Chemistry, Use and Risks of Glyphosate Herbicide

What is glyphosate? some readers may ask. It is a broad-spectrum systemic herbicide, which in layman's terms means it is a poison designed to kill a large variety of weeds. It is not new, but for years was the primary ingredient of the herbicide Roundup®. Glyphosate is chemically relatively simple. Is it well-known? In 2001, the EPA listed it as the most used herbicide in America.¹

Chemistry

Glyphosate is a chemical derivative of glycine, the smallest amino acid found in proteins. The formula of glycine is

HOOC-CH₂-NH₂

Replacing one glycine's amino hydrogen atoms with a phosphonomethyl group, the formula for glyphosate is

HOOC-CH₂-NH-CH₂-P(O)(OH)₂

One really nice feature of this simple molecule is its water-solubility. It does not require an oil base. It readily forms a solution greater than 1 percent by weight, which may be increased through the addition of surfactants and other substances. Generally, glyphosate is provided in salt form, e.g. the ammonium salt or the isopropyl amine salt.

Manner of Action

Glyphosate interferes with the enzymatic production of certain essential amino acids, hence the plant must be actively growing. The chemical is usually absorbed through the leaves. Although glyphosate works very well, there are a number of requirements that must be met in order to achieve useful results.

Difficulties in Use

The time of day and the weather greatly affect the results obtained. A rain-free period after application is most important. Sunny, hot days work best. Dust can be a negative factor, since glyphosate is readily absorbed by dust particles. Water high in calcium and magnesium ions lessens potency, requiring an increase in glyphosate concentration. Heavy dew may or may not be a significant factor. Some plants absorb the herbicide better than others, with some plants such as certain varieties of clover, being little affected.

Negative Effects Partly from Overuse

Another growing difficulty is that of glyphosate-resistant weeds. If glyphosate is overused as a cure-all, without a complete weed management program in effect, resistant weeds can result. This has led in some areas to the combining of weed killers employing different modes of action. Unfortunately, multiply-resistant weeds pose a real threat. Demonstrating this, the article "Facts About Glyphosate - Resistant Weeds"² indicates the number of glyphosate-resistant weed species tripled in just the two years between 2003 and 2005! This is despite the fact that the gene responsible for resistance is relatively rare.

¹ EPA - About Pesticides - Table 3.6

² "Facts About Glyphosate - Resistant Weeds," by Chris Boerboom, University of Wisconsin, and Michael Owen, Iowa State University

Additional References:

"Understanding Glyphosate to Increase Performance," by Bob Hartzler, Iowa State U., Chris Boerboom, U. Wisconsin, Glenn Nice, Purdue U., and Peter Sikkema, U. Guelph