weed. It can't, or weed wouldn't be resistant-it would just be hard to kill, for whatever reason.

Anyway-you might look @ this nice little publication put out by good ISU folks. The Horseweed (marestail) publication is the first link offered:

http://www.weeds.iastate.edu/mgmt/2006/NCWSSglystewardship.htm