Glyphosate is unique among herbicides because glyphosate acid is combined with different cations and sold in different formulations. A large number of brand names can lead to confusion about the amount of a particular product to apply per acre.
Some guidelines usually express pesticide activity in terms of pounds of active ingredient per acre (lbs ai/A), but this is inadequate to fully describe the activity of different formulations of glyphosate.
Instead we refer to the lbs ae/A (ae = acid equivalent) of the formulated product. The lbs ae/gal refers to the amount of glyphosate acid contained in a gallon of product.
Water quality, especially hardness, can significantly reduce the efficacy of glyphosate. Transition metal ions in water - including iron, zinc, calcium, and magnesium - form complexes with glyphosate that reduce foliar absorption and translocation (movement) in plants.
Glyphosate activity can be increased by reducing carrier volume from 15 to 25 gallons of water per acre down to 5 to 10 gallons per acre. This reduces the ratio of cations to glyphosate molecules in the water.
Glyphosate activity is also increased by adding ammonium sulfate (AMS) to the spray solution. With AMS, calcium sulfate precipitates from the drying spray droplet preventing it from forming a complex with glyphosate. Similar effects may occur with other cations in the spray droplet.
AMS also increases glyphosate activity since the interaction of the ammonium ion with glyphosate increases absorption.
Steps to maximize glyphosate effectiveness:
1 - use clean water free of silt (dirt will bind to and inactivate glyphosate herbicides);
2 - add almost all of the water to the spray tank;
3 - add spray-grade ammonium sulfate at 8.5 to 17 pounds per 100 gallons while agitating the spray solution;
4 - add the glyphosate herbicide;
5 - add remaining water to the spray tank; and
6 - apply in a carrier volume close to 10 gallons per acre.