Herbicide

Weeds controlled with herbicide

Herbicides, also commonly known as weedkillers, are chemical substances used to control unwanted plants.[1] Selective herbicides control specific weed species, while leaving the desired crop relatively unharmed, while non-selective herbicides (sometimes called total weedkillers in commercial products) can be used to clear waste ground, industrial and construction sites, railways and railway embankments as they kill all plant material with which they come into contact. Apart from selective/non-selective, other important distinctions include persistence (also known as residual action: how long the product stays in place and remains active), means of uptake (whether it is absorbed by above-ground foliage only, through the roots, or by other means), and mechanism of action (how it works). Historically, products such as common salt and other metal salts were used as herbicides, however these have gradually fallen out of favor and in some countries a number of these are banned due to their persistence in soil, and toxicity and groundwater contamination concerns. Herbicides have also been used in warfare and conflict.

Modern herbicides are often synthetic mimics of natural plant hormones which interfere with growth of the target plants. The term organic herbicide has come to mean herbicides intended for organic farming. Some plants also produce their own natural herbicides, such as the genus Juglans (walnuts), or the tree of heaven; such action of natural herbicides, and other related chemical interactions, is called allelopathy. Due to herbicide resistance - a major concern in agriculture - a number of products combine herbicides with different means of action. Integrated pest management may use herbicides alongside other pest control methods.

In the US in 2007, about 83% of all herbicide usage, determined by weight applied, was in agriculture.[1]:12 In 2007, world pesticide expenditures totaled about $39.4 billion; herbicides were about 40% of those sales and constituted the biggest portion, followed by insecticides, fungicides, and other types.[1]:4 Smaller quantities are used in forestry, pasture systems, and management of areas set aside as wildlife habitat.

History

Prior to the widespread use of chemical herbicides, cultural controls, such as altering soil pH, salinity, or fertility levels, were used to control weeds.[2] Mechanical control (including tillage) was also (and still is) used to control weeds.

First herbicides

2,4-D, the first chemical herbicide, was discovered during the Second World War.

Although research into chemical herbicides began in the early 20th century, the first major breakthrough was the result of research conducted in both the UK and the US during the Second World War into the potential use of herbicides in war.[3] The first modern herbicide, 2,4-D, was first discovered and synthesized by W. G. Templeman at Imperial Chemical Industries. In 1940, he showed that "Growth substances applied appropriately would kill certain broad-leaved weeds in cereals without harming the crops." By 1941, his team succeeded in synthesizing the chemical. In the same year, Pokorny in the US achieved this as well.[4]

Independently, a team under Juda Hirsch Quastel, working at the Rothamsted Experimental Station made the same discovery. Quastel was tasked by the Agricultural Research Council (ARC) to discover methods for improving crop yield. By analyzing soil as a dynamic system, rather than an inert substance, he was able to apply techniques such as perfusion. Quastel was able to quantify the influence of various plant hormones, inhibitors and other chemicals on the activity of microorganisms in the soil and assess their direct impact on plant growth. While the full work of the unit remained secret, certain discoveries were developed for commercial use after the war, including the 2,4-D compound.[5]

When 2,4-D was commercially released in 1946, it triggered a worldwide revolution in agricultural output and became the first successful selective herbicide. It allowed for greatly enhanced weed control in wheat, maize (corn), rice, and similar cereal grass crops, because it kills dicots (broadleaf plants), but not most monocots (grasses). The low cost of 2,4-D has led to continued usage today, and it remains one of the most commonly used herbicides in the world. Like other acid herbicides, current formulations use either an amine salt (often trimethylamine) or one of many esters of the parent compound. These are easier to handle than the acid.

Further discoveries

The triazine family of herbicides, which includes atrazine, were introduced in the 1950s; they have the current distinction of being the herbicide family of greatest concern regarding groundwater contamination. Atrazine does not break down readily (within a few weeks) after being applied to soils of above neutral pH. Under alkaline soil conditions, atrazine may be carried into the soil profile as far as the water table by soil water following rainfall causing the aforementioned contamination. Atrazine is thus said to have "carryover", a generally undesirable property for herbicides.

Glyphosate (Roundup) was introduced in 1974 for nonselective weed control. Following the development of glyphosate-resistant crop plants, it is now used very extensively for selective weed control in growing crops. The pairing of the herbicide with the resistant seed contributed to the consolidation of the seed and chemistry industry in the late 1990s.

Many modern chemical herbicides used in agriculture and gardening are specifically formulated to decompose within a short period after application. This is desirable, as it allows crops and plants to be planted afterwards, which could otherwise be affected by the herbicide. However, herbicides with low residual activity (i.e., that decompose quickly) often do not provide season-long weed control and do not ensure that weed roots are killed beneath construction and paving (and cannot emerge destructively in years to come), therefore there remains a role for weedkiller with high levels of persistence in the soil.

Other Languages
العربية: مبيد أعشاب
asturianu: Yerbicida
azərbaycanca: Herbisidlər
беларуская: Гербіцыды
български: Хербицид
bosanski: Herbicid
català: Herbicida
čeština: Herbicid
dansk: Herbicid
Deutsch: Herbizid
eesti: Herbitsiid
Ελληνικά: Ζιζανιοκτόνο
español: Herbicida
Esperanto: Herbicido
euskara: Herbizida
فارسی: علف‌کش
français: Herbicide
Gaeilge: Luibhicíd
galego: Herbicida
ગુજરાતી: નિંદામણનાશક
한국어: 제초제
հայերեն: Հերբիցիդներ
हिन्दी: शाकनाशक
hrvatski: Herbicidi
Bahasa Indonesia: Herbisida
italiano: Diserbante
ქართული: ჰერბიციდები
қазақша: Гербицидтер
Кыргызча: Гербициддер
Latina: Herbicida
lietuvių: Herbicidai
magyar: Herbicid
मराठी: तणनाशक
Bahasa Melayu: Racun rumpai
монгол: Гербицид
မြန်မာဘာသာ: ပေါင်းပင်သတ်ဆေး
Nederlands: Herbicide
नेपाली: शाकनाशी
日本語: 除草剤
norsk: Herbicid
oʻzbekcha/ўзбекча: Gerbitsidlar
polski: Herbicydy
português: Herbicida
română: Erbicid
русский: Гербициды
Scots: Yerbicide
Simple English: Herbicide
slovenčina: Herbicíd
slovenščina: Herbicid
српски / srpski: Хербициди
srpskohrvatski / српскохрватски: Herbicid
Basa Sunda: Hérbisida
suomi: Herbisidi
svenska: Herbicid
тоҷикӣ: Алафкуш
Türkçe: Herbisit
тыва дыл: Гербицид
українська: Гербіциди
Tiếng Việt: Thuốc diệt cỏ
粵語: 除草劑
中文: 除草剂