Herbicide Formulations and Calculations: Active Ingredient or Acid Equivalent?

April 7, 2000
Most people who routinely use pesticides are familiar with the term active ingredient. The active ingredient of a pesticide formulation is the component responsible for its toxicity (phytotoxicity for herbicides) or ability to control the target pest. The active ingredient is always identified on the pesticide label, either by common name (atrazine or bentazon, for example) or chemical name (2,4-dichlorphenoxy acetic acid or diglycolamine salt of 3,6-dichlor-o-anisic acid, for example). The active ingredient statement may also include information about how the product is formulated and the amount of active ingredient contained in a gallon or pound of formulated product. For example, the Basagran label indicates the active ingredient (bentazon) is formulated as the sodium salt, and one gallon of Basagran contains 4 pounds of active ingredient.

Usually when an herbicide trade name is followed by a number and letter designation (4L, 75DF, 7EC, etc.), the number indicates how many pounds of active ingredient are in a gallon (for liquid formulations) or pound (for dry formulations) of the formulated product. The formulation designations for Basagran 4L, AAtrex 90DF, and Prowl 3.3EC indicate Basagran 4L contains 4 pounds of active ingredient (bentazon) per gallon of formulated product, AAtrex 90DF contains 0.90 pound of active ingredient (atrazine) per pound of formulated product, and Prowl 3.3EC contains 3.3 pounds of active ingredient (pendimethalin) per gallon of formulated product, respectively.

Some herbicides (atrazine, for example) have specific maximum-per-year application rates that cannot be exceeded. These maximum-per-year application rates are generally presented in terms of the total amount of active ingredient that can be applied per year. How would you calculate the pounds of active ingredient applied at a given product use rate? There are several calculations that can be used to determine the amount of active ingredient applied at a given product use rate. One of the easiest calculations is

Using this equation, we can calculate the amount of active ingredient (bentazon) that is applied when we apply 2 pints (0.25 gallon) per acre of Basagran 4L:

Sometimes, however, the numbers preceding the formulation designation (L, EC, DF, etc.) do not indicate pounds active ingredient per gallon or pound but rather the acid equivalent per gallon or pound. The term acid equivalent is one that many people are less familiar with. Acid equivalent may be defined as that portion of a formulation (as in the case of 2,4-D ester, for example) that theoretically could be converted back to the corresponding or parent acid. Another definition of acid equivalent is the theoretical yield of parent acid from a pesticide active ingredient that has been formulated as a derivative (esters, salts, and amines are examples of derivatives). For instance, the acid equivalent of the isooctyl ester of 2,4-D is 66 percent of the ester formulation but 88 percent of the ethyl acetate ester formulation. Why would an herbicide (one that has the acid as the parent molecule) be formulated as a derivative (ester, salt, amine, etc.) of the parent acid?

An herbicide molecule may sometimes be altered to impart some property other than herbicidal activity. Herbicidal activity refers to the ability of a particular herbicide to effectively bind to a target site within the plant and exert some type of lethal effect (i.e., you apply the herbicide to the plant and the plant eventually dies). Such alterations are possible with herbicide molecules that are acids (for example, molecules that have a carboxyl group as part of their structure). The acidic carboxyl hydrogen is replaced by the desired ions to form a salt or reacted with an alcohol to form an ester. Why would this be done? For example, due to the chemical characteristics of a particular herbicide molecule, the parent acid may not be readily absorbed into a plant, because it's not able to effectively penetrate the waxy cuticle covering the leaf. Somehow altering the parent acid may increase the ability of the herbicide to penetrate through the leaf much more effectively. For some postemergence herbicides, formulating the parent acid as an ester or salt is frequently done to facilitate absorption through the leaf. Other formulations or derivatives of the parent acid may increase the water solubility of the herbicide. 2,4-D (2,4-dichlorphenoxy acetic acid) is commonly formulated as an ester or amine. The ester formulation increases the lipid solubility of the herbicide, which allows it to more easily penetrate the waxy cuticle of the plant leaf. The amine formulation greatly increases the water solubility of the herbicide, which may be desirable if the product needs to be moved into the soil solution for root uptake (brush control, for example).

If an herbicide is formulated as a derivative of the parent acid, it is important to remember that the parent acid is the herbicidally active portion of the formulation. The parent acid is what binds to the herbicide target site within the plant and causes plant death. The salt or ester portion of the formulated product may allow for greater absorption into the plant but plays no role in binding to the herbicide target site. For example, when an ester herbicide penetrates the cuticle, enzymes convert the ester back to the parent acid, so following absorption, the ester part of the formulation plays no role in herbicidal activity. Modification of the parent acid (formulation as a salt, ester, or amine) may increase the amount of active ingredient in a formulation, because the amount of active ingredient listed on a product label includes both the weight of the parent acid and the weight of the salt or ester. Modification does not always, however, increase the amount of acid (herbicidally active portion) in the formulation. The acid equivalent represents the original acid portion of the molecule and is used for "apples-to-apples" comparisons of different formulations containing the same acid. Another example will hopefully alleviate some the confusion.

2,4-D can be formulated as various esters. The chain length of the ester can be varied but is most commonly eight carbon atoms long (isooctyl ester). Let's assume we have two ester formulations of 2,4-D: the first has only two carbon atoms forming the ester, and the second has eight carbons forming the ester. The parent acid is the same in these two formulations; the only difference is the length of the ester. These can be visualized in the following diagrams.



The structure on the left is the parent acid of 2,4-D. The second diagram is the parent acid, formulated with a 2-carbon side chain (the two added carbons are in bold text), and the third diagram is the parent acid, formulated with an 8-carbon side chain (again, the added carbon atoms are in bold text). While these added carbon atoms may modify some aspect of herbicide performance (the isooctyl ester is the most commonly used ester formulation of 2,4-D), it is the parent acid (the one depicted in the left diagram) that acts at the target site within the plant. The added carbon atoms of the esters add weight to the formulation and may increase the amount of active ingredient of a formulation, but they do not increase the amount of parent acid in the formulation. If these two formulations were commercially available, and someone wanted to know how much of the parent acid each formulation contained, the calculation to use would be based on the acid equivalent of the formulations, not the active ingredient of the formulations.

Let's assume that both the 2,4-D 2-carbon ester formulation and the 8-carbon ester formulation were commercially available and each contains 4 pounds of active ingredient per gallon. The application rate on the label is 1 pint per acre of either formulation. Since the application rates and the pounds of active ingredient per gallon are identical for each formulation, the amount of active ingredient applied would be the same for each formulation. If you doubt this, plug in the appropriate numbers for each formulation in the formula given previously for calculating the amount of active ingredient applied. Even though the amount of active ingredient applied is the same for each formulation, the amount of acid applied is not the same. Remember that it is the parent acid that binds to the target site to control the weed; the ester portion of the formulation is not involved in binding to the target site. How would we calculate the amount of acid applied?

The first step is to determine the amount of acid equivalent contained in a gallon of formulated product. Some labels indicate both the amount of active ingredient and acid equivalent contained in the formulation, while others list only active ingredient. If the pounds acid equivalent is specified on the product label, all you need to do to determine the pounds acid equivalent applied per acre is substitute pounds acid equivalent for pounds active ingredient in the equation presented previously for calculating the pounds active ingredient applied. For this example, however, let's assume that neither of these 2,4-D ester formulation labels indicates the amount of acid equivalent.

The formula that can be used to calculate the amount of acid equivalent contained in a gallon of formulated product is

We now need to provide some molecular weights (i.e., how much the molecule weighs) to complete these calculations. The molecular weight of the parent 2,4-D acid is 221.04. The molecular weight of the 2-carbon ester formulation is 29.02 (weight of the two carbons and five hydrogens) + 221.04 (weight of the parent acid) = 250.06. The molecular weight of the 8-carbon ester formulation is 333.25.

The acid equivalent of the 2-carbon ester formulation is

So the amount of acid equivalent in 1 gallon of formulated product is

The acid equivalent of the 8-carbon ester formulation is

So the amount of acid equivalent in 1 gallon of formulated product is

Again we applied 1 pint (0.125 gallon) per acre of each formulation, and because they both contain 4 pounds active ingredient per gallon, the amount of active ingredient applied is equal. The amount of acid applied (that part of the formulation that actually controls the weed) for each formulation is not equal.

The amount of acid applied per acre with the 2-carbon ester formulation is

The amount of acid applied per acre with the 8-carbon formulation is

This example demonstrates that there was more acid applied with the 2-carbon ester formulation than with the 8-carbon formulation. In practical terms, more of the part of the formulation that actually controls the weeds was applied with the 2-carbon ester formulation. To compare the herbicidally active portion of two ester, salt, or amine formulations, product equivalents should be based on the acid equivalent of a salt or ester formulation.

This exercise was done to illustrate that, to calculate equivalent rates of salt or ester formulations, the acid equivalent calculation should be used. If there is only one formulation of a salt or ester product commercially available, it wouldn't really matter if you calculated active ingredient or acid equivalent. For example, Pursuit is formulated as the ammonium salt of imazethapyr, but currently only one manufacturer markets Pursuit. There are, however, several commercial formulations of 2,4-D and glyphosate. Referring to Table 5, you can see there are over 30 different commercial formulations of glyphosate available today, and more will likely be available in the future. Not all these formulations contain the same amount of acid equivalent, so if you want to determine equivalent rates of two glyphosate-containing formulations with respect to how many molecules of glyphosate are applied, you must calculate these rates based on acid equivalent. Table 6 lists some calculations of acid equivalents, based on an application rate of 1 pound active ingredient per acre. This table illustrates that, when calculations are based on equivalent active ingredient, the amount of acid applied may not always be equal. It is the acid portion of a salt formulation that binds at the target site.

The purpose of this article is to illustrate how to calculate differences in formulations based on either active ingredient or acid equivalent. Will differences in the amount of acid equivalent applied between two formulations result in weed-control differences? You might argue that, if the difference in amount of acid applied is large enough, differences in weed control might result and might be noticed on weeds against which the herbicide is "marginal." However, it is difficult to make an all-inclusive statement that weed-control differences will always result if differing amounts of acid are applied, especially when the difference in amount of acid applied is small. Labeled application rates are established by herbicide manufacturers based on product testing. It does not seem likely that a herbicide manufacturer would market an herbicide at an application rate that would consistently result in reduced weed control compared to a competitive formulation.-- Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague