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Equation Used to Calculate Rate of AMS (07/02/15)

Question: Several of my growers have been using the formula listed in section #11 on page 72 in the 2015 ND Weed Control Guide to assess their water quality and AMS requirements:

Equation Used to Calculate Rate of AMS

Question: Several of my growers have been using the formula listed in section #11 on page 72 in the 2015 ND Weed Control Guide to assess their water quality and AMS requirements:  lbs AMS/100 gal = (0.002 X ppm K) + (0.005 X ppm Na) + (0.009 X ppm Ca) + (0.014 X ppm Mg) + (0.042 X ppm Fe).

Our good spray water quality has resulted in many cutting out or only using AMS at 0.5 lb/100 gallons of water for a water conditioner for Roundup. The 0.5 lb of AMS is much less than the 8.5 to 17 lbs/100 gallons of water that is recommended on the Roundup labels. How reliable is this formula? Is there a minimum amount of AMS recommended that we should be using? It seem like the growers are putting a lot of faith in this equation and I want to make sure that it works.

Short Answer: The information on page 72 in the 2015 ND Weed Control Guide does recommend using the equation listed above to calculate the amount the AMS need to condition spray water but there is additional information which is equally important, “Glyphosate labels suggest AMS at 8.5 to 17 lb/100 gallons of water. However, analysis of water across the U.S. shows AMS rates of 4 to 6 lbs/100 gal are adequate to overcome most hard water.” We recommend the 4 to 6 lb rate of AMS – See Long Answer.

Long Answer: Two reactions occur with AMS. First reaction: sulfate (from AMS) will bind with cationic minerals in the spray water to form Ca-sulfate, Mg-sulfate, Fe-sulfate, etc. Second reaction: ammonium (from AMS) binds with glyphosate to form glyphosate-ammonium. After antagonistic minerals are neutralized by the sulfate then the ammonium can bind with most postemergence herbicides and form the optimum ammonium-herbicide salt (glyphosate-NH4, dicamba-NH4, Poast-NH4, 2,4-D-NH4, etc.). This ammonium-herbicide molecule can more effectively pass through the cuticle resulting in greater absorption and greater herbicide efficacy. The equation listed above can be used to calculate the amount of sulfate in AMS that is needed to overcome antagonistic minerals in the spray water.  Some water in this region may have low mineral content and when using the equation only a small amount of AMS will be calculated to nullify the hard water. Section #11 on page 72 contains another important fact: “The formula does not account for cationic minerals (Ca) on leaf surfaces (lambsquarters, sunflower, velvetleaf, others) that can antagonize glyphosate.” Some plants contain Ca on the leaf surface and additional sulfate is needed to nullify that mineral source. It is not practical to calculate the amount of Ca on weed leaf surfaces and will require increasing the amount of AMS needed just for that mineral source.

It is important to remember that the function of ammonium is just as important as role of sulfate. The equation in section #11 does not address the amount of ammonium in AMS needed to enhance/optimize the herbicide. **Important point: Even in water with no or low antagonistic cationic salt content where no sulfate is needed, a generous amount of ammonium is required to form ammonium-herbicide for optimum herbicide activity. All weak-acid herbicides are enhanced by ammonium. Always add a nitrogen source if allowed and not restricted by the label!

Water conditioning trials have been conducted at NDSU for well over 30 years and researchers have repeatedly found that AMS is the most economical and effective water conditioner for glyphosate and many other herbicides. It is from this research and also from an extensive water quality testing campaign conducted by Winfield with thousands of water samples tested across the U.S that we have concluded with a general recommendation to “Always add AMS at 4 to 6 lbs/100 gal of water”.

Water quality in the mid-west and west is notoriously bad. Mineral level in water increases to over 1,600 ppm in western ND. ND water often contains a combination of sodium, calcium, magnesium, and iron and these cations are generally additive in the antagonism of herbicides. Water in ND, SD, and MT is often high in sodium bicarbonate which does not normally occur in other areas of the U.S. Calcium levels above 150 ppm and sodium bicarbonate levels above 300 ppm in spray water can reduce weed control in all situations. Water with 1600 ppm sodium bicarbonate may occur in ND, but total hardness levels can exceed 2,500 ppm. Water quality must be known to adjust AMS rate. Growers using the equation and adding AMS only at 0.5 lbs/100 gal may overcome herbicide antagonism from low mineral content in water but the low amount of AMS will not provide sufficient ammonium to optimize herbicide activity – the 0.5 lbs of AMS may have enough sulfate to bind minerals but it does not contain enough ammonium to enhance the herbicide.

Last thought – A great scientist (Dr. John Nalewaja) once said the following, “The best adjuvant for Roundup is more Roundup”. Most growers (should) use full glyphosate rates to delay resistance in weeds. When using high glyphosate rates the requirement for AMS diminishes and complete control may occur on susceptible weeds if AMS is used or not used. The value of AMS will be clearly evident not only when spray water contains antagonistic levels of minerals, but also on those weeds that are less susceptible, weeds larger than recommended, weeds hardened by drought, weeds stressed by environment or previous herbicide applications, or when using herbicide rates that are lower than required for complete kill.

Rich Zollinger

Extension Weed Specialist

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