Which glyphosate product is best?
by Bob Hartzler

Updated January 26, 2001 -- Whereas in the past Monsanto was the exclusive manufacturer and distributor of glyphosate products (Roundup, Rodeo, etc.), in recent years several other companies have entered the glyphosate market.  While most companies with glyphosate alternatives simply market their products as generic formulations, Syngenta has tried to differentiate Touchdown from the Roundup products sold by Monsanto.  Touchdown does differ from Roundup and other generics in that Touchdown is formulated as a different salt of glyphosate than Roundup (Figure 1).  The parent acid of glyphosate has a negative charge and salts with a positive charge are formulated with glyphosate to produce a finished product.  Roundup uses the isopropylamine salt, whereas Touchdown is formulated as either a trimethylsulfonium or a diammonium salt.

Does the salt used in the formulation significantly impact herbicide performance?  Probably not.  The salts used are selected to make sure the formulated product handles well, is compatible with other products that might be included in the spray tank, and will not cause adverse crop responses.  The primary reason Syngenta developed a new formulation of Touchdown (Touchdown IQ) is to get away from the burning the original Touchdown formulation caused on Roundup Ready crops.  This adverse response was primarily due to the trimethylsulfonium salt used in the formulation, and was viewed as undesirable by many farmers who planted Roundup Ready soybeans. 

The salt is held to the glyphosate molecule by a relatively weak electrostatic charge.  Because of this, once the product is added to a spray tank the formulated salt (isopropylamine, etc.) can easily be replaced by positively charged salts present in the water.  Thus, the glyphosate that reaches the leaf surface often is not associated with the salt that it is formulated with.  Interactions between glyphosate and calcium salts found in water are the primary reason for adding AMS to the spray tank.

While there occasionally may be performance differences between glyphosate products, these differences are much more likely to be caused by the differences in surfactants formulated with the product, rather than the salt used in the formulation.  When equivalent rates of glyphosate are used and label recommendations for additives are followed the likelihood of differences in performance of different glyphosate products are small.  No product has proven consistently better than another.

Selecting the proper rate for the situation and using the appropriate additives are the key considerations in obtaining consistent control with glyphosate products.  Several different concentrations of glyphosate are now being marketed, so it is important to adjust rates according to the product   used.  Glyphosate labels often state the concentration in two ways:  a) lbs per gal of formulated glyphosate and b)  lbs per gal of acid equivalent of glyphosate.  For example, Roundup Ultra contains 4 lbs per gal of the isopropylamine salt of glyphosate but only 3 lbs per gal acid equivalent of glyphosate.  The first value includes the weight of the salt formulated with glyphosate, whereas the second only measures how much glyphosate is present.  Since the salt does not contribute                                                                                                                                       to weed control, the acid equivalent is a more accurate method of expressing                                                                                                                                       concentrations.  Table 1 lists formulations of several glyphosate products.

Table 1.  Comparison of Roundup Ultra and other glyphosate formulations.

The types and amount of surfactants vary in different glyphosate products.  Monsanto does not recommend the use of additional surfactants with products marketed as Roundup Ultra, whereas surfactants may be required with other Roundup products.  The Touchdown IQ label and most generics state that a non-ionic surfactant may be used. 

In summary, the major differences affecting the performance of glyphosate products are the surfactants included in the package rather than the salt formulated with the product.  As with different amine salts of 2,4-D, we do not expect significant differences in performance of different salts of glyphosate.  We also do not expect consistent differences among products when used at equivalent rates and label recommendations for additives are followed.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University