April 20, 2007

A myriad of AMS substitutes (sometimes called water conditioners) and other adjuvants have been developed in response to difficulties associated with handling granular AMS and getting it to dissolve in the spray tank. (See related story, AMS - What Is It Doing in My Tank). Most of these substitutes contain a small amount of AMS in addition to a number of proprietary products, such as surfactants, anti-foaming agents, etc.

The effectiveness of AMS substitutes in hard water depends largely upon the amount of AMS in the substitute product.

In university trials designed to test the limits of adjuvant effectiveness, many water conditioners or adjuvants containing AMS did not perform as well as granular AMS. Figure 1 depicts data generated in one such experiment. Spray solutions were prepared in distilled water to avoid the interference of iron (Fe) and calcium (Ca). The "no adjuvant treatment" represents glyphosate by itself. In distilled water, all the water conditioners (AMS substitutes) worked as well as glyphosate alone, or increased activity slightly (solid black bars). However, when 160 ppm iron was added to the spray solution, only two water conditioners were as effective as 1% AMS, others had no effect, and some appeared to reduce control further.

Why do some water conditioners work better than others? One factor is the amount of AMS in the water conditioner (AMS substitute). Extensive university research has shown that 8.5 lb AMS/100 gal is necessary for optimum glyphosate efficacy in most water sources. In some cases, 17 lb/100 gal is necessary because the water is harder or the weeds are more difficult to control. These rates represented the amount of NH4+ from AMS needed to block Ca2+ and Fe3+ from binding to the glyphosate. When there is less AMS in solution, more glyphosate binds to the Ca2+ and Fe3+, and glyphosate activity is reduced.

Figure 1. Velvetleaf control 14 days after treatment with tank mixtures of glyphosate, water conditioners, and iron. Spray solutions were prepared in distilled water. 100% represents dead velvetleaf.

Common use rates for AMS substitutes are 0.125-2.5% v/v. Most are 0.25-1.0% v/v. Due to incomplete labeling requirements, it is not possible to know exactly how much AMS is in these products, but given the solubility of AMS in water (5.89 lb/gal at 32 F), and the fact that AMS is only a portion of the active ingredient in the AMS substitute, we can predict that the amount of AMS added probably ranges from about 0.4 lb/100 gal (at 0.125% v/v) up to 8.5 lb/gal (at 2.5% v/v). At typical water conditioner use rates (1% v/v or less), the amount of AMS added is usually less than 3.5 lbs/100 gals. That is substantially less than the 8.5-17 lbs AMS/100 gal shown to provide maximum glyphosate activity. Using a football analogy, AMS acts like the offensive line to protect the quarterback — glyphosate. A low rate of AMS could be compared to an average offensive lineman weight of 150 lbs, not enough to stop an aggressive defender from tying up the quarterback.

In Figure 1, at the 2.5% v/v rate of Class Act Next Generation (Class Act NG) resulted in glyphosate performance similar to glyphosate+AMS. At that rate, it provides 8.5 lb AMS/100 gal. However, at the lower rates (all other water conditioners), especially below 1% v/v, there was not enough AMS (less than 3.4 lb/100 gal) to adequately counteract the Fe3+ in the water, with the exception of NTANK.

What is the take-home lesson of this research? Producers should be aware of how much AMS they are getting with an AMS substitute, and should make sure it is enough to adequately counteract the Ca2+ and Fe3+ present in their water. To get enough AMS from an AMS substitute, a rate of 2.5 gal/100 gal or higher may be necessary, if allowed by the product label.. This represents an amount far greater than the 1-4 qts/100 gal recommended on most labels.

Similar research was conducted by weed scientists at Kansas State University. Their data may be viewed in a brief research summary at http://www.oznet.ksu.edu/weedmanagement/Dallas%20Files/MS200501&8%20FD.pdf (PDF 19.6 KB)

Mark Bernards
Extension Weed Specialist