By Ramdas Kanissery, Steve Futch, Brent Sellers and Camille McAvoy
Glyphosate is a popular post-emergent herbicide among Florida citrus growers for its broad-spectrum of weed control under trees and in row middles. The use of glyphosate as a “burn-down” application alone, or in combination with other herbicides, is a standard practice in citrus groves.
Various glyphosate-containing product formulations are available on the market to citrus growers. Regardless of the product, the active ingredient that controls the emerged weeds is the same: glyphosate. Nevertheless, the differences among various formulations of glyphosate-based herbicides can be confusing. This article is intended to help citrus growers better understand their glyphosate products and accomplish the goal of effective weed control.
Unlike most herbicides used in citrus production, glyphosate-containing products are formulated as salts. The major variability between glyphosate products is the type of glyphosate salt in the formulation. Another difference is the additives (e.g., surfactants) in the products (Figure 1).
Chemically, glyphosate is a weak acid which equates to a weak negative charge. When the glyphosate acid is added to a base that has a positive charge, the salt is formed. When it is absorbed, the salt is disassociated at the site of action within the plant tissues. It is only glyphosate acid that binds to the target enzyme and is responsible for herbicidal activities.
Glyphosate products use several forms of glyphosate salts. The major salts in glyphosate products are potassium, isopropylamine, ammonium and trimesium. Potassium refers to the salt form of glyphosate acid from monopotassium (K). Isopropylamine salt has a “propyl chain” (3-carbon chain) and an amine group. Ammonium salt has one (mono) or two (di) ammonium groups. Trimesium is a short name for trimethyl-sulfonium, which means three methyl groups and sulfur. The information on the salt component of the glyphosate is provided on the product label.
For instance, the Roundup Powermax label states its active ingredient as“Glyphosate, N-(phosphonomethyl) glycine, in the form of its potassium salt.” Similarly, Gly Star Plus states its active ingredient as “Glyphosate, N-(phosphonomethyl) glycine, in the form of isopropylamine salt.” In both products, glyphosate is the common name of the active ingredient, and the rest of the information describes the chemical structure.
Glyphosate salts vary in molecular weight and have differences in the glyphosate acid-to-salt ratio. These changes cause variations in the amounts of active ingredient (glyphosate acid + salt) and acid equivalent (glyphosate acid alone) in the formulations. Also, different glyphosate salts may have different water solubility. Thus, different glyphosate salts are purported to have variable effects on stability in formulation, absorption and translocation within the weed.
However, research communities have yet to find a noteworthy effect of glyphosate salt on weed-control efficacy. Most reports suggested that differences in weed control are inconsistent and do not support a single superior glyphosate-salt formulation.
Since the acid component of the material is what binds at the active site within the plant, glyphosate rates are often referred to in terms of pounds (lbs.) acid equivalent (a.e.) per acre instead of lbs. active ingredient (a.i.) per acre. A compiled list of several glyphosate products and the product use rates for equivalent amounts of glyphosate acid per acre is presented in Table 1.
Other components (e.g., adjuvants, surfactants, carrier solvents, etc.) are contained within the formulation of glyphosate products. These inert materials are components that improve the handling, safety and solubility of products. They promote the dispersion, emulsifying, absorbing, spreading, sticking or penetration of the spray droplets. They can also facilitate movement of glyphosate into plants. Differences in performance among glyphosate products could be attributed to these components.
While many products have a built-in surfactant, the type of surfactants used in the formulated product may be dependent upon the manufacturer. Consequently, even if the active ingredient is identical between two products, a significant difference in the performance may solely be dependent upon the inert materials. In general, it is difficult to statistically separate these differences with replicated field studies.
Adding an appropriate surfactant is helpful while using a glyphosate product without a built-in surfactant system. Such glyphosate formulations are usually utilized in aquatic situations. Make sure to read the herbicide product label to see requirements and specifications for additional surfactants. For products where the addition of a surfactant is suggested, consider adding a high-quality, non-ionic surfactant at rates of 0.25 to 1.0 percent volume per volume (0.32 to 1.28 ounces of surfactant per gallon of spray solution) depending on the label specifications.
If the water source for spray mix is “hard,” or in other words, if it contains large amounts of dissolved salts like calcium (Ca), magnesium (Mg), iron and aluminum, glyphosate activity could be reduced. These salts, particularly Ca and Mg, will displace the potassium, isopropylamine or other salt components in the glyphosate-salt complex in the formulated product. The glyphosate activity will be reduced as weeds absorb less glyphosate bound with Ca or Mg salts than the formulated salt of glyphosate.
To prevent this, most glyphosate product labels suggest adding ammonium sulfate (AMS) during the preparation of herbicide tank mix. AMS prevents antagonistic salts from interacting with glyphosate. The negatively charged sulfate portion of AMS will bind with the positively charged Ca and/or Mg. AMS should always be added to the water prior to adding glyphosate product to avoid the potential formation of less active glyphosate-salt complexes with antagonistic cations.
Ramdas Kanissery is an assistant professor at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Southwest Florida Research and Education Center (SWFREC). Steve Futch is a multi-county Extension agent at the UF/IFAS Citrus Research and Education Center. Brent Sellers is associate professor and associate director of the UF/IFAS Range Cattle Research and Education Center. Camille McAvoy is a laboratory technician at the UF/IFAS SWFREC.