This article was written by Alan York from North Carolina State University. It was originally available via the NC Cooperative Extension Service. Our thanks to Dr. York and NCSU for this very informative piece.
Things used to be simple. Glyphosate was originally produced exclusively by Monsanto, and it was formulated as a 4 lb/gal isopropylamine salt. After the patent on glyphosate isopropylamine salt expired, a number of generic brands of glyphosate isopropylamine salt came into the market. Except for dealing with several brand names, things remained relatively simple. A quart of one brand was pretty much equal to a quart of another brand. All formulations basically suggested the use of a nonionic surfactant. Then, Roundup Ultra entered the marketplace. This particular product contained adequate surfactant in the formulation, and there was no benefit from adding more. Things really became confusing when Touchdown 5, Touchdown, and Roundup UltraMax came on the market. These products either contained a greater amount of glyphosate isopropylamine salt per gallon or they were formulated as different salts.
With many brand names and different concentrations and different salts now available, how does one go about comparing the products?
The parent acid of glyphosate has a negative charge, and salts with a positive charge are formulated with glyphosate to produce a finished product. Commercial formulations of glyphosate are available as the isopropylamine salt, the diammonium salt, and the trimethylsulfonium salt. Concentrations of the formulated products range from 3.57 lb/gal of the trimethylsulfonium salt to 5 lb/gal of the isopropylamine salt.
Does the salt used in the formulation significantly impact herbicide performance? In the case of Roundup Ready crop response, the answer is yes. Touchdown 5, formulated as the trimethylsulfonium salt, could be used for burndown. It was also registered for application to Roundup Ready soybeans. There was minor injury (basically chlorosis or foliar burn) to the soybeans, but the effect was only cosmetic. This response was due to the trimethylsulfonium salt, not the active portion of the molecule. Touchdown 5 was not registered for postemergence application to Roundup Ready cotton or corn because these crops were much more severely damaged by the trimethylsulfonium salt. Because of this problem with Roundup Ready crop damage with the trimethylsulfonium salt, that particular formulation is being phased out of the market. The new Touchdown, introduced in 2001, is formulated as a diammonium salt. Research has shown no differences in response of Roundup Ready corn, cotton, or soybeans to Touchdown and the various brands containing the isopropylamine salt of glyphosate.
Research has shown very little to no difference in weed response to the various formulations of glyphosate. The salt is held to the glyphosate molecule by weak bonds. Because of this, once the product is added to the spray tank, the salt ion is easily dissociated from the glyphosate parent molecule. Thus, the glyphosate that reaches the leaf surface is often not associated with the salt it was formulated with. In other words, the weed really doesn’t know the difference among the various brands and salt formulations.
Differences in weed control with the various products, if observed, are much more likely to be caused by differences in the adjuvants included in the formulated products rather than the salt used in the formulation. Adjuvant recommendations vary according to the particular product used. When equivalent rates of glyphosate are used and label recommendations for additives are followed, the likelihood of seeing a difference in weed control among glyphosate products is very small. Note that the manufacturers claim the adjuvant systems in some of the newer products, Roundup Ultra, Roundup UltraMax, and Touchdown in particular, reduce the necessary rainfast period.
How does one determine what rate of one product is equivalent to another product? Because the salt used in the formulated product does not contribute to weed control, one should compare products (or rates of formulated products) on the basis of acid equivalency.
The labels of some glyphosate products will list both the rate of formulated salt and the acid equivalent. For example, the label for Roundup Ultra says the product contains “4 pounds per U.S. gallon of the active ingredient glyphosate, in the form of its isopropylamine salt.” Then the label says that is “equivalent to 3 pounds per U.S. gallon of the acid, glyphosate.” Labels for other products, Roundup UltraMax for example, express the ingredients only as the amount of salt formulation per gallon. The Roundup UltraMax label says the product contains “5 pounds per U.S. gallon of the active ingredient glyphosate, in the form of its isopropylamine salt”. Other products may list the active ingredients only on the basis of glyphosate acid equivalent. For example, the Touchdown label says the product “contains 3 pounds of glyphosate acid in each gallon, in the diammonium salt form.”
When comparing products, one should not be concerned with the amount of formulated salt per gallon. Rather, focus on the amount of glyphosate acid equivalent per gallon. For example, Roundup Ultra contains 4 pounds of the isopropylamine salt per gallon while Touchdown contains only 3.57 pounds of the diammonium salt per gallon. This does not mean Touchdown is a “weaker” product. Both products contain 3 pounds of glyphosate acid equivalent per gallon. The isopropylamine ion is heavier than the diammonium ions, hence the isopropylamine salt of glyphosate is heavier than the equivalent amount of diammonium salt of glyphosate.
When the various products are compared on the basis of glyphosate acid equivalent per acre and the label directions for use of adjuvants are followed, weed control should be essentially identical regardless of the product used. The rate of product to achieve a given rate of glyphosate acid equivalent will differ among the various products.
Registered uses of the various glyphosate products do vary. For example, all commonly available glyphosate products can be used for burndown in no-till situations. Additionally, all of the commonly available products can be applied postemergence to Roundup Ready soybeans. However, only certain brands and formulations are registered for postemergence application to Roundup Ready corn or cotton, or for application to cotton using hooded sprayers.
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