Herbicides

A herbicide is a pesticide used to kill unwanted plants. Selective herbicides kill certain targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often based on plant hormones. Herbicides used to clear waste ground are nonselective and kill every plant with which they come into contact. 

History of Herbicides

The first widely used herbicide was 2,4 dichlorophenoxyacetic acid, often abbreviated 2,4-D. It first saw widespread production and use in the late 1940s. It is easy and inexpensive to manufacture, and kills many broadleaf plants while leaving grasses unaffected. Its low cost has led to continued usage today. Like other acid herbicides, current formulations utilize either an amine salt (usually trimethyl amine) or one of many esters (ester) of the base compound. These are easier to handle than the acid. 

There are earlier examples of cultural controls, such as altering soil pH, salinity, or fertility levels to control weeds. 

2,4-D exhibits relatively poor selectivity, meaning that it causes stress to non-target plants. It is also less effective against some broadleaf weeds, including sedges and many vinous plants. Many other herbicides have been developed to address these limitations. 

The 1970s saw the introduction of atrazine, which has the dubious distinction of being the herbicide of greatest concern for groundwater contamination. 

Glyphosate, frequently sold under the brand name Roundup, was introduced in the late 1980s for non-selective weed control. It is now a major herbicide in selective weed control in growing crop plants due to the development of crop plants that are resistant to it. The pairing of the herbicide with the resistant seed led to the consolidation of the seed and chemistry industry in the late 1990s. 

Uses 

Herbicides are widely used in management of landscape turf and in agriculture. They are used in total vegetation control [tvc] programs for maintenance of way for highways and railroads. Relatively smaller quantities are used in forestry, pasture systems, and management of set-aside areas for wildlife habitat. 

Classification of herbicides 

Herbicides can be grouped by chemical family, mode of action, and type of vegetation controlled. 

They are also classified by their activity: 

Contact herbicides destroy only that plant tissue in contact with the chemical spray. Generally, these are the fastest acting herbicides. They are ineffective on perennial plants that are able to re-grow from roots or tubers. 

Systemic herbicides are foliar-applied and are translocated through the plant and destroy a greater amount of the plant tissue. 

Soil-borne herbicides are applied to the soil and are taken up by the roots of the target plant. 

Pre-emergent herbicides are applied to the soil and prevent germination or early growth of weed seeds. 

Their classification by mechanism of action (MOA) indicates the first enzime or protein or biochemical step affected in the plant, after application. The main MOA of herbicides are: 

ACCase inhibitors, are coumpounds that kill grasses. This enzyme is the first step in the lipid synthesis. Thus, the herbicides with this MOA affect the cell membrane production in the meristems of the grass plant. The ACCase of grasses are sensitive to the herbicide, whereas the ACCase of dicot plants are not. 

ALS inhibitors ALS enzime is the first step in the synthesis of the amino acids Val, Leu, and Isoleu. The herbicides with this MOA also affects the cell membrane at the meristem. They affetc grasses and dicots alike. 

EPSPs inhibitors This enzime is in the metabolic route to make the amino acids Trp, Phenylalanine and Tirosine. The effect os this group of compounds are noted on the whole plant. They affetc grasses and dicots alike. 

Sintetic auxin These were the herbicides that originated the era of organic herbicides in the world. They were discovered about 1940's after a long pursue of the plant growth regulator AUXIN. They have several point of action on the cell membrane, They are affective on the control of dicot plants. 

PHOTOSYSTEM II inhibitors this group of herbicides are responsible for reducing electron flow from water to NADPH2+ at the photochemical step in photosynthesis. They bind to the Qb site on the D2 protein, and prevent that quinone binds to this site. Therefore, this group of compounds make an accumulation of ellectron on the chlorophyll. As consequence, there is oxidative reactions in excess of the normally tolerated by the cell, and the plant dies. 

Application 

Most herbicides are applied as water-based sprays using ground equipment. Ground equipment varies in design, but the greatest number of acres is sprayed with self-propelled sprayers equipped with a long boom (often 60-80 feet) with flat fan nozzles spaced about every 20". Towed, handheld, and even horse-drawn sprayers are also used. 

Herbicides can also be applied aerially using helicopters or airplanes, and can be applied through irrigation systems (chemigation). 

Terminology 

Control is the destruction of unwanted weeds, or the damage of them to the point where they are no longer competitive with the crop 
Suppression is incomplete control still providing some economic benefit, such as reduced competition with the crop 
Crop Safety, for selective herbicides, is the relative absence of damage or stress to the crop. Most selective herbicides cause some visible stress to crop plants. 

Some major herbicides in use today 

Glyphosate, a systemic nonselective herbicide used in no-till burndown and for weed control in crops that are genetically modified to resist its effects. It is an example of EPSPs inhibitor. 

Paraquat, a nonselective contact herbicide used for no-till burndown and in aerial destruction of marijuana and coca plantings. More acutely toxic to people than any other herbicide in widespread commercial use. 

2,4-D, a broadleaf herbicide in the phenoxy group used in turf and in no-till field crop production. Now mainly used in a blend with other herbicides that act as synergists. It is an example of synthetic auxin. 

clopyralid, is a broadleaf herbicide in the pyridine group, used mainly in turf, rangeland, and for control of noxious thistles. Notorious for its ability to persist in compost. It is another example of synthetic auxin. 

metoalachlor, a pre-emergent herbicide widely used for control of annual grasses in corn and sorghum; has largely replaced atrazine for these uses 

dicamba, a persistent broadleaf herbicide active in the soil, used in turf and field corn. It is another example of synthetic auxin. 

picloram, a pyridine herbicide mainly used to control unwanted trees in pastures and edges of fields. It is another example of synthetic auxin. 

atrazine, a triazine herbicide used in corn and sorghum for control of broadleaf weeds and grasses. Still used because of its low cost and because it works as a synergist when used with other herbicides. it is an example of a photosystem II inhibitor. 

Other herbicides of historical interest 

2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) was a widely used broadleaf herbicide until being phased out starting in the late 1970s. While 2,4,5-T itself is of only moderate toxicity, the manufacturing process for 2,4,5T contaminates this chemical with trace amounts of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). TCDD is extremely toxic to people. With proper temperature control during production of 2,4,5-T, TCDD levels can be held to about .005 ppm. Before the TCDD risk was well understood, early production facilities lacked proper temperature controls. Individual batches tested later were found to have as much as 60 ppm of TCDD. 

2,4,5-T was withdrawn from use in the USA 1983, at a time of heightened public sensitivity about chemical hazards in the environment. Public concern about dioxins was high, and production and use of other (non-herbicide) chemicals potentially containing TCDD contamination was also withdrawn. These included pentachlorophenol (a wood preservative) and PCBs (mainly used as a stabilizing agent in transformer oil). 

Some feel that the 2,4,5-T withdrawal was not based on sound science. 2,4,5-T has largely been replaced by dicamba and triclopyr. 

Agent Orange was a herbicide blend used by the U.S. military in Viet Nam between January 1965 and April 1970 as a defoliant. It was a mixture of 2,4,5-T, 2,4-D, and picloram. Because of TCDD contamination in the 2,4,5-T component, it caused serious illnesses in many veterans who were exposed to it. Agent Orange often had much higher levels of TCDD than 2,4 5 T used in the US. The name Agent Orange is derived from the orange color-coded stripe used by the Army on barrels containing the product. It is worth noting that there were other blends of synthetic auxins, at the time of the Viet Nan war, which containers were known by the blue, white, and other colors.