Iowa State University

Alternative Additives for Glyphosate
by Bob Hartzler

September 24, 2003 -  With glyphosate dominance of the soybean herbicide market, additive manufacturers have developed numerous products to be used with this herbicide.  One tactic has been to develop products that include a combination of ingredients, such as an ammonium salt and surfactant.  The need for these components varies with glyphosate formulation, quality of water used as a carrier, target species and environmental conditions.  Researchers at Michigan State University evaluated several of these products and compared their efficacy to AMS, the most widely used additive with glyphosate (Pratt et al.  2003).  Roundup Ultra was the glyphosate formulation used in the studies, a product with a label that suggested a non-ionic surfactant might improve performance under certain conditions.  Velvetleaf was the target weed, a species that is especially responsive to AMS additions to glyphosate.  The characteristics of the additives are described in Table 1.

Table 1.  Characteristics of additives used in experiments.

The addition of AMS to both deionized water and tap water improved velvetleaf control, but the effect was greater in tap water than deionized (Table 2).   This would be expected since the deionization process removes the salts from the water that antagonize glyphosate.  There was no difference between the two concentrations of AMS evaluated.   While the primary benefit to AMS is believed to be in neutralizing the antagonistic effects of salts in the carrier, the leaf surface of velvetleaf has a high concentration of calcium that may also antagonize glyphosate.  This might explain the benefit of AMS in deionized water in this experiment.  A low rate of glyphosate was used to allow differentiation of the additive affect (i.e. its hard to determine whether AMS is having an effect if 100% control is achieved with all treatments).

Table 2.  Effect of carrier water and spray additives on velvetleaf control
with 11 oz Roundup Ultra in the greenhouse.

^aThe tap water contained 427 mg CaCO₃ and 0.5 mg FE/ml.

In studies comparing AMS to various additives, AMS was equal to or better than all formulated products  (Table 3).  In the greenhouse studies there was no difference between 1 and 2% AMS, so this data was pooled.  In the field however, 2% AMS provided a higher lever of velvetleaf control than the lower concentration.  Dryve and Class Act Next Generation contain the highest concentration of N of the additives compared, and these products generally provided the greatest improvement in velvetleaf control.  The rates of Dryve and Class Act evaluated provided similar concentrations of N as the 1% AMS treatment.  Products with the lowest concentration of N generally provided the least benefit in velvetleaf control.

Table 3.  Effect of different additives on glyphosate activity in
greenhouse and field studies on velvetleaf control.

^aDeionized water with 500 mg/L CaCO₃ was used as carrier in the field study.

This study highlights the importance of additive selection with any herbicide.  Additives should be purchased for a specific purpose and need.  In this situation, the need was a nitrogen source to neutralize the antagonistic effects of salts in the carrier and the velvetleaf leaf surface.  Although some of the additives evaluated contained AMS and therefore could be used as a substitute, the user would end up paying a premium for the additional ingredients in these products.  If there is a need for a combination of adjuvants, these preformulated products may simplify the mixing process.  However, routine use of these products to 'make things simple' may end up wasting money and may compromise herbicide performance.

Pratt, D. J. J. Kells and D. Penner.  2003.  Substitutes for ammonium sulfate as additives with glyphosate and glufosinate.  Weed Technol.  17:576-581.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University