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Role of AMS with glyphosate
by Bob Hartzler
Updated on February 15, 2001 - Numerous products containing glyphosate are now being marketed, and while recommendations for surfactants vary among the products, all the labels recommend that ammonium sulfate (AMS) be included in the spray tank. The role of surfactants and crop oil concentrates is to improve retention on the leaf surface and increase movement of the herbicide from the leaf surface into the leaf. The purpose of AMS is considerable different than traditional additives, but under many situations can be just as important.
Glyphosate is a type of chemical known as a weak acid, which basically means the molecule possesses a negative charge (see related article). Manufacturers formulate the parent acid with a positively charged cation (salt) in order to make a commercial product that is easy to handle, mixes well with other products, and has good herbicidal properties. When the product is in the herbicide jug the negatively charged glyphosate molecule stays closely associated with the positively charged salt. However, once the herbicide is added to the spray tank with water things can happen that may reduce the performance of glyphosate.
All water sources contain salts such as calcium, magnesium and others. Water hardness is a rating of how much salt is present in the water. The salts found in water can interact with glyphosate by substituting for the salt (isopropylamine, diammonium, etc.) used in the formulated product. Research has shown that calcium and magensium salts commonly found in water reduce the activity of glyphosate. These salts interact with charged portions of the glyphosate molecule to form complexes that are not absorbed as quickly by plants as other salts of glyphosate.
Two mechanisms have been proposed for the beneficial effects of AMS used with glyphosate products. Electron micrographs of leaf surfaces treated with glyphosate and AMS reveal the presence of crystals on the leaf surface (Nalewaja, J.D. and R. Matysiak. 1993. Pesticide Sci. 38:77-84). These crystals were not present when AMS was not included in the spray solution. The authors speculated that the sulfate ion in AMS combines with calcium to form calcium sulfate crystals, therefore preventing the formation of the calcium-glyphosate complexes. In addition, the positively charged ammonium ions in AMS may compete with the calcium and magnesium for the charged sites on the glyphosate molecule, therefore reducing the amount of glyphosate tied up by the antagonistic salts.
How much AMS is needed to inactivate the antagonistic salts found in water? The Roundup Ultra label recommends 8.5 to 17 lbs per 100 gallons, whereas the Touchdown label states 4.25 to 17 lbs per 100 gallons. The specific amount required is determined by the hardness of the water used as the carrier. Water hardness is an indicator of salt concentration. Calcium in water can antagonize glyphosate at concentrations as low as 150 ppm. Researchers at North Dakota State University developed the following equation to calculate the amount of AMS needed to counteract the negative affects of hard water: AMS (lbs per 100 gal water) = 0.009 (ppm calcium) + 0.005 (ppm sodium) + 0.002 (ppm potassium) + 0.014 (ppm magnesium). In order to use this equation it is necessary to have the water tested in order to determine the amount of salts present.
The 17 lb rate of AMS is probably higher than needed for most water sources found in Iowa. NDSU states that 8.5 lbs of AMS is sufficient for most water sources, and this recommendation should hold up in Iowa also. However, the only way to know for sure is to have the water tested.
There are certain weeds in which AMS can improve glyphosate performance even if water hardness is not an issue. Velvetleaf is the most common species in Iowa where this response is seen. I have heard some weed scientists state that the response is due to high concentrations of calcium on the velvetleaf leaf surface, therefore forming glyphosate complexes that are not readily absorbed. Since velvetleaf is found in most fields, the inclusion of AMS in the tank may improve the consistency of glyphosate treatments. The lower rate of AMS should be sufficient for this function also.
Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University
more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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