You can also increase glyphosate's effectiveness by adding a little ammonium sulfate (a common nitrogen fertilizer) to the mix.
The ammonium sulfate (spray grade of a fertilizer) is a water conditioner. So if you use it, you add it to the water before the glyphosate or any surfactants.
Glyphosate (active ingredient in Roundup) is degraded very badly by hard water. The various minerals in the water tie up the glyphosate very rapidly. In fact there have been some studies that have shown that using distilled water made Roundup work significantly better than lesser quality water.
So having clean water is important for Roundup type products. Of course for homeowner use where the amounts are rather small, simply using the upper end of the rate will offset some of the chemical tie-up. It may be easier and simpler just to use a little more glyphos than go to the trouble of mixing the fertilizer. But when spraying on a commercial scale we always add a water conditioner before using any glyphosate (either Roundup or generic) product. It just makes sense to have the product be as effective as it can for a given rate of application.
There are liquid products made specifically to condition spray water that are a lot handier than using ammonium sulfate (a granular fertilizer that has to be dissolved in the water). We have pretty much gotten away from using the fertilizer and have went to the liquid formulations, although the cost per acre is slightly higher. But no need to worry about agitation and getting the fertilizer dissolved so it will do its job.
More than most probably wanted to know.