How Herbicides Work

All pesticides work by disrupting some natural mechanism within the biology of the targeted plant, insect or animal species. Most of these natural or man-made chemicals kill their targets. Some protect crops or livestock by repelling pests. The ultimate goal for pesticide researchers is to find chemicals that kill or repel the target pests without affecting other organisms in the environment or humans.

Herbicides – chemicals that kill weeds – are the most widely used pesticides in farming. Each year, they account for about 70 percent of all agricultural pesticide use in the U.S.

In the 60s and 70s, many of today’s common herbicides were introduced. Scientists could document that the chemical worked, but they often didn’t understand how the pesticide worked.

Modern herbicides can be grouped by first by the way they act, then by the way they’re used, and then by how they kill the weeds they’re meant to kill.

First, herbicides are active in the weed either through contact or in a systemic way.

Contact herbicides destroy only the plant tissue that is in contact with the chemical. Generally, these are the fastest acting herbicides. They are less effective on perennial plants because they can grow new tops from their roots, tubers or rhizomes.

Systemic herbicides can move through the target plant. So, if the herbicide is applied to the tips, it can then move to the roots, and vise versa. These herbicides can control perennial plants and are ultimately more effective than contact herbicides, but they are slower acting.

Second, herbicides can be categorized by their use.

Spray applied herbicides are usually contact chemicals and they are mixed with liquid and sprayed on the field.

Soil applied herbicides are injected into the ground where the roots can take up the chemical. There are three types of soil applied herbicides. First, “pre-plant incorporated herbicides” (as the name suggests) are applied to the ground before the crops are planted. Second, “pre-emergent herbicides” are applied to the soil before the crop germinates and emerges from the ground; the goal here is to kill weeds also before they emerge. Third, “post-emergent herbicides” are applied after the crop has emerged.

Third, herbicides can be classified by their mechanism of action. In other words, herbicides work on different enzymes, proteins or biochemical steps.

• The synthetic auxin class herbicides were some of the first chemical herbicides in the 40s and 50s. The widely-used 2, 4-D is a synthetic auxin. These chemicals work on broadleaf or dicot plants by mimicking plant hormones. They make the plant grow uncontrollably, breaking down critical structures like the cell walls. Other growth inhibitors include Banvel, Tordon and Paramount.
• Photosystem II inhibitors reduce the flow of electrons from one chemical to another during the process of converting light energy into food through photosynthesis. Atrazine and other trazine herbicides, as well as urea derivatives (like diuron) are of this type. They also work against broadleaf or dicot plants. These chemicals don’t break down in the environment readily, and so have been linked to problems of groundwater contamination.
• EPSPS inhibitors kill all kinds of plants (grasses and broadleaves) by disrupting the plant’s ability to synthesize critical amino acids like tryptophan. Roundup or glyphosate is perhaps the best-known EPSPS inhibitor on the market. Liberty herbicide inhibits the glutamine synthesis pathway. These chemicals break down when they reach the soil, and their use has exploded after seed companies introduced GMO (genetically modified organism) versions of crops that can resist the herbicides. Roundup Ready versions of corn, soybeans, cotton and other crops have come to dominate the market.
• ALS inhibitors also attack a plant’s ability to synthesize important amino acids, and they affect both grasses and broadleaves. Common trade names include Arsenal, Pursuit, Ally, Beacon, Peak, Everest and Python. Since the plants can’t produce critical amino acids, they slowly starve to death and the chemicals can inhibit the production of DNA. However, the ALS chemical reaction pathway exists only in plants, so these chemicals are thought to be among the safest to humans and animals.
• ACCase inhibitors kill only grasses. ACCase is an enzyme that is needed in the first steps of the lipid formation process in grasses. Lipids are used in the formation of cell membranes, so the chemicals break down cellular structures. Trade names include Discovery, Hoelon, Acclaim, Fusilade and Select.

There are other classes of herbicides that inhibit the growth of weed seedlings and disrupt cell membranes in other ways.

All herbicides affect the environment and humans in some form, although there is a lot of debate over how significant and damaging the effects of individual compounds are. The problems with herbicides can range from skin rashes to death. For instance, phenoxy herbicides are often contaminated with dioxins and research has suggested that exposure to dioxin can cause a rise in cancer risk. Triazine exposure has been implicated in an increased risk of breast cancer, but there’s debate over a direct causal relationship. Other studies suggest that both herbicides and insecticides could result in Parkinson’s disease. And the herbicide Paraquat – often used to kill marijuana and coca plants – has also been linked to Parkinson’s.

Written by Bill Ganzel, the Ganzel Group. First published in 2009. A partial bibliography of sources is here.