Influence of Adjuvants on Absorption of Foliar-Applied Herbicides
No discussion of foliar absorption would be complete without some mention of adjuvants. Manipulating herbicide spray solutions with various adjuvants represents one opportunity to improve performance of foliar-applied herbicides under a variety of environmental conditions; however, there are limits. The term adjuvant encompasses a wide range of compounds that have been used to improve herbicide performance. The general properties of these compounds are illustrated in Figure 4. Adjuvants can improve herbicide performance by influencing a number of factors that are involved in herbicide absorption.
Increase droplet coverage and spray retention. One of the best-known properties of surfactants is their ability to reduce the surface tension of the spray solution. Reducing the surface tension of the spray solution means that spray droplets will spread beyond their initial contact area. Increasing the coverage area increases herbicide absorption. ’Stickers’ are materials that increase the probability that droplets will adhere to the leaf surface rather then acting like little ping-pong balls and bouncing off leaves. Stickers are often water-soluble polymers, acrylic latex or resins. Wetting or spreading agents are often combined with stickers to improve coverage. The term spreader-sticker was often used to describe these adjuvants, but does not represent a logical term since ‘sticking’ is the opposite of ‘spreading’.
Increase spray droplet drying time. A material that increases the drying time of a spray droplet is called a humectant. Humectants are commonly water-soluble materials that resist drying even after the aqueous solution has evaporated. They accomplish this by drawing water from the atmosphere. Glycerin, propylene glycol, diethylene glycol, polyethylene glycol, urea and ammonium sulfate are common humectants. Oil-based compounds, like COC (crop oil concentrate)or MSO (methylated seed oil) also resist drying. Since absorption can occur only when the herbicide is in solution, slowing the rate at which droplets dry will allow more time for absorption. Not all herbicides are compatible with oil-based humectants. Theoretically, adding a humectant should improve herbicide performance under hot, dry conditions; however, herbicide performance rarely increases with the addition of a spray adjuvant if weeds are under severe moisture stress.
Increase cuticle penetration. Crop oil concentrates (COC) and vegetable oils (methylated seed oil, MSO) fall into the category of penetrants. This type of surfactant can improve cuticular penetration by softening, plasticizing, or dissolving cuticular waxes and allowing herbicide movement to the more hydrophilic regions underneath. Penetrant adjuvants are often a complex mixture of surfactant and oil (paraffinic petroleum or modified vegetable).
Increase cellular accumulation. The cellular absorption of weak acid herbicides can be improved with the addition of UAN (urea ammonium nitrate) or AS (ammonium sulfate) to the spray solution. Weak acid herbicides have a carboxylic acid or other functional group that accepts or loses a hydrogen ion depending on the pH of the surrounding solution. This means that the herbicide’s solubility will change with changes in solution pH. Under normal conditions, ammonium ions (NH4+) are actively transported into the cell across the cell membrane. The addition of UAN or AS to the spray solution increases ammonium accumulation in the cell. This accumulation alters the cell’s pH because once inside the cell the ammonium ion becomes ammonia (NH3+) plus a free hydrogen(H+). The increase of hydrogen inside the cell would decrease cellular pH if allowed to accumulate. To maintain cytoplasmic pH in the range of 7.5-8.0, hydrogen ions are pumped across the cell membrane into the cell wall. The movement of ammonium into the cell and subsequent pumping of hydrogen ions into the cell wall causes the cell wall pH, which is already acidic, to become even more acidic, as low as pH 4.5. For weak acid herbicides, acidic conditions cause more of the herbicide molecules to be present in their non-ionized, lipophilic form. The nonionized form passes through the cell membrane and into the cell in response to a concentration gradient, increasing the herbicide concentration in the cell due to a process called ion trapping (the process is llustrated in the animation on cellular absorption).
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The bottom line is that adjuvants can help improve herbicide performance under adverse weather conditions; however, it is not possible to substitute adjuvants for good growing conditions. Adjuvants may improve herbicide movement in the cuticle or leaf, but the problem lies in the effects of moisture stress on the plant’s physiology. If the herbicide requires translocation to meristemic regions or requires active metabolic process in the plant, moisture stress will significantly reduce herbicide efficacy regardless of the adjuvant system.