Herbicides and forestry
Herbicides are chemical compounds used to control unwanted plant growth, primarily targeting grasses, weeds, and other plant pests. In forestry and agriculture, herbicides play a crucial role in managing vegetation, clearing rights-of-way, and facilitating crop harvesting by either killing plants or inhibiting their growth. They can be classified into selective and nonselective types, with selective herbicides designed to target specific plants without harming crops, while nonselective herbicides affect all plant growth in treated areas. Historically, herbicides have been in use since ancient times, evolving significantly in the 20th century with the introduction of synthetic compounds like 2,4-D and 2,4,5-T, which have been utilized not only in agriculture but also in military operations. The use of herbicides is regulated to mitigate their environmental and health impacts, with oversight from agencies such as the U.S. Environmental Protection Agency (EPA), which ensures that products are tested for safety before being released for public use. The application of herbicides raises diverse opinions, with discussions focusing on their benefits in crop management versus potential negative effects on ecosystems and human health.
Herbicides and forestry
Herbicides are a class of pesticide used to kill or otherwise control unwanted vegetation. They are frequently employed in agriculture and forestry.
Background
Herbicides are used for the control of grasses, weeds, and other plant pests. These chemical compounds kill plants or inhibit their normal growth. In general, herbicides work by interfering with photosynthesis, so that a plant dies from lack of energy, or by a combination of defoliation (leaf removal) and systemic herbicidal action.
Herbicides are used to clear rights-of-way beneath power lines and along railways and roads. In agriculture and forest management, they are used to control weeds or to remove the leaves from some crop plants to facilitate harvesting. While herbicides may be employed in lieu of tillage, their use is more often in conjunction with tillage and other agronomic practices. During wartime, defoliants and other herbicides have been used to destroy plants that an enemy uses for cover during battle or for food.
Types of Herbicides
Herbicides may be selective or nonselective. Selective herbicides, such as amitrole, atrazine, monuron, pyridine, 2,4-dichlorophenoxyacetic acid (2,4-D), and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), target a particular plant pest and will kill or stunt weeds among crop plants without injuring the crop. For example, 2,4-D targets soft-stemmed plants, while 2,4,5-T is effective against woody plants. Cereals are crops particularly suited for treatment with 2,4-D, since the compound does not harm narrow-leafed plants but kills broad-leaved weeds. Selective toxicity minimizes the environmental impact of an herbicide. Nonselective herbicides (also called broad-spectrum or general-usage herbicides) are toxic to all plants. Examples include dinoseb, diquat, paraquat, and arsenic trioxide. Nonselective compounds are best suited for areas where all plant growth is to be suppressed, such as along railroad rights-of-way.
Some compounds, known as contact herbicides, kill only those plant parts to which they are directly applied. Others, called systemic herbicides, are absorbed through the plant’s foliage or roots and carried to other parts of the plant. When mixed with the soil, some herbicides kill germinating seeds and small seedlings.
Popular inorganic herbicides include ammonium sulfate, sodium chlorate, sulfuric acid solutions, and borate formulations. Among the organic herbicides are the organic arsenicals, substituted amides and ureas, nitrogen heterocyclic acids, and phenol derivatives. Phenoxyaliphatic acids and their derivatives, a major group of organic herbicides, are selective poisons that readily travel from one part of a plant to another.
History
Agricultural societies have used simple chemical herbicides such as ashes and common salts for centuries. In 1896, a fungicidal compound known as Bordeaux mixture (a combination of copper sulfate, lime, and water) was found also to be effective against some weeds. Subsequently, copper sulfate was employed as a selective weed killer in cereal crops. By the early 1900’s, sodium arsenate solutions and other selective inorganic herbicidal mixtures had been developed. In 1932, dinitrophenol compounds were introduced.
In the early 1940’s, a new generation of herbicidal compound emerged. In an attempt to mimic natural plant hormones, the defoliant 2,4-D was created. At low concentrations 2,4-D promotes retention of fruit and leaves; at higher concentrations, it overstimulates plant metabolism, causing the leaves to drop off. A related chemical, 2,4,5-T, came into general use in 1948. The years after World War II saw the first large-scale application of herbicides in agriculture and other areas. The new defoliants rapidly gained acceptance because of their effectiveness against broad-leaved weeds in corn, sorghum, small grains, and grass pastures.
A few years after their development, these defoliants were employed as chemical weapons. During its conflict with Communist guerrillas in Malaya during the late 1940’s and early 1950’s, Britain sprayed 2,4,5-T on crops and jungle foliage to deprive the guerrillas of food and cover. The United States conducted a similar antifood and antifoliage campaign in South Vietnam during the 1960’s. In this campaign, dubbed “Operation Ranch Hand,” massive quantities of herbicidal mixtures were sprayed from aircraft onto Vietcong food plantations, infiltration routes, staging areas, and bases. The quantity and frequency of the spraying greatly exceeded recommended levels; in addition, mechanical problems or military need often forced aircraft to dump their loads all at once, drenching the jungle below. Soldiers, civilians, and the environment were subjected to unusually high concentrations of defoliants. One of the herbicides used in this campaign was Agent Orange, a mixture that included 2,4-D and 2,4,5-T. Commercial preparations of 2,4,5-T contain varying amount of dioxin, a highly toxic contaminant. Agent Orange has been implicated in the increased incidence of still births and birth defects among the Vietnamese living in the areas sprayed, in the cancers and other illnesses suffered by American and Australian soldiers who were involved in the operation, and in birth defects among the children of these veterans. In 1970, the United States placed severe restrictions on domestic and agricultural use of 2,4,5-T, at about the same time the defoliation campaign was halted.
U.S. Regulation of Herbicides
In 1947, the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) authorized the United States Department of Agriculture (USDA) to oversee registration of herbicides and other pesticides and to determine their safety and effectiveness. In December, 1970, the newly formed United States Environmental Protection Agency (EPA) assumed statutory authority from the USDA over regulations. Under the Federal Environmental Pesticide Control Act of 1972, an amendment to FIFRA, manufacturers must register all marketed pesticides with the EPA before the product is released. Before registration, the chemicals must undergo exhaustive trials to assess their potential impact on the environment and human health. The EPA’s decision to grant registration is based on the determination that unreasonable adverse effects on human health or the environment are not anticipated within the constraints of approved usage. Beginning in October, 1977, the EPA classified all pesticides to which it has granted registration as either a restricted-usage (to be applied only by certified pest control operators) or unclassified (general-usage) pesticide.
In 2024, the EPA used its emergency authority to ban the use of dimethyl tetrachloroterephthalate, better known as Dacthal. Prior to this incident, the EPA had not exercised its emergency authority on pesticides in almost forty years. The EPA stated that use of the chemical caused an "imminent hazard," and was linked to fetal hormone disruption.
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