Herbicides Kill Weeds

Weeds compete with crop plants for valuable nutrients. The traditional method for controlling weeds is to plow them under the soil, where in decomposing they release the nutrients they absorbed while they were alive. Plowing also aerates the soil, but it is either labor-intensive or energy-intensive and can lead to topsoil erosion. In the early 1900s, farmers noted that certain fertilizers, such as calcium cyanamide, CaNCN, selectively kill weeds while causing little harm to crops. This prompted a broad search for chemicals that act as herbicides. Today, a farmer can choose from hundreds of herbicides, many tailored for a specific weed. Farmers in the United States apply almost 600 million pounds of herbicides annually, which is about three times more than the amount of insecticides they apply. 

Glyphosate is a nonselective herbicide that affects a biochemical process common to all plants—the biosynthesis of the amino acids tyrosine and phenylalanine. Glyphosate has low toxicity in animals because most animals do not synthesize these amino acids, obtaining them from food instead. Glyphosate is the active ingredient of the herbicide Round-up. 

As decomposers, fungi play an important role in soil formarion, but they can also harm crops. Most of rhe harm they cause occurs during a plants early growth stages. Fungj can also spoil. stored food and arc particularly devastating to the world s fruit harvest. 

During the last 60 years, pesticides have benefited our society by preventing disease and increasing food production. Our need for pesticides will continue, but greater. specificity will certainly be demanded. Furthermore, it is becoming increasingly apparent that the benefits of using pesticides must be considered in the context of potential risks. 

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Triazines are selective herbicides used to control a wide spectrum of grass and broadleaf weeds in cereal, oilseed, and horticultural crops. Triazine herbicides kill weeds by interfering with the electron transport chain in photosystem II (PS II). These herbicides bind to the QB protein in the PS II reaction center and block the flow of electrons through the photosynthetic electron transport chain. 

Herbicides kill weeds Insecticides kill insects Fungicides kill fungi Bactericides kill bacteria Nematocides kill nematodes... 

The herbicide kills weeds in cotton crops rather than the cotton plant itself ... 

Herbicide Kill weeds and other plants that grow where they are not wanted... 

Many commercial herbicides kill weeds by interfering with the action of photosystem II or photosystem I. Inhibitors of photosystem II block electron flow, whereas inhibitors of photosystem I divert electrons from the terminal part of this photosystem. Photosystem II inhibitors include urea derivatives such as diuron and triazine derivatives such as atrazine. These chemicals bind to the Qg site of the D1 subunit of photosystem II and block the formation of plastoquinol (QH2). 

Herbicides kill weeds (unwanted plants), grasses, and other plants. 

Pesticides are chemicals that kill pests, and are categorized by the types of pests they kill. For example, insecticides kill insects, herbicides kill weeds, bactericides kill bacteria, fungicides kill fungi, and algicides kill algae. 

Plants can also be pests that need to be controlled, particulady noxious weeds infesting food crops. Prior to 1900, inorganic compounds such as sulfuric acid, copper nitrate, sodium nitrate, ammonium sulfate, and potassium salts were used to selectively control mustards and other broadleaved weeds in cereal grains. By the early 1900s, Kainite and calcium cyanamid were also used in monocotyledenous crops, as well as iron sulfate, copper sulfate, and sodium arsenate. Prom 1915 to 1925, acid arsenical sprays, carbon bisulfate, sodium chlorate, and others were introduced for weed control use. Total or nonselective herbicides kill all vegetation, whereas selective compounds control weeds without adversely affecting the growth of the crop (see Herbicides). 

There are three potential solutions for annual and perennial weed problems with drip irrigation. One is the use of preemergence herbicides that have residual weed control properties, particularly under continuous moisture. Another is the use of contact herbicides on the emerged weeds in the wet spot or persistent herbicides with contact activity. A third approach is the use of herbicides through the drip system when allowed by the product label (i.e., the use of herbicides capable of killing weeds as they germinate). With all three approaches it is essential that perennial weeds... 

Pesticide Any substance or agent employed to destroy a pest organism. Common pesticides include insecticides (to kill insects), herbicides (to kill weeds), fungicides to kill (fungi), and nematicides (to kill nematodes). 

Herbicides, or weed killers, may be classified as pesticide chemicals. They can kill plants on contact, or they can be translocated (i.e absorbed by one part of the plant and carried to other parts where they exert their primary toxic effect). Most commonly used herbicides have a low toxicity and have caused few adverse effects in users. Some herbicides pose more serious problems to the central nervous system (CNS) and can cause depression. The skin absorption of herbicides also may cause skin irritation, dermatitis, and photosensitization in addition to peripheral motor neuropathies. 

Glyphosate is toxic to plants and free-living microorganisms because it inhibits aromatic amino acid biosynthesis. On the other hand, it is extremely nontoxic to humans and animals because humans derive their amino acids from the diet. Additionally, it is broken down in the soil, so it is non-persistent. The only problem with glyphosate herbicides is that they will kill crop plants as readily as weeds. Recently, genetically engineered crop varieties have been introduced which are resistant to the herbicide, allowing weeds to be killed preferentially. 

Pesticides are chemicals or biological substances used to kill or control pests. They fall into three major classes insecticides, fungicides, and herbicides (or weed killers). There are also rodenticides (for control of vertebrate pests), nematicides (to kill eelworms, etc.), molluscicides (to kill slugs and snails), and acaricides (to kill mites). These chemicals are typically manmade synthetic organic compounds, but there are exceptions which occur naturally that are plant derivatives or naturally occurring inorganic minerals. 

Chemicals used to control weeds are called herbicides. They kill plants by contact or systemic action. Contact herbicides kill only the plant parts which the chemical touches. Systemic herbicides are absorbed by roots or foliage and carried throughout the plant. Systemic herbicides are particularly effective against perennial weeds because the chemical reaches aU parts of the plant—even deep roots and woody stems, which are relatively inaccessible. Contact herbicides are usually used to control annuals and biennials and are characterized by the quick die-back they cause. Systemics may take a longer time to provide the desired results—up to 2 or 3 weeks, or even longer for woody perennials. 

Herbicide activity is either selective or nonselective. Selective herbicides are used to kill weeds without significant damage to nearby plants. They are used to reduce weed competition in crops, lawns, and ornamental plantings. Nonselective herbicides are chemicals that kill all plants present if apphed at an adequate rate. They are used where no plant growth is wanted, such as fence rows, ditch banks, driveways, roadsides, parking lots, and recreation areas. 

Dichlobenil, bromoxynil, and ioxynil are contact herbicides used to control hard-to-kill weeds. 

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