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The Editors of Encyclopaedia Britannica Last Updated: Sep 1, 2024 • Article History Table of Contents
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herbicide, an agent, usually chemical, for killing or inhibiting the growth of unwanted plants, such as residential or agricultural weeds and invasive species. A great advantage of chemical herbicides over mechanical weed control is the ease of application, which often saves on the cost of labor. Most herbicides are considered nontoxic to animals and humans, but they can cause substantial mortality of nontarget plants and the insects that depend on them, especially when applied aerially.
Chemical weed control has been used for a very long time: sea salt, industrial by-products, and oils were first employed. Selective control of broad-leaved weeds in fields of cereal crops was discovered in France in the late 1800s, and this practice soon spread throughout Europe. Sulfates and nitrates of copper and iron were used, and sulfuric acid proved even more effective. Application was by spraying. Soon sodium arsenite became popular both as a spray and as a soil sterilant. On thousands of kilometers of railroad right-of-way, and in sugarcane and rubber plantations in the tropics, the hazardous material was used in tremendous quantities, often resulting in the poisoning of animals and occasionally humans.
More From Britannica poison: HerbicidesSinox, the first major organic chemical herbicide, was developed in France in 1896. In the late 1940s new herbicides were developed out of the research during World War II, and the era of the “miracle” weed killers began. Within 20 years more than 100 new chemicals were synthesized, developed, and put into use. Chemical weed control superseded both plant-disease and insect-pest control in economic impact. In particular, the year 1945 was key to the development of selective chemical weed control. Introduced then were 2,4-D (2,4-dichlorophenoxyacetic acid), 2,4,5-T (2,4,5-trichlorophenoxyacetic acid), and IPC (isopropyl-N-phenylcarbamate)—the first two selective as foliar sprays against broad-leaved weeds, the third selective against grass species when applied through the soil.
The new herbicides were revolutionary in that their high toxicity allowed for effective weed control at dosage rates as low as one to two kilograms per hectare (one or two pounds per acre). This contrasted with carbon bisulfide, borax, and arsenic trioxide, which were required at rates of up to 2,242 kilograms per hectare (one ton per acre), and with sodium chlorate, required at rates of about 112 kilograms per hectare (100 pounds per acre). However, some of those early herbicides, including 2,4,5-T, were later deemed unsafe for humans and the environment and were discontinued in many countries. Effective herbicides have continued to be developed, and some, such as glyphosate, are widely used around the world.
Since the mid-1980s, certain agricultural plants, known as herbicide-resistant crops (HRCs), have been genetically engineered for resistance to specific chemical herbicides, notably glyphosate. These genetically modified organisms (GMOs) enable effective chemical control of weeds, since only the HRC plants can survive in fields treated with the corresponding herbicide. Such crops have been especially valuable for no-till farming, which helps prevent soil erosion. However, because these crops encourage increased application of chemicals to the soil rather than decreased application, they remain controversial with regard to their environmental impact and general safety. In addition, in order to reduce the risk of selecting for herbicide-resistant weeds, farmers must use multiple diverse weed-management strategies.
In 2024 the U.S. Environmental Protection Agency (EPA) took emergency action to suspend the use of dimethyl tetrachloroterephthalate, also known as DCPA or Dacthal, after the herbicide was found to cause irreversible damage to fetuses exposed in utero, including low birth weight, impaired brain development, decreased IQ, and impaired motor skills later in life. A preemergent herbicide targeting annual grasses, DCPA was commonly applied to broccoli, Brussels sprouts, and cabbage, among other crops. The emergency order was the EPA’s first in almost 40 years.
