Herbicide
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What is a herbicide?
A herbicide is a chemical agent designed to kill or inhibit the growth of unwanted plants, like weeds and invasive species. Chemical herbicides offer ease of application compared to mechanical methods, which can save labor costs.
Herbicides are categorized as either selective, affecting specific plants, or nonselective, affecting plants generally. They are also classified based on application: foliage-applied or soil herbicides. Contact herbicides kill only the parts they touch, while translocated herbicides target roots and other organs by moving from treated surfaces. In terms of timing, they can be preplant, preemergence, or postemergence.
However, most herbicides can cause mortality of nontarget plants and the insects that depend on them, especially when applied aerially.
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What are herbicides used for?
Herbicides are agents, typically chemicals, designed to kill or inhibit the growth of unwanted plants like weeds and invasive species. They offer an advantage over mechanical methods due to their ease of application, which can reduce labor costs.
Modern herbicides fall into two main categories: selective, which target specific plant species, and nonselective, which affect plants in general. They are also classified based on application method, such as foliage-applied and soil herbicides. With respect to planting time, herbicides are classified as preplant, preemergence, or postemergence weed killers.
Weed control is crucial in agriculture because weeds can lower yields, raise production costs, disrupt harvesting, and diminish product quality.
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How do herbicides kill or control weeds?
Herbicides kill or control weeds through various mechanisms depending on the herbicide type. Contact herbicides like sulfuric acid kill only the plant parts they touch, while translocated herbicides like 2,4-D move throughout the plant to kill roots and other organs.
Glyphosate inhibits an enzyme called EPSP synthase, disrupting the synthesis of aromatic amino acids needed for plant growth.
Herbicides are categorized by application time: preplant, preemergence, or postemergence. The ease of chemical application, especially via aerial spraying, offers a significant advantage over mechanical methods.
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What are the main types of herbicides?
Herbicides fall into two main categories: selective and nonselective. Selective herbicides target specific plant species, while nonselective herbicides affect plants generally. Both categories are further divided into foliage-applied and soil herbicides. Contact herbicides like sulfuric acid and paraquat kill only the plant organs they touch. Translocated herbicides such as amitrole and 2,4-D are transported to and effective against roots or other organs from treated surfaces. Additionally, herbicides are classified by planting time as preplant, preemergence, or postemergence weed killers.
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How do farmers safely apply herbicides?
To safely apply herbicides, farmers must integrate their use into the overall farm program, considering factors such as crop stage, weed stage, and weather conditions. When used as directed, herbicides are generally safe for the operator, wildlife, and livestock.
Spraying is the most common application method because it allows extremely small amounts to be applied uniformly. However, the greatest risk lies in accidental injury to crops from drift and residues in the soil, especially if residues enter water courses.
Because chemical pesticides can harm people, wildlife, and the environment, governments have implemented strict controls over their sale and use. Practically all aspects of chemical usage are now regulated or restricted by government.
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What are some possible effects of herbicides on the environment?
Herbicides can have several negative effects on the environment. They can contaminate soil and water, harming agriculture and aquatic ecosystems. Some herbicides persist in the environment, posing long-term threats to invertebrate animals and potentially affecting the food web.
The use of herbicides can lead to the contamination of plants, which can be deadly or harmful to insects, including pollinators. Some herbicides have been correlated with declines in bird populations. While natural processes in the soil can detoxify chemicals, the complexity of herbicide compounds can lead to long residence times in the soil.
Regulations and restrictions on herbicide use are increasing due to environmental and health concerns.
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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.
History
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.
Sinox, 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.
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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.
Application
Modern weed killers are put in two categories: selective (affecting specific plant species) and nonselective (affecting plants generally). These, in turn, are classified as foliage-applied and soil herbicides. Contact herbicides (e.g., sulfuric acid, diquat, paraquat) kill only the plant organs with which they are in contact. Translocated herbicides (e.g., amitrole, picloram, and 2,4-D) are effective against roots or other organs to which they are transported from aboveground treated surfaces (i.e., soil). With respect to planting time, herbicides are also classified as preplant, preemergence, or postemergence weed killers. Preplant herbicides may be applied to the soil or to weeds before crop planting.
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Many millions of hectares are treated from the air each year, particularly cereal croplands, pastures, rangelands, forests, and other situations where an airplane can be used. Modern equipment for treating row cropland with herbicides has made weed control increasingly convenient. Sprayers, soil incorporation equipment, and spreaders for pelleted herbicides have all added to the convenience of, and removed uncertainty from, herbicide application. Machinery is available that simultaneously builds up beds, plants the seed, sprays with insecticide, and incorporates fertilizer and preemergence herbicide all in one operation.
Certain herbicides (e.g., sodium arsenite) are sometimes applied by the jar method, whereby the tops of weeds are bent over and immersed in jars of poisonous solution. The herbicide is drawn into the rest of the plant and into connecting plants, gradually killing the entire system. Wild morning glory, poison oak, and camel thorn are sometimes treated in this manner. Chlorinated benzene and certain aromatic solvents are used to control aquatic weeds by adding them directly to the watery medium. See also defoliant.