COVER STORY
A ROUNDUP ON GLYPHOSATE: HISTORY, USE, ISSUES, AND ALTERNATIVES By Jeffrey Derr, Ph.D. Professor of Weed Science, Virginia Tech
G
lyphosate has been in the news quite a bit recently. You probably have seen the ads on television concerning glyphosate and human health. In this article I discuss some background information on this herbicide as well as scientific reports on toxicity of the chemical.
A. What is glyphosate?
The glyphosate molecule (Figure 1) is similar to the amino acid glycine. The chemical name is N-(phosphonomethyl) glycine, a relatively small molecule compared to newer herbicides. It is the active ingredient in products such as Roundup PROMAX, as well as in generic products sold under a variety of trade names. Since the chemical is off patent (patent expired around 2000), other companies besides Monsanto/Bayer can sell this herbicide under their own trade name. Some examples of other trade names for products that contain glyphosate include Accord, Gly Star Pro, Glyphomate 41, Razor, Rodeo and Touchdown, among others.
B. Formulation
The initial formation of this herbicide was an isopropylamine salt of glyphosate, which is still widely used, although other salt formulations are available now, such as the potassium salt and the dimethylamine salt. This becomes important when comparing products. We need to look at the amount of glyphosate acid in products, not the amount of active ingredient, since the different salt formulations differ in their molecular weight. For example, Roundup PROMAX contains 5.5 pounds per gallon of the potassium salt of glyphosate (ai) or 4.5 lbs/gallon glyphosate acid (ae). Roundup Pro contained 4.0 lbs/gallon of the isopropylamine salt of glyphosate (ai) or 3.0 lbs/gallon glyphosate acid (ae). So Roundup PROMAX contains 50% more glyphosate acid than Roundup Pro and applications rates in terms of fluid ounces of product per acre are lower than for Roundup Pro. Besides glyphosate, there are other chemicals in commercial formulations, usually water and a surfactant. As an example of a formulation, Roundup PROMAX contains the potassium salt of glyphosate at 48.7% by weight, surfactant(s) approximately 9%, and water plus minor ingredients at approximately 42%.
FIGURE 1: glyphosate isopropylamine salt
12 • MISSISSIPPI TURFGRASS • WINTER 2020
Surfactants can aid in the absorption of herbicides by weeds. One example would be POEA (polyoxyethylene tallow amine). Concerns have been raised about the toxicity of this surfactant to aquatic organisms such as tadpoles, salamanders, and frogs. Aquatic formulations of glyphosate either do not contain a surfactant or contain a surfactant that has not shown to be an issue. That is why certain formulations of glyphosate are not labeled for aquatic use – not due to the toxicity of glyphosate but due to toxicity concerns about the surfactant to amphibians and other aquatic organisms. For glyphosate products that do not contain a surfactant, one generally needs to add an approved one for optimum weed control.
C. History
Glyphosate was found to control weeds by a Monsanto scientist in 1970. The herbicide was patented in 1971 and was introduced commercially as Roundup in 1974. Glyphosate use has increased over time, especially when Roundup-Ready crops were introduced, starting with Roundup Ready soybeans in 1996. These soybean lines, and later corn, cotton, and other crop cultivars, were developed to be resistant to glyphosate through biotechnology. For these geneticallymodified crops, glyphosate could be applied overtop for selective weed control.
D. Mode of action
Glyphosate inhibits the enzyme enolpyruvyl shikimate-3-phosphate (EPSP) synthase, needed for synthesis of the aromatic amino acids tryptophan, tyrosine, and phenylalanine in plants. Animals, including people, do not make these amino acids, so they need to get these chemicals in their diet. Plants have to make these amino acids, which are the building blocks of proteins. So glyphosate inhibits a process that occurs in plants but not in animals, resulting in a chemical that controls plants with low acute toxicity to animals, including people.
E. Acute toxicity of glyphosate
A measure of toxicity is the LD50 value, the dose required to kill 50% of the test animal, usually rats. The higher the LD50, the lower the toxicity. The acute oral LD50 for glyphosate in rats is greater than 5,000 mg/kg – practically non-toxic. The acute dermal LD50 rat is greater than 5,000 mg/kg – practically non-toxic. (taken from
