Herbicides

Herbicides are a subclass of pesticides that are designed to kill unwanted plants or inhibit their growth. Herbicides contain a wide variety of chemicals, including both inorganic and organic compounds, with different abilities to interrupt specific physiological processes of plants for their survival, growth, and development. The use of herbicides provides a cost-effective method to control the growth of unwanted plants in order [Page 1421]to decrease labor costs and increase crop production. Herbicides are extensively applied in agriculture, forestry, and urban areas for unwanted plant management. In addition, herbicides are often used to control nuisance aquatic plant growths or algae blooms in water bodies.

Initially, most chemical herbicides were inorganic compounds that were usually very toxic to both desirable crops and unwanted plants as well as to mammals. The first synthetic organic herbicide was developed in France in 1932 for use in cereals. The rapid development of synthetic organic herbicides was stimulated by the need for increasing food production and by the Allies’ search for potential chemical warfare agents during World War II. Since then, the development and use of herbicides have increased steadily. Indeed, herbicides are the most commonly used pesticides in the world today. Most of the heavy herbicide use takes place in North America, Europe, Japan, and Australia.

There are various herbicide classification schemes based on different herbicidal properties. According to their activity, herbicides can be simply classified as (1) selective herbicides, which are used to kill unwanted plants without harming others, or (2) nonselective herbicides, which kill all plants. According to their movement within the plant, herbicides can also be classified as (1) contact herbicides, which destroy only the plant tissue in contact with the herbicide, or (2) systemic herbicides, which are absorbed either by the roots or the foliar parts of a plant and are then carried throughout the plant to injure it. According to application timing, herbicides are classified as (1) preplanting herbicides, which are applied before a crop is planted; (2) preemergent herbicides, which are applied before the emergence of unwanted plants; or (3) postemergent herbicides, which are applied after crops or unwanted plants have emerged. Herbicides with similar chemical structure often disrupt the same biochemical process in plants. Therefore, herbicides are commonly grouped by their chemical structure and mechanism of action. Understanding a herbicide's mechanism of action is helpful in choosing the appropriate type of herbicide for effective plant management.

Herbicide use plays an important role in modern agriculture by increasing crop production at low labor cost and decreasing soil erosion through reduced tillage. Although most herbicides are specific plant poisons and have a very low toxicity in animals, some herbicides can be highly toxic to humans and other animals. Herbicides can potentially affect ecosystems by having direct toxicity to nontarget organisms, altering vegetation compositions and habitat structure, or reducing food sources. In addition, some water-soluble herbicides may contaminate surface and groundwater resources, with adverse effects on aquatic fauna. Improper herbicide use can cause negative impacts on the environment and pose a health threat to herbicide applicators and others exposed to herbicides via drinking water or food. Despite their contribution to food production, herbicides must be used appropriately to reduce their risks.

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