North Texan

I would add it every time I used glyphosate. Calcium and magnesium will bond with glyphosate, so something like hard water will reduce effectiveness, or plants that contain an abundance of either. When ammonium sulfate is added, it does two things. First, the sulfate acts to bind and tie up some of the calcium and magnesium in the tank water, and even some in the treated plants. Second, the glyphosate has a tendency to bind with ammonium. Plants uptake the ammonium-glyphosate ions a little better, making the glyphosate more effective. The key is to use spray grade ammonium sulfate and to add it to the tank before adding the glyphosate.

You'll still need to use a surfactant. Some people try to use the ammonium sulfate in place of surfactant. That's not what it is for.

I would not mix the glyphosate with a product like Crossfire, which is 2,4-D + triclopyr unless you are specifically having trouble with woody plants in the food plot. I use triclopyr a lot for rangeland restoration, generally tank-mixed with a clopyralid. It works fairly well for certain species of brush, and does provide substantial broadleaf control of western ragweed, common broomweed, and many other forbes (and unfortunately legumes). Would make me want to be very cautious about applying to a food plot.