Researchers at Monsanto Company recognized the potential of glyphosate for weed control in 1971, and the company began marketing it in 1974. Since then, it has become the world's leading herbicide and is the active ingredient in many commercial products. Because these products may contain a number of ingredients besides glyphosate, and because the concentration of glyphosate acid -- the ingredient lethal to plants -- can vary, these products are not equal.
Glyphosate functions by moving through a plant's foliage to areas of new growth and blocking an essential enzyme, preventing growth and causing the plant to wilt and die. This makes it a systemic herbicide. It is also non-selective, meaning it kills any plant with which it comes in contact, unless that plant has been genetically modified to resist it. The U.S. Environmental Protection Agency places glyphosate in Toxicity Category III, which is only one class above the category with substances that are virtually non-toxic. It notes that some glyphosate products are in higher toxicity categories.
Products vary in the concentrations of glyphosate acid they contain, and these concentrations are largely determined by the glyphosate salt each one contains. Glyphosate can occur as an isopropylamine or diammonium salt, as well as one containing potassium or sodium -- which have similar chemical properties -- and each serves different purposes. The IPA and DA salts are the ones found in most herbicides, but the potassium salt can be used to modify and regulate plant growth and to make fruit ripen sooner. It is often applied to peanut and sugarcane crops.
Besides the active ingredient, glyphosate herbicides may include an additive to hasten absorption of the product by the plant. The leaves of many plants are covered with small hairs, and to prevent the product from beading on these hairs and simply evaporating, manufacturers include a surfactant. The concentration of such surfactants, such as polyethoxylated tallowamine, is important, because if the plant is overexposed to them, it can die before the glyphosate kills it, and it can re-sprout from the roots. Ammonium sulphate is a recommended additive when mixing glyphosate with hard water. It prevents the salts in the water from binding with the glyphosate and making it unavailable to the plant.
Concentrations of glyphosate acid and the presence of surfactants like POEA create significant differences among the various formulations of glyphosate herbicide. The label of each product provides the proper mixing instructions and intended uses for that product, but it also provides information regarding the concentration of glyphosate. The label does not have to specify the names of the inert ingredients, such as surfactants, but it does list their concentrations. It's important to remember that an inert ingredient is not necessarily benign. "Scientific American" magazine reports that people who drank a glyphosate-based herbicide in Japan in the 1980s and died were poisoned, not by glyphosate, but by POEA.
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