Issue No. 7, Article 3/May 11, 2007
Formulations of 2,4-D: Acids, Esters, and Amines
The herbicide active ingredient 2,4-D, originally discovered in the 1940s, continues to find utility across a diversity of landscapes. The herbicide is a popular tool among homeowners for the selective control of certain broadleaf weed species in turf, and it is frequently a component of burndown herbicide applications in agronomic cropping situations. A large selection of commercially available 2,4-D formulations, trade names, and so on exist from which weed management practitioners can select. However, not all formulations and products are identical.
One characteristic of 2,4-D-containing products that is of particular importance is the "type" of formulation. There are several ways to define formulation, but let's say the formulation consists of the active ingredient and all associated inert components that aid in handling, mixing, application, and absorption. Most often, 2,4-D products are available as one of three formulations: acid, amine, or ester. Each type of formulation has unique characteristics that can influence where and how a particular product is used. This article compares characteristics of these formulations to help explain how and why different formulations are used for different applications.
Figure 1 illustrates the chemical structure of the herbicide 2,4-D. The molecule is considered a weak acid because the carboxyl hydrogen atom (the one to the far right) can dissociate, imparting a net negative charge to the molecule. In the dissociated (negatively charged) form, the acid molecule is very soluble in water but is not readily absorbed through a plant leaf. The waxy cuticle that covers the leaf surface is composed of many noncharged substances, which reduce the ability of a charged molecule to penetrate and enter the plant. Somehow altering the parent acid form can influence how quickly and thoroughly it enters a plant through the leaf. These alterations produce derivatives that have physical and chemical properties different than the parent acid, such as increased ability to penetrate through a waxy leaf or increased water solubility for enhanced root uptake. The two most common derivatives of 2,4-D acid are amines and esters.
Esters are formed by reacting the parent acid with an alcohol, while amine salts are formed when the parent acid is reacted with an amine. Figures 2 and 3 illustrate the chemical structures of 2,4-D amine (dimethylamine) salt and isooctyl ester, respectively. The isooctyl ester is a very common ester formulation of 2,4-D, while the ammonium salt is perhaps the most common amine formulation. Other esters and amine salt formulations, however, are available.
Figure 3. Isooctyl ester of 2,4-D.
As mentioned, these different types of derivatives impart different characteristics to the formulation. For example, the isooctyl ester formulation is more soluble in hydrophobic ("water-avoiding") substances, like waxes, while amines are more soluble in hydrophilic ("water-loving") substances. In practical terms, esters are better able to penetrate the waxy leaf surface of weeds (and crop plants) than amines, while amines are more easily moved into the soil by rainfall for root uptake (an important characteristic in certain brush-control applications).
Table 1 provides some general comparisons between the amine and ester formulations of 2,4-D. These comparisons are somewhat relative since the specific type of amine salt or ester chain length can influence some of these characteristics. For example, ester formulations are considered more volatile (the change from a liquid state to a vapor state) than amine formulations, but the actual volatility potential of the ester formulation is influenced by the length of (i.e., the number of carbon atoms in) the ester chain. Also remember that different derivatives can impact the amount of "active ingredient" contained in a quantity of formulated product.
To accurately compare among various products, calculations of "equivalency" should be based on the amount of acid equivalent contained in the formulation rather than the active ingredient. For instance, the acid equivalents of the isooctyl and ethyl acetate ester formulations of 2,4-D are 66% and 88%, respectively. Several years ago, we discussed the concept of acid equivalent as it applies to herbicides such as glyphosate ("Herbicide Formulations and Calculations: Active Ingredient or Acid Equivalent?," Issue No. 2, April 7, 2000). The same concepts and applications can be used to make comparisons among various 2,4-D formulations.
2,4-D is frequently used as a burndown tankmix prior to corn or soybean planting. Both the amine and ester formulations are labeled for burndown applications prior to soybean planting, but the ester formulation is usually preferred over the amine formulation. The low water solubility of an ester reduces the potential for it to be moved into the soil by precipitation, where it could cause severe injury to germinating soybean seed. Also, the ability of esters to better penetrate the waxy leaf surfaces of weeds often results in better control of large weeds and better control during periods of cool air temperatures. The labels of many 2,4-D ester formulations (3.8 lb acid equivalent per gallon) allow applications of up to 1 pint per acre 7 days prior to soybean planting; increasing the rate to more than 1 pint increases the waiting interval to 30 days. In addition to waiting intervals, labels sometimes also indicate that tillage operations should not be performed for at least 7 days after application and that the seed furrow must be completely closed during the planting operation or severe crop injury may result. Factors that increase the likelihood of the 2,4-D coming in direct contact with the crop seed increase the probability of severe crop injury.--Aaron Hager