Herbicide
[2] The combined effects of herbicides, nitrogen fertilizer, and improved cultivars has increased yields (per acre) of major crops by three to six times from 1900 to 2000.
[4]: 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.
[4]: 5 Herbicide is also used in forestry,[5] where certain formulations have been found to suppress hardwood varieties in favor of conifers after clearcutting,[6] as well as pasture systems.
In 1940, his work with indoleacetic acid and naphthaleneacetic acid indicated that "growth substances applied appropriately would kill certain broad-leaved weeds in cereals without harming the crops,"[11][12] though these substances were too expensive and too short-lived in soil due to degradation by microorganisms to be of practical agricultural use; by 1941, his team succeeded in synthesizing a wide range of chemicals to achieve the same effect at lower cost and better efficacy, including 2,4-D.[13] In the same year, R. Pokorny in the US achieved this as well.
[14] 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.
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.
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.
Foliage-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 can interfere with absorption.
[18] 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.
[19] The so-called cyclohexanedione herbicides, which are used against grasses, include the following commercial products cycloxydim, clethodim, tralkoxydim, butroxydim, sethoxydim, profoxydim, and mesotrione.
[26] Knowing about herbicide chemical family grouping serves as a short-term strategy for managing resistance to site of action.
[28] Eventually the Herbicide Resistance Action Committee (HRAC)[29] and the Weed Science Society of America (WSSA)[30] developed a classification system.
[31][32] Groups in the WSSA and the HRAC systems are designated by numbers and letters, inform users awareness of herbicide mode of action and provide more accurate recommendations for resistance management.
On large areas, herbicides may also at times be applied aerially using helicopters or airplanes, or through irrigation systems (known as chemigation).
Weed-wiping may also be used, where a wick wetted with herbicide is suspended from a boom and dragged or rolled across the tops of the taller weed plants.
In Wales, a scheme offering free weed-wiper hire was launched in 2015 in an effort to reduce the levels of MCPA in water courses.
Unlike with annuals however, application is mostly unnecessary thereafter and is thus mostly used to decrease the delay between productive economic cycles of lumber crops.
Although herbicidal warfare uses chemical substances, its main purpose is to disrupt agricultural food production or to destroy plants which provide cover or concealment to the enemy.
Deployment of herbicides and defoliants served the dual purpose of thinning jungle trails to prevent ambushes and destroying crop fields in regions where the MNLA was active to deprive them of potential sources of food.
For example, a 1995 panel of 13 scientists reviewing studies on the carcinogenicity of 2,4-D had divided opinions on the likelihood 2,4-D causes cancer in humans.
Herbicide properties that increase likelihood of transport include persistence (resistance to degradation) and high water solubility.
Chemical manufacturer Monsanto Company agreed to change its advertising after pressure from New York attorney general Dennis Vacco; Vacco complained about misleading claims that its spray-on glyphosate-based herbicides, including Roundup, were safer than table salt and "practically non-toxic" to mammals, birds, and fish (though proof that this was ever said is hard to find).
[64] This caused incredibly strong selective pressure upon weeds, encouraging mutations conferring glyphosate resistance to persist and spread.
Specific mutations that have the most helpful effect for the plant have been shown to occur in separate instances and dominate throughout resistant weed populations.
Herbicides should be used to their greatest potential by ensuring that the timing, dose, application method, soil and climatic conditions are optimal for good activity.
This has been cited as one reason why ALS-resistant Stellaria media has evolved in Scotland recently (2000), despite the regular use of a sequence incorporating mecoprop, a herbicide with a different mode of action.
[83] By 1983, patches of ryegrass had become immune to Hoegrass (diclofop-methyl), a family of herbicides that inhibit an enzyme called acetyl coenzyme A carboxylase.
In 2013, only two herbicide classes called Photosystem II and long-chain fatty acid inhibitors, were effective against ryegrass.
