Iowa State University

 

leftbar.JPG (7146 bytes) rightbar.jpg (2335 bytes)

Which glyphosate product is best?
by Bob Hartzler
blueline.jpg (1822 bytes)

Updated March 13, 2003  -  In an effort to differentiate themselves from competing glyphosate products, manufacturers and distributors continue to develop new formulations and/or marketing strategies.  Regardless of the marketing claims, only so much can be done in developing new formulations.  The three primary things that can be changed in a glyphosate formulation are:  1) the salt included in the formulation; 2) surfactants and other 'inert' ingredients; and 3) the concentration of the parent acid placed in the product.

gyphos.JPG (23022 bytes)

Glyphosate is a weak acid herbicide that in its original form has a hydrogen ion held to a phosphorous group by a weak electrostatic charge.  (In the diagram to the left the P should have another -OH group attached) This parent acid of glyphosate is effective as a herbicide, but doesn't make a stable formulation or mix well with other products.  To make a better agricultural product, manufacturers replace the hydrogen ion with a salt.  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 other positively charged salts present in the water used as a carrier.  Thus, the glyphosate that reaches the leaf surface often is not associated with the salt that it is formulated with. 

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) was 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. 

In 2002 Monsanto test marketed the WeatherMax formulation of glyphosate and have gone with a full release for 2003.  WeatherMax contains the potassium (K) salt of glyphosate.  While Monsanto's marketing campaign touts the all-weather performance of WeatherMax, probably the biggest advantage of this formulation is the higher concentration of glyphosate found in the formulated product (Table 1). 

While there occasionally may be performance differences between glyphosate products, these differences are  more likely to be caused by the differences in surfactants formulated with the product, rather than the salt used in the formulation.   Monsanto does not recommend the use of additional surfactants with products marketed as Roundup UltraMax or WeatherMax, whereas surfactants may be required with other Roundup formulations.  The Touchdown IQ label and most generics state that a non-ionic surfactant may be used.  University trials have consistently demonstrated similar performance among glyphosate products when equivalent rates are applied and label recommendations for surfactants are followed.

A generic formulation that has made a name for itself in the market is ClearOut 41.  Several factors have led to its notoriety, including low price, a law suit between its distributor and Monsanto, and the presence of a Danger signal word rather than the Caution found on most glyphosate products.  The ClearOut label states that the product causes irreversible eye damage, whereas most glyphosate products (including Roundup WeatherMax) warn that the product causes moderate eye irritation.  Obviously the higher level signal word on the ClearOut 41 label is due to differences in 'inert' ingredients since the active ingredient in ClearOut is identical to that in other glyphosate products.  I can speculate that ClearOut 41 has a higher concentration of a tallow amine surfactant than found in other glyphosate products.  The tallow amine surfactants are especially active with glyphosate, but unfortunately have the unfavorable characteristic of being corrosive to eyes and also are toxic to aquatic organisms.  Because of these negative traits, most companies have chosen to reduce the concentrations of this type of surfactant in formulated products.  Roundup Original, still marketed by Monsanto, also carries a Warning signal word on its label.   ClearOut or other glyphosate formulations carrying a Danger or Warning signal word should not pose undue risks to applicators if standard precautions for handling pesticides are used.

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 usually 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 and weed killing ability.

Table 1.  Comparison of several glyphosate formulations.

Formulation Salt Active ingredient
(lb/gal)
Acid equivalent
(lb/gal)
Equivalent rates of product
(ae basis)
Roundup Ultra isopropylamine 4 3 32 oz
Roundup UltraMax isopropylamine 5 3.7 26 oz
Roundup WeatherMax potassium 5.5 4.5 22 oz
Touchdown 5 trimethylsulfonium 5 3.4 28 oz
Touchdown IQ diammonium 3.6 3 32 oz
Touchdown 6 trimethylsulfonium 6 4.1 23 oz
Glyphomax isopropylamine 4 3 32 oz

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 when label recommendations for additives are followed.

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

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
This site designed and managed by Brent Pringnitz.
Submit questions or comments here.  

Copyright 1996-2003, Iowa State University, all rights reserved  

Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.