herbicide herbicide herbicides (us: /ˈɜːrbɪsaɪdz/, uk: /ˈhɜːr-/), also commonly known as weed killers, are substances used to control undesired plants, also known as weeds. selective herbicides control specific weed species while leaving the desired crop relatively unharmed, while non-selective herbicides (sometimes called total weed killers 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. herbicide being sprayed onto crops weeds controlled with herbicide modern herbicides are often synthetic mimics of natural plant hormones that interfere with the 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 united states in 2012, about 91% of all herbicide usage, determined by weight applied, was in agriculture.: 12 in 2012, world pesticide expenditures totaled nearly $24.7 billion; herbicides were about 44% of those sales and constituted the biggest portion, followed by insecticides, fungicides, and fumigants.: 5 herbicide is also used in forestry, where certain formulations have been found to suppress hardwood varieties in favor of conifers after clearcutting, as well as pasture systems, and management of areas set aside as wildlife habitat. history prior to the widespread use of herbicides, cultural controls, such as altering soil ph, salinity, or fertility levels, were used to control weeds. 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 herbicides began in the early 20th century, the first major breakthrough was the result of research conducted in both the united kingdom and the united states during the second world war into the potential use of herbicides in war. 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, r. pokorny in the us achieved this as well. 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. 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, was 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 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 afterward, 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 weed killer with high levels of persistence in the soil. terminology herbicides are classified/grouped in various ways; for example, according to the activity, the timing of application, method of application, mechanism of action, and chemical family. this gives rise to a considerable level of terminology related to herbicides and their use. intended outcome 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 and still provides some economic benefits, 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. defoliant, similar to herbicides, but designed to remove foliage (leaves) rather than kill the plant. selectivity (all plants or specific plants) the basis of selectivity is based on physical or biological factors. some biological factors include morphology, physiology, metabolism, and biochemical factors. there are some climatic factors affecting absorption including humidity, light, precipitation, and temperature. foliar-applied herbicides will enter the leaf more readily at high humidity by lengthening the drying time of the spray droplet and increasing cuticle hydration. light of high intensity may break down some herbicides and cause the leaf cuticle to thicken, which reduces absorption. precipitation may wash away or remove some foliar-applied herbicides but it will increase root absorption of soil-applied herbicides. drought-stressed plants are less likely to translocate herbicides. as temperature increases, herbicides' performance may decrease. absorption and translocation may be reduced in very cold weather. selective herbicides control or suppress certain plants without affecting the growth of other plant species. selectivity may be due to translocation, differential absorption, or physical (morphological), or physiological differences between plant species. surfactants alter the physical properties of the spray solution and the overall phytotoxicity of the herbicide, increasing translocation. 2,4-d, mecoprop, and dicamba control many broadleaf weeds but remain ineffective against turf grasses. non-selective herbicides are not specific in acting against certain plant species and control all plant material with which they come into contact. they are used to clear industrial sites, waste grounds, railways, and railway embankments. paraquat, glufosinate, and glyphosate are non-selective herbicides. timing of application preplant: preplant herbicides are nonselective herbicides applied to the soil before planting. some preplant herbicides may be mechanically incorporated into the soil. the objective for incorporation is to prevent dissipation through photodecomposition and/or volatility. the herbicides kill weeds as they grow through the herbicide-treated zone. volatile herbicides have to be incorporated into the soil before planting the pasture. agricultural crops grown in soil treated with a preplant herbicide include tomatoes, corn, soybeans, and strawberries. soil fumigants like metam-sodium and dazomet are in use as preplant herbicides. preemergence: preemergence herbicides are applied before the weed seedlings emerge through the soil surface. herbicides do not prevent weeds from germinating but they kill weeds as they grow through the herbicide-treated zone by affecting the cell division in the emerging seedling. dithiopyr and pendimethalin are preemergence herbicides. weeds that have already emerged before application or activation are not affected by pre-herbicides as their primary growing point escapes the treatment. postemergence: these herbicides are applied after weed seedlings have emerged through the soil surface. they can be foliar or root absorbed, selective or nonselective, and contact or systemic. application of these herbicides is avoided during rain since being washed off the soil makes it ineffective. 2,4-d is a selective, systemic, foliar-absorbed postemergence herbicide. method of application soil applied: herbicides applied to the soil are usually taken up by the root or shoot of the emerging seedlings and are used as preplant or preemergence treatment. several factors influence the effectiveness of soil-applied herbicides. weeds absorb herbicides by both passive and active mechanisms. herbicide adsorption to soil colloids or organic matter often reduces the amount available for weed absorption. positioning of the herbicide in the correct layer of soil is very important, which can be achieved mechanically and by rainfall. herbicides on the soil surface are subjected to several processes that reduce their availability. volatility and photolysis are two common processes that reduce the availability of herbicides. many soil-applied herbicides are absorbed through plant shoots while they are still underground leading to their death or injury. eptc and trifluralin are soil-applied herbicides. foliar applied: these are applied to a portion of the plant above the ground and are absorbed by exposed tissues. these are generally postemergence herbicides and can either be translocated (systemic) throughout the plant or remain at a specific site (contact). external barriers of plants like cuticles, waxes, cell walls etc. affect herbicide absorption and action. glyphosate, 2,4-d, and dicamba are foliar-applied herbicides. persistence residual activity: an herbicide is described as having low residual activity if it is neutralized within a short time of application (within a few weeks or months) – typically this is due to rainfall, or reactions in the soil. an herbicide described as having high residual activity will remain potent for the long term in the soil. for some compounds, the residual activity can leave the ground almost permanently barren. mechanism of action herbicides are often classified according to their site of action because as a general rule, herbicides within the same site of action class will produce similar symptoms on susceptible plants. classification based on the site of action of the herbicide is preferable as herbicide resistance management can be handled more effectively. classification by mechanism of action (moa) indicates the first enzyme, protein, or biochemical step affected in the plant following application. list of mechanisms found in modern herbicides accase inhibitors: acetyl coenzyme a carboxylase (accase) is part of the first step of lipid synthesis. thus, accase inhibitors affect cell membrane production in the meristems of the grass plant. the accases of grasses are sensitive to these herbicides, whereas the accases of dicot plants are not. als inhibitors: acetolactate synthase (als; also known as acetohydroxyacid synthase, or ahas) is part of the first step in the synthesis of the branched-chain amino acids (valine, leucine, and isoleucine). these herbicides slowly starve affected plants of these amino acids, which eventually leads to the inhibition of dna synthesis. they affect grasses and dicots alike. the als inhibitor family includes various sulfonylureas (sus) (such as flazasulfuron and metsulfuron-methyl), imidazolinones (imis), triazolopyrimidines (tps), pyrimidinyl oxybenzoates (pobs), and sulfonylamino carbonyl triazolinones (scts). the als biological pathway exists only in plants and microorganisms (but not animals), thus making the als-inhibitors among the safest herbicides. epsps inhibitors: enolpyruvylshikimate 3-phosphate synthase enzyme (epsps) is used in the synthesis of