Herbicides ( Latin herba 'herb', 'grass' and Latin caedere 'kill') or weed killers are substances that are supposed to kill off disturbing plants. They are mainly used in agriculture , but also on non-cultivated land.
Cultivated plants compete with weeds for water, nutrients and light. Dense weed growth can make harvesting very difficult and significantly reduce it. Weeds can be decimated manually, with machines or with herbicides.
A distinction is made between selective herbicides, which act against certain plants, and broad-spectrum or total herbicides , which act against a large number of plants. During the Vietnam War , in the course of Operation Ranch Hand, herbicides (particularly Agent Orange ) were also used as defoliants for military purposes .
The need for plant protection began for humans with the systematic cultivation of crops. While the control of insects and fungal diseases has only recently become possible with the help of chemical agents, purely mechanical removal of weeds has been common for thousands of years. The United States, most advanced in large-scale agricultural production, was particularly receptive to the introduction of selective herbicides. The strong competition in agricultural production with the need to increase yields, save on planting material and seeds consequently promoted the control of weeds in order to avoid crop failures. The rise in the cost of human labor, and in part also the shortage of labor, were powerful promoters for the introduction of predominantly selective herbicides with the elimination of manual or purely mechanical weed removal. In the course of the rationalization of the cultivation of large monocultures, many work processes had to be carried out by machine. Potatoes, cotton or even grain can only be harvested economically with harvesting machines. A machine harvest, e.g. B. cotton or potatoes, however, requires the pretreatment of the crop. Potato plants are pretreated with haulm killers and desiccants, only then can machines harvest relatively easily. The killing and drying of potato greens as a special form of herbicides also suppresses fungal infection from rotting tissue. Large monocultures also encourage the development of certain weeds. So the control of weeds was z. B. in cereal crops more and more urgent. Since harvesting of the grain with combine harvesters can only be done relatively late, this not only favors the spread of precocious weeds, but also weeds in general, since chaff and weed seeds remain on the field and are not transported away with the harvested material as was previously the case. This inevitably leads to a further increase in the use of selective herbicides in grain cultivation. Although the problem of weeds that has become resistant has not yet arisen seriously, there is a proliferation of weeds that are difficult to control and perhaps so far less important. This forces research to search for new herbicides with a different spectrum of activity and to use herbicide combinations with a broader spectrum of activity. Demands for extremely low pesticide residues make research and the introduction of new insecticides and, in some cases, fungicides difficult. This does not occur to the same extent with herbicides. On the one hand, almost without exception, the practically herbicidal compounds (with the exception of sodium arsenite) are very little toxic for warm-blooded animals; on the other hand, the herbicides are mainly used before or shortly after sowing and in the first stage of development of a crop, but hardly later. Nevertheless, residue regulations must also be submitted for herbicides and maximum quantities have been specified in the harvested crop since 1968 (in the Federal Republic of Germany: maximum quantity regulation for plant protection).
In 1942, 2,4-dichlorophenoxyacetic acid (2,4-D) was developed as the first highly effective herbicide.
Thiocarbamates and phenylureas followed between 1945 and 1960 . Between 1960 and 1980 triazines (e.g. atrazine ), diphenylether herbicides , nitriles (e.g. bromoxynil ), carbamates and chloroacetamides (e.g. alachlor ) were used as herbicides. Sulphonylureas , amino acid derivatives (e.g. glyphosate , glufosinate ) and imidazolines followed between 1980 and 1990 . Newer herbicides such as metazachlor can also be applied to the crop that has already emerged (e.g. winter oilseed rape), which has been bred to be resistant to this treatment.
Application of herbicides
It is applied to the field in aqueous suspensions. Manufacturers offer suspensions of active ingredients in emulsifiers and powders that are diluted with water. If the herbicides are used before or during sowing, they are called pre-emergence herbicides . Post-emergence herbicides are used after the first cotyledons have formed.
Systemic herbicides distribute themselves in the plant after ingestion. If an active ingredient is only mobile in the xylem , it can be shifted upwards but not downwards.
|log K ow|
|mobility||-3 to 0 ( hydrophilic )||0 to 3||3 to> 6 ( lipophilic )|
|mobile in the xylem||Triazines , phenylureas||Diflufenican , diphenylether herbicides|
|mobile in xylem and phloem||Glyphosate , glufosinate , amitrole||Growth substances, sulfonylureas , imidazolinones , DIMs|
Important groups of herbicides
In a plant cell , the chloroplasts are responsible for the conversion of carbon dioxide and water into carbohydrates with the release of oxygen ( photosynthesis ). The majority of herbicides intervene in this metabolism.
Broad spectrum herbicides
Also known colloquially as total herbicides, with a few exceptions have an unselective effect against all plants.
This group of active substances interfere with the conversion of light into chemical energy in photosystem I . They include paraquat and diquat , which are applied as contact herbicides (active ingredient absorption through the leaves).
Inhibitor of amino acid synthesis in plants
Glyphosate (trade name including Roundup ) is an example of a broad spectrum herbicide against weeds (e.g. couch grass ). Glyphosate is one of the most widely used herbicides in the world, the share of sales in 2001 was around US $ 3 billion. This herbicide ingests a plant through green parts of the plant (i.e. not through the roots ).
Glyphosate prevents the biosynthesis of the aromatic amino acids L - phenylalanine , L - tyrosine and L - tryptophan by inhibiting the shikimic acid pathway . Since the shikimic acid pathway does not occur in animal metabolism, the herbicide is only effective against plants. Glyphosate is u. Because of its non-specific effect. a. used as a pre-emergence herbicide .
Other well-known broad spectrum herbicides are the sulfonylureas ( amidosulfuron and sulfometuron-methyl (trade name: Oust)) and imidazolines (imidazolinone) such as imazapyr (trade name: Arsenal). These substance classes act on the biosynthesis of branched amino acids such as L - valine , L - leucine and L - isoleucine . These herbicides are absorbed through the roots and leaves, and are often used on soybean and grain arable land .
In earlier decades in particular, the growth substances, chemically chlorophenoxyacetic acids such as 2,4-D , 2,4,5-T or MCPA , were an important class of herbicides. These stimulate rapid growth, but the broad-leaved weeds then die due to a lack of food. The narrow-leaved grain has a higher tolerance threshold and is not damaged.
Selective herbicides, post-emergence herbicides
An important group of post-emergence herbicides are the 1,3-cyclohexanediones. Important representatives are Cycloxydim and Sethoxydim . These herbicides inhibit the fatty acid synthesis in plants and are often used against grasses.
Other groups are the thymine / uracil herbicides (eg. Bromacil , use in weeds in soybean crops), the benzothiadiazoles (eg. Bentazone , trade name: Basagran) which Phenylpyridazine (eg. Pyridate ) and phenoxypropionic acids ( e.g. 2,4-DB or Fenoxaprop - very good effect against grasses).
To increase the selectivity, safeners are used together with the appropriate herbicides. They reduce the harmful effects (also known as herbicide stress) on crops. The phytotoxic effect against weeds, however, remains unaffected. A distinction is made between seed, soil and leaf safeners. It is a chemical alternative to genetic modification of seeds to increase yields per hectare in herbicide applications. The effect is based on the induction of enzyme reactions within the plant, which enables rapid detoxification. An important seed safener is, for example, fluxofenim , an oxime ether. In Europe, an assessment and approval of the substances in a positive list is legally stipulated by Regulation (EC) No. 1107/2009 (Plant Protection Products Regulation) .
Trends in herbicide development
In the 1950s and 1960s, a simple synthesis of various organic molecules was used to test whether a substance was harmful to a weed. Since the 1980s, the biochemical metabolism of plants has been increasingly clarified. Important enzymes of plant biosynthesis are isolated and the inhibition of the enzymes is investigated with various synthetic substances (enzyme assays). Many modern methods, e.g. Gas chromatography, for example, allows important biochemical metabolic products to be identified quickly.
Another tendency is to reduce the amount of active ingredient. The researchers also want to keep the amount of active ingredient herbicides per hectare low while minimizing toxicity for humans and animals.
While in 1950 about 12 kg of sodium chlorate or 7 kg of atrazine had to be used on one hectare of arable land for weed control, in 1970 it was only 1 kg to 2 kg of bentazone for 1 hectare . From 1980 onwards, only 20 g of chlorosulfuron were sufficient for one hectare of arable land. Recent developments investigate an inhibition of the shikimate pathway by carbohydrates such as 7-deoxy-sedoheptulose .
Spectrum of activity of herbicides
|abbreviation||Effect against weeds / grass weeds|
|Cellulose synthesis inhibitors|
|Photosystem II inhibitor (PS inhibitor)|
|weed||Tepraloxydim||Glufosinate||Flufenacet||Napropamide||Isoxaben VA||Fluazifop-P-butyl NA||Metamitron VA + NA||Propyzamide||Phenmedipham||Clopyralid||glyphosate||Clethodim NA||Chloridazon||Pendimethalin|
|Blade of wind||+||++++||+++||+++||-||++||++||++||-||-||++++||+++||+++||++|
Substance groups of herbicides
Important herbicides Substance group A connection of this group % of world sales
(S. Group, 2000)
Amino acid derivatives
Stunted growth of a dicotyledon from herbicide application
With a turnover share of 39% (2007), herbicides are the most economically important pesticides . The total turnover for herbicides in the world market was US $ 15.5 billion.
The largest economic sectors for herbicides are in the protection of soy (approx. 10% of world sales of pesticides, abbreviation: WUAPSM, approximate dates 1985), grain (10% of WUAPSM), wheat (6% of WUAPSM), fruit and vegetables (3.5% of the WUAPSM), rice (3.7% of the WUAPSM), cotton (2.2% of the WUAPSM), sugar beet (2.2% of the WUAPSM).
Around 90% of the soy fields, 71% of the grain fields, 63% of the wheat fields, 69% of the sugar beet fields, 35% of the rice fields, 17% of the fruit and vegetable plantations are treated with herbicides worldwide.
In 2017, 50,092 tons of herbicides were released in Germany.
Through the multiple use of a single group of active ingredients over several years, resistant weeds can be selected, especially in monocultures . This phenomenon was observed with almost all drug groups. Plants with a high reproductive rate become particularly resistant. A current example is the field foxtail in Germany. The HRAC is working intensively on how to curb the development of resistance. However, the development of resistance is less in weeds than in insects or fungi. However, resistances can also be cultivated in a targeted manner, so that herbicides tailored to the crops can also be used after the crops have emerged (post-emergence). However, the problem then is to eliminate the useful crop (e.g. winter rape ) from the stand when the crop rotation is changed . In the meantime it can be assumed that the development of resistance will sooner rather than long lead back to combined weed control methods.
Sometimes herbicide-resistant weeds are also called superweeds , then mainly in the mass media and political discussions.
The effect of synthetic pesticides such as herbicides on humans and the environment is controversial. By eliminating mechanical weeding of the soil, e.g. B. in beet cultivation, humus should accumulate in the soil, which, together with the elimination of weed competition, enables reduced nitrogen fertilization. In addition, birds are less disturbed, especially during the breeding phase, when the fields are no longer tilled. On the other hand, herbicides, if blown away, can damage neighboring crops and decimate the number of wild plants in the area. Herbicides that get into the soil can change the species composition of soil organisms without reducing the total number of soil organisms. Large-scale weed control measures have an indirect effect on fauna. By combating certain weeds, certain types of insects (e.g. parasitic wasps) can be massively reduced locally, which in turn affects the animals in the food chain (e.g. partridges). The effects on humans range from the argument that the broad use of herbicides is affordable for the general population and cancer rates are therefore declining and insignificant in comparison to epidemic cancers caused by tobacco smoke and obesity and chronic food infections (mainly in developing countries) to critical voices such as that of Rachel Carson , who with her book The Silent Spring drew attention to the effects of insecticide and herbicide use on large areas in the United States. The fear of chemical food residues tends to dominate the European population . The majority of respondents rate the risk as higher than the health effects of bacterial contamination of food or an unbalanced diet. Pesticides and herbicides have been well studied toxicologically, but due to the diverse chemical structures of these substances, it cannot be ruled out that some of these compounds will prove to be more dangerous in future studies than currently assumed.
Effects on food chains and biodiversity
Herbicides not only cause the shoots of certain plants to die, but also their roots . If herbicides are used over a large area, the range of plant species in the agricultural landscape can be greatly reduced. Since more or less many insect species are dependent on each plant species and other animals (especially birds , migratory birds ) are dependent on them via the food chains , there is a risk of general species impoverishment in the field landscape. The massive decline in species (loss of biodiversity ) in Europe's agricultural landscapes is primarily a consequence of these interrelationships. However, the share of herbicide and insecticide use in agriculture in the causes of the decline in species cannot be clearly determined.
There are current indications that the widespread use of broad spectrum herbicides in some regions of Germany (e.g. in Saxony) has continued to spread in recent years. The increased use occurs particularly in the course of the mulch sowing process . Mechanical weed control is dispensed with (so-called no-plow tillage ) and instead broad-spectrum herbicides (e.g. glyphosate / Roundup) are used intensively . The abandonment of plowing the soil is a departure from fallow land , which was an important habitat for the traditional inhabitants of the cultural landscape such as skylark and partridge . Something similar occurs with the conversion of flowering and species-rich hay meadows into high-yield, but species-poor intensive grassland (killing of the previous plant population - new sowing of fast-growing, protein-rich grasses without any changes). Although these are soil-conserving methods, which are useful on slopes at risk of erosion , the consequences for biodiversity are not yet foreseeable if they are used almost everywhere in agriculture.
A position paper of the Federal Office for Consumer Protection and Food Safety from October 2019 comes to the conclusion: “The fundamental conflicting goals between high and safe harvest yields on the one hand and the preservation of the Segetal flora on the other hand can be resolved to a limited extent. For sustainable arable farming, a changed perspective and plant protection practice - also and in particular - with regard to weed control are required. The motto may apply: Dare more weeds! "
Drift - moving the medium even to areas that are far away
The widespread use of herbicides also makes the active ingredients detectable in surface and groundwater. Another danger arises from active substances with a particularly high vapor pressure (in relation to other active substances) , since they can spread via the gas phase . One example is clomazone , which according to guidelines from 2012 may only be applied at outside temperatures below 25 ° C. The agent was initially banned after residents reported health problems and discoloration of the leaves on adjacent areas.
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