Glyphosate confusion: What are the differences in formulations?

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

There are over 30 different glyphosate products that growers in Michigan can choose from for weed control in glyphosate-tolerant (Roundup Ready) crops. With all of these choices, it is important to keep in mind that not all glyphosate formulations are created equal. Knowing your glyphosate product is essential to achieving optimum weed control. While all of these options contain the same active ingredient (glyphosate), differences exist in the formulated products that can influence overall weed control. Below is a general explanation of some of the differences that you may observe in the different glyphosate products that you can purchase in Michigan.

Differences in the concentration of glyphosate

One of the biggest differences in the glyphosate products that are available in Michigan is the concentration of the glyphosate acid in the formulation (the glyphosate acid is what kills the weed). This concentration is expressed as pounds acid equivalent per gallon (lb a.e./gal). Unlike other herbicides, glyphosate containing products are formulated as salts to enhance absorption into the plant. Several types of salts have been used to formulate different glyphosate products over the years and each salt has a different molecular weight. Primary salts that have been used to formulate glyphosate products include: potassium, isopropylamine, monoammonium, diammonium, and trimesium salts. Because of the differences in the molecular weights of these salts there are changes in the glyphosate acid to salt ratio in the different glyphosate products. These changes cause differences in the amounts of active ingredient (glyphosate acid + salt) and acid equivalent in of many of these formulations. Once absorbed into the plant the salt is cleaved off and the glyphosate acid binds to the site of action which ultimately leads to plant death in susceptible plant species. Because the glyphosate acid is the portion of the formulation that ultimately controls the weed, we often refer to glyphosate rates not in terms of pounds active ingredient (a.i.) per acre, but in terms of pounds acid equivalents (a.e.) per acre. For example, a normal use rate for postemergence weed control in glyphosate-tolerant crops is often 0.75 lb a.e./A, this rate is equivalent to 32 fl oz/A for a 3 lb a.e./gal product like Buccaneer Plus and equivalent to 22 fl oz/A for a 4.5 lb a.e./gal product like Roundup WeatherMax. Even though the product rates are different for each of the formulations in this example they are both delivering the same amount of glyphosate acid to the plant. To help sort out some of the confusion for several of the products available in Michigan

Table 10 in the MSU Weed Control Guide for Field Crops (E-434) is a compiled list of several glyphosate products and the product use rates for equivalent amounts of glyphosate acid per acre.

Addition of a surfactant

Another difference in glyphosate products is whether a surfactant needs to be added to the spray solution or if the formulated glyphosate product has a built-in adjuvant package. Products like Roundup WeatherMax, Touchdown Total, Glyphomax XRT and several others have built-in adjuvant systems. Even though all of these products have a built-in adjuvant system, there can and many times are differences in the type of surfactant formulated in the product. These differences may equate to differences in weed control under extreme conditions. With some of these products under adverse conditions the addition of 0.25% v/v of a non-ionic surfactant may help improve glyphosate activity. However, under most conditions there are no differences between these products especially when they are used at the correct rates at the appropriate application timings. For products where the addition of a surfactant is recommended add a high quality non-ionic surfactant at 0.25% to 1.0% v/v, depending on product. Table 10 in the MSU Weed Control Guide for Field Crops (E-434) includes information on whether the addition of a surfactant is recommended for a particular product. Whether a surfactant needs to be added or not, virtually all glyphosate products recommend the addition of ammonium sulfate (AMS). We recommend adding dry spray grade AMS at 17 lbs/100 gal. or the equivalent of 17 lbs/100 gal. of liquid AMS products. The addition of AMS minimizes the negative effect of hard water on glyphosate activity and is important for velvetleaf control, regardless of water quality.

Dr. Sprague's work is funded in part by MSU's AgBioResearch.

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