Glyphosate, best known by the commercial brand name Roundup, is a systemic herbicide that affects virtually all herbaceous plants and many woody plants. Applied to foliage or to recently cut stumps, glyphosate works by progressing through the plant to its actively growing areas and inhibiting protein synthesis. Similar in chemical structure to an amino acid, glyphosate prevents plants from creating three amino acids required for growth.
Glyphosate was introduced in 1974 by Monsanto Company, and is manufactured today by the company for use in over a half-dozen herbicide products. A broad-spectrum herbicide, glyphosate does not target particular types of plants but works equally on any type of plant it is applied to. The formulation of the herbicide includes a surfactant, which helps the herbicide better adhere to plant foliage.
Glyphosate and Herbaceous Plants
On herbaceous plants---that is, perennials and annuals with relatively soft, green stems---glyphosate is most effective during the warm months as this is the time when plants are actively growing. Applied to the leaves of the plant, the herbicide moves through plant tissues to the active growing sites, known as apical meristems. Within several days, the plant's green tissues yellow and die, and starved of energy, the roots also begin to deteriorate.
Glyphosate and Woody Plants
Glyphosate is most effective on woody plants like trees and shrubs when applied to recently cut stumps in the fall, just as the plant is entering winter dormancy. The herbicide is transported into the specimen's active growing sites when the plant attempts to break dormancy in spring, and a plant will display effects of the herbicide by growing stunted, yellowed and deformed foliage, if any appears at all.
Effects on Soil
According to Monsanto and other chemical analyses of the product, glyphosate binds tightly to most types of soil, where it is broken down primarily by microbes. Although the company claims that the chemical usually disappears from soils within 2 weeks, the U.S. Environmental Protection Agency (EPA) states that average half-life in soils is 60 days. Because it binds to soil particles, glyphosate in the soil is not available for uptake by plant roots; leaching from the soil is less likely because of this as well.
Effects in Water
Although glyphosate is approved for use on water-borne plants, Roundup-brand formulations are less effective than a newer product Monsanto developed specifically for use on aquatic vegetation, which does not contain surfactants. Glyphosate itself is water-soluble, and disappears rapidly from this environment. Tests on glyphosate used under aquatic conditions indicate low toxicity to fish and other aquatic organisms.
Toxicity to Other Organisms
According to the EPA, the main risks of excessive glyphosate intake in humans are the possibility of congested respiration, increased breathing rate, kidney damage and reproductive system damage. Risk of cancer is low, according to laboratory tests done on rats and dogs, and the chemical does not pass through milk or eggs when fed to cows and chickens. Fish and mammals are largely unaffected by glyphosate, although the chemical has been shown to present some toxicity risk to wild birds.