Editing Herbicide (section)

==Terminology==
Herbicides can be classified/grouped in various ways; for example, according to their activity, the timing of application, method of application, mechanism of their action, and their chemical structures.

===Selectivity===
Chemical structure of the herbicide is of primary affecting efficacy. 2,4-D, [[mecoprop]], and [[dicamba]] control many broadleaf weeds but remain ineffective against turf grasses.<ref name="Vats">{{cite book|last1=Vats|first1=S.|editor1-last=Lichtfouse|editor1-first=E.|title=Sustainable Agriculture Reviews 15|date=2015|publisher=Springer International Publishing|pages=153–192|chapter=Herbicides: history, classification and genetic manipulation of plants for herbicide resistance}}</ref>

Chemical additives influence selectivity. [[Surfactant]]s alter the physical properties of the spray solution and the overall phytotoxicity of the herbicide, increasing translocation. [[Herbicide safener]]s enhance the selectivity by boosting herbicide resistance by the crop but allowing the herbicide to damage the weed.

Selectivity is determined by the circumstances and technique of application. Climatic factors affect absorption including [[humidity]], light, precipitation, and temperature. 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. Precipitation may wash away or remove some foliage-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.

===Non-selective herbicides===
Non-selective herbicides, generally known as [[defoliant]]s, are used to clear industrial sites, waste grounds, railways, and railway embankments. [[Paraquat]], [[glufosinate]], and glyphosate are non-selective herbicides.<ref name=Vats />

===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 (chemistry)|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. Crops grown in soil treated with a preplant herbicide include tomatoes, corn, soybeans, and strawberries. Soil fumigants like [[Metam sodium|metam-sodium]] and [[dazomet]] are in use as preplant herbicides.<ref name=Vats/>
* '''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.<ref name=Vats/>
* '''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.<ref name=Vats />

===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 [[colloid]]s 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. [[S-ethyl dipropylthiocarbamate|EPTC]] and [[trifluralin]] are soil-applied herbicides.<ref name=Vats/>
* '''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 [[Cuticle (botany)|cuticles]], waxes, [[Plant cell wall|cell walls]] etc. affect herbicide absorption and action. Glyphosate, 2,4-D, and dicamba are foliar-applied herbicides.<ref name=Vats />

===Persistence===
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. A 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.{{cn|date=December 2023}}