If you have hard water (lots of minerals) it can really reduce the effectives of your gly as the minerals bond with the gly as soon as you mix it in the tank. I have been using spray grade Ammonium Sulfate as a water conditioner and had great success with it. The recommended mixing rate is 17lb of powder per 100 gallons of water. If you condition your water first, there's not much that will survive a 3 or 4% gly solution. But I would also trust the recommendation above for 2,4D. If you are growing some gly resistant weeds, that should help end the problem. I spray a mixture of 1% gly and 1% 2,4D on the sides of our forest roads and I can't see anything that survives it.