Spray adjuvants are used with postemergence herbicides to help overcome the barriers that impede movement of the herbicide from the leaf surface to the interior of the cell. The Weed Science Society of America defines an adjuvant as any substance in a herbicide formulation or added to the spray tank to modify herbicidal activity or application characteristics. Some products are formulated with sufficient additives such that the user usually does not need to add them to the tank (Roundup Ultra), whereas other products require addition of adjuvants for all uses (Pursuit, Basis Gold, etc.). This article will discuss the types of adjuvants commonly used with postemergence products to improve herbicide absorption, and describe the mechanisms by which they function. Other additives commonly used with postemergence herbicides, such as drift retardants, compatibility agents, etc., will not be covered in this paper. These products often are called utility adjuvants.
There are three primary types of adjuvants used to enhance herbicide performance: surfactants, crop oil concentrates and ammonium fertilizers. Surfactants are a class of adjuvant widely used with herbicides in corn and soybean production. The word surfactant is derived from the term surface active agent, and describes the ability of these compounds to function at the interface between compounds with different solubilities (Figure 1). Surfactant molecules have two distinct components, one is hydrophilic (water soluble) whereas the other is lipophilic (oil soluble). The lipophilic portion of the molecule typically is a long alkyl chain. The two portions of the surfactant molecule allow it to associate with liquids having wide ranging solubilities. In the figure the surfactant allows the oil to go into suspension in water by creating an emulsion.
There are several types of surfactants available, but most products marketed for use with postemergence herbicides are classified as nonionic because they have a neutral charge. Cationic surfactants are formulated with several herbicides, including most glyphosate products. A measurement frequently used to describe surfactants is the HLB (hydrophilic/lipophilic balance). The HLB describes the ability of the surfactant to associate with hydrophilic and lipophilic compounds. Surfactants with a high HLB balance associate better with water soluble compounds than with oil soluble compounds. Most surfactants used with postemergence herbicides have HLB values of 12 or greater. In recent years silicone surfactants have been introduced for agricultural uses. The carbon-based lipophilic chain is modified with silicone in silicon surfactants to dramatically change their characteristics.
Crop oil concentrates (COC) are a combination of a surfactant and a non-phytotoxic oil. Most COC's contain between 15 and 20% emulsifier. COC's are frequently classified by the type of oil used to manufacture them, either a petroleum-based oil or a modified vegetable oil. A methylated seed oil (MSO) is manufactured with a vegetable based oil that has been chemically altered by attaching methanol units to the oil. The attachment of the methanol to the oil alters the HLB of the oil to an optimum level. Methylated seed oils seem to have the greatest advantage over traditional COC's in situations where weeds are under stress from environmental conditions.
The final class of additive used are the nitrogen based fertilizers. Ammonium sulfate, 28% N and 10-34-0 have all been used at some time for this purpose. It is believed that the ammonium ion is largely responsible for the beneficial effect of fertilizers on herbicide performance.
Adjuvants can enhance herbicide activity in several different ways. The effect of surfactants on the surface tension of spray droplets is well documented. The epicuticular wax on the surface of leaves repels water, resulting in beading of spray droplets as they land on leaves. In some situations a high percentage of spray droplets may simply bounce off leaves, resulting in the herbicide falling harmlessly to the ground. Surfactants reduce the surface tension of spray droplets, increasing spray retention and allowing the spray droplets to spread over a larger area (Figure 2). An increase in spray coverage is especially important with contact herbicides that do not move within plants. In most situations the optimum effect of surfactants is reached at concentrations higher than needed to minimize droplet surface tension. This indicates that the effect of surfactants on herbicide activity is due to more than a simple reduction in spray droplet surface tension.
Silicon surfactants reduce the surface tension of water much more than traditional surfactants, resulting in a rapid spreading of spray droplets. These products have been found to increase rainfastness with some products. While in some situations silicon surfactants may have advantages over traditional surfactants, they can also be deleterious to herbicide absorption. Silicone surfactants do not always perform well with herbicides that require small, concentrated spray deposits to maximize uptake.
While the effect of surfactants and crop oil concentrates on spray retention and spread are well documented, the other mechanisms by which they enhance herbicide absorption are less clear. It is believed these products are primarily involved in aiding herbicide movement through the cuticle, but there is evidence that they may also facilitate movement across the cell membrane. Some additives have been shown to disrupt the integrity of the epicuticular wax layer on leaf surfaces. The wax platelets on the surfaces of many leaves may be softened or disrupted by the oils in COC's. It is speculated that some additives act as humectants. As spray droplets dry on the leaf surface the herbicide molecules may form solid crystals. It has been demonstrated that herbicide absorption from a solid is much slower than from a herbicide in solution. An additive that reduces the rate at which the spray droplet evaporates may enhance absorption by keeping the herbicide in solution for a longer time.
Ammonium salts are also widely used with postemergence herbicides. The effect of AMS on reducing antagonism of glyphosate by calcium and other salts present in water has been discussed in an earlier article. Two other possible benefits of ammonium salts on postemergence herbicides have also been described. Most postemergence herbicides are weak acids, and their polarity is dependent upon the pH of the solution they are in. At lower pH's (acid) these herbicides become more lipophilic and better suited to penetrate through cell membranes. Plants are efficient at absorbing ammonium due to their need for nitrogen. An active transport mechanism has been proposed where ammonium absorption is driven by the transfer of hydrogen ions (H+) to the outside of the cell (Figure 3). Pumping hydrogen ions to the extracellular space will reduce the pH of this area. Thus, use of ammonium as a spray additive may enhance herbicide absorption by creating a pH gradient across the cell membrane that favors absorption due to the behavior of weak acid herbicides.
The benefit of ammonium salts on herbicide absorption is frequently more consistent on velvetleaf than other weed species. This may be due to the fact that the leaf surface of velvetleaf is alkaline (high pH). The pH of a neutral water droplet placed on the leaf surface of velvetleaf rapidly increased whereas on a lambsquarter leaf there was no change in pH of the droplet (Figure 4). The specific benefit of the ammonium ion for enhancing herbicide absorption in velvetleaf has yet to be elucidated, but it may be due to preventing the formation of calcium salts of the herbicide parent acid.
Selecting the appropriate adjuvants can be confusing even for experts. There are many products available, and since adjuvants are not regulated, manufacturers usually do not provide specific information about the composition of their products. This makes comparing products difficult, if not impossible. Because of this, it is usually best to purchase adjuvants from persons directly involved in the sale of herbicides. It is in the best interest of these persons to sell products that will work well with the products they sell.
The herbicide label is the best source of information and specifies the legal requirement for the type of adjuvant to be used with that product. Herbicide manufacturers expend considerable effort to determine what adjuvants provide the most consistent product performance. Sometimes labels give users options on what adjuvant to use with respect to environmental conditions, target species, and tank-mix partners. For example, hot, dry weather hardens plants off and reduces herbicide absorption. Under these conditions the higher rates of a COC or surfactant would be beneficial. Under cool, wet conditions the cuticle often is thinner and less of a barrier to absorption (Figure 5). A surfactant might be more appropriate than a COC under these conditions in order to reduce the risk of crop injury. Ammonium based fertilizers are especially important for many herbicides when velvetleaf is present. Also be sure to consider the impact of the spray additive on tank-mix partners. The use of spray additives for one product may not be appropriate for another product. Finally, in recent years several products have been introduced that contain more than one type of additive. Before using combination products determine the components and the rates of each component to ensure that they are appropriate for the intended use.
Proper use of spray adjuvants is a critical step in successful postemergence weed control. Select adjuvant types specifically recommended on the herbicide label and manufactured for use with postemergence herbicides. Remember there are no miracle products that allow reductions in herbicide rates. While reduced herbicide rates often can provide acceptable results, success depends upon timely application and understanding the susceptibility of target weeds, not because one spray additive is superior to another.
Acknowledgment: Jerry Green, DuPont, reviewed this article and made numerous helpful suggestions. His willingness to assist in its preparation is greatly appreciated.