Weed management today brings better yields tomorrow
By Jason Mefford
Missouri State University
Cutting back on weed management this year may create a monster next year. The increase in the seed bank and the proliferation of possibly herbicide-resistant weed biotypes nearby means that weed management for the next growing season will be crucial, and it may take years to return the seed bank to a previously lower level. In the upcoming growing season, the use of adjuvants may be well worth including in your herbicide arsenal.
To understand how adjuvants work, it's important to know why weed management was tough this past season. All leaves have a waxy covering (cuticle) surrounding the outside of the leaf. The covering reduces fungal problems by allowing water to run off the leaf and helps protect the leaf under harsh environmental conditions which will thicken the cuticle. Herbicides have to pass through this waxy barrier and a thicker cuticle means potentially less herbicide entering the plant.
When plants are affected by drought, they go into survival mode, shutting down or reducing any function not necessary to maintain life. As the cuticle thickens to reduce moisture loss, plants begin closing off leaf pores where moisture leaves the plant (stomata). This closure slows down water movement, which in turn causes nutrient/herbicide movement to slow within plants. In perennials or weeds with taproots, this results in less herbicide reaching the root system before the plant can metabolize the herbicide.
Adjuvants play a role in aiding herbicide movement into the plant. There are three types of adjuvants that work directly with herbicide chemistry: surfactants, concentrated crop oils, and fertilizer salts. The herbicide a grower uses will determine which adjuvant is recommended.
Surfactants have many different roles depending on the chemistry used. Cationic and anionic (positive and negatively charged) surfactants are great wetting agents and some cationic surfactants are preformulated with herbicides (e.g., Roundup). However, nonionic surfactants do not have a charge and can be used with a wide variety of herbicides and are not affected by ions in hard water. Nonionic surfactants are good dispersing agents, soluble and stable in cold water, and have low toxicity to plants and animals.
Silicone-based surfactants work even better than nonionic surfactants. They disperse water droplets better and are humectants (humidity creating). Combining these qualities increases the amount of herbicide entering the plant in addition to reducing the time for herbicides to become rainfast. However, silicon surfactants are not compatible with herbicides that need small concentrated deposits, such as glyphosate.
Regardless of the surfactant used, all disperse water droplets. This is done by breaking the surface tension of water allowing the droplets to spread out rather than bead up. This is especially important when using contact herbicides such as Cobra. Contact herbicides do not move through the plant, so coverage area is extremely important.
Concentrated crop oils (and methylated seed oils) enhance uptake of herbicides by penetrating the cuticle. The waxy cuticle is made up of fatty organic compounds as are these oil-based adjuvants. Because they have similar chemical properties, the oils and cuticle are chemically attracted to one another allowing better penetration of the herbicide through the cuticle. Oils also keep herbicides in a liquid state longer, further aiding herbicide uptake. Methylated seed oils are smaller less complicated molecules, which makes them lighter and more effective at penetrating the cuticle. Crop oils and methylated seed oils must also contain an emulsifier in addition to requiring agitation to keep the oil suspended in water preventing buildup on the water surface.
Ammonium sulfate, 28 percent nitrogen, and 10-34-0 fertilizer solutions enhance uptake of herbicides by reducing the pH on leaf surfaces. Lowering the pH of most herbicides makes them more fat (lipid) soluble, which means they can penetrate the cuticle better. It is also thought that as plants take in some of the ammonium ions, pathways are created for herbicides to enter the plant. AMS also "conditions" hard water high in magnesium and calcium ions for herbicides, especially glyphosate which binds to these ions reducing its effect. Adding AMS before adding glyphosate to the mix can protect the herbicides active ingredient against binding. In addition to these benefits, fertilizer salts draw moisture, keeping the herbicide in a liquid state on the leaf surface longer. allowing better uptake. AMS will work well in cooler temps because salts build humidity easier at lower temps.
The chemistry behind herbicide adjuvants make them worth a second look in the next growing season. Knowing how adjuvants work and their role in plant physiology will help you become a better weed manager.
Always follow herbicide instructions and use the recommended adjuvants labels suggest.