User:Smartse/hcide

Classification
Herbicides are classified in various ways depending on how they are used and how they function. This results in considerable terminology related to herbicides and their use. An individual herbicide will fall into multiple categories, for example glyphosate is systemic, non-selective, post-emergence, non-residual and an EPSPS inhibitor.

Selectivity

 * Selective herbicides only affect particular species or groups of species of plants such as grasses or broadleaf species. Selectivity may be due to translocation, differential absorption, or physical (morphological), or physiological differences between plant species. 2,4-D, mecoprop, and dicamba control many broadleaf weeds but remain ineffective against turf grasses.
 * Non-selective herbicides affect all plant species. Paraquat, glufosinate and glyphosate are non-selective herbicides.

Application timing

 * Preplant herbicides are applied to soil before a crop is planted and sometimes 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 herbicides are applied before the weeds appear at the soil surface. They do not prevent weeds from germinating but kill weeds as they grow through the herbicide-treated zone. 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 herbicides are applied after weed seedlings have appeared above the soil. 2,4-D is a selective, systemic, foliar-absorbed postemergence herbicide.

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. 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.

Systemic action

 * Contact herbicides only affect the plant tissue that the herbicide is applied to. They are generally faster-acting than systemic herbicides, but less effective on perennial plants, which are able to regrow from rhizomes, roots or tubers.
 * Systemic herbicides are absorbed and transported through the plant, either from foliar application down to the roots, or from soil application up to the leaves. They are capable of controlling perennial plants and may be slower-acting taking days or weeks to have an effect, but are ultimately more effective than contact herbicides.

add to usage
Non-selective herbicides are used to clear industrial sites, waste grounds, railways, and railway embankments. Surfactants alter the physical properties of the spray solution and the overall phytotoxicity of the herbicide, increasing translocation.


 * 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.
 * Use of multiple herbicides at the same time, i.e. contact to kill already emerged weeds + residual for later control.


 * 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.

Application method

 * 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.

Unsure

 * The basis of selectivity is based on physical or biological factors. Some biological factors include morphology, physiology, metabolism, and biochemical factors.