How an Herbicide Works

Technical articles

Herbicides Active ingredients Herbicide application Adjuvants Weed management

Herbicides are active ingredients or formulated products that have the property of killing plants. The name of a herbicide generally corresponds to the commercial name of a product from a distributor or manufacturer. Herbicides can have different compositions and formulations and possess different modes of action.

Composition

A herbicide is composed of 2 types of constituents :

• The active ingredients, which have herbicidal properties.
• The formulants, which can be fillers or solvents and which improve the preparation via :
  • The quality (stability (emulsifier, dispersant), presentation (color, scent), ease of use (emetic)).
  • The physical behavior (wetting agent, adhesive).
  • The biochemical activity (surfactant, phytoprotector).

Formulation

The formulation of a herbicide corresponds to the physical form in which the herbicide is sold on the market. This formulation is obtained by the mixing of active ingredients with formulants. Herbicides can be presented in different forms, solid or liquid :

• Solid : Soluble Granules (SG), Wettable Powders (WP), Capsule Suspensions (CS), Water Dispersible Granules (WG).
• Liquid : Soluble Concentrates (SL), Emulsifiable Concentrates (EC), Aqueous Emulsions (EW), Suspension Concentrates (SC).

The formulation is very important in the handling of products, from their manufacture and transport to their storage and preparation of sprays. For example, suspension concentrates tend to sediment, so it is necessary to shake them before use.

Modes of action and resistance

Mode of action

Different types of herbicides are distinguished according to their penetration route and movement within the plant :

• Herbicides with root penetration : Applied to the soil, penetration through roots, seeds or seedlings, these are pre-emergence herbicides.
• Herbicides with foliar penetration : Applied on leaves, penetration through leaves, petioles or stem, these are post-emergence herbicides.
• Contact herbicides : Act after penetration into the plant without migrating from one organ to another.
• Systemic herbicides : Act after penetration and can migrate from one organ to another within the plant.

Example : Glyphosate = systemic herbicide with foliar penetration.

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Herbicides act on different processes of plant growth and development such as its physiology (photosynthesis & root permeability), its growth (cell division, elongation), and its biosyntheses (lipids, amino acids).

Resistance

Continuous use of the same herbicide products inevitably leads to flora selection, that is, often monospecific populations composed of species on which these active ingredients are not effective. Resistance phenomena to herbicides have appeared in weeds over the past decades. There are about 270 weed species resistant to herbicides.

The HRAC classification helps prevent resistance and alternate herbicides to maintain their effectiveness. The lower the number, the higher the resistance to the herbicide.

Different chemical families and active herbicidal substances are distinguished according to their modes of action and HRAC classification. Here is a non-exhaustive list of the most widespread resistances :

Mode of action	Chemical family	Active substance (example product)	Known resistance	HRAC classification
Acetyl-CoA Carboxylase (ACCase) inhibitors	FOP	Clodinafop-propargyl (Celio)	Blackgrass, Ryegrass, Wild Oat, Teosinte, etc.	1
	DEN	Pinoxaden (Axial-pratic)
Acetolactate Synthase (ALS) inhibitors	Triazolopyrimidine	Florasulam (Primus)	Poppy , Blackgrass, Ryegrass, Common Groundsel, Common Bentgrass, etc.	2
	Sulfonylureas	Mesosulfuron (Allie)
		Iodosulfuron (Atlantis Pro)
Auxins	Phenoxy-carboxylates	2,4-D (Lonpar)	Poppy	4
		MCPA (Ariane New)
5-EnolylPyruvyl-Shikimate 3-Phosphate Synthase (EPSPS) inhibition		Glyphosate (Roundup Flash Plus)	Ryegrass, Annual Bluegrass, etc.	9
Very-long-chain fatty acid synthesis (VLCFAS) inhibition	alpha-Oxyacetamide	Flufenacet (Fosburi)	First cases	15

Selectivity of herbicides

Herbicides are called selective when, used under normal conditions of use, they respect certain crops and allow control of certain weeds in these crops. They are called total when, used at the recommended doses for this use, they are likely to destroy or prevent the development of all vegetation with variable persistence of action.

Various types of selectivity are distinguished :

• Position selectivity : The pre-emergence herbicide, applied on the surface, only distributes in the superficial soil layer a few centimeters deep. This is the zone where most weed species germinate, whose seeds are small. In contact with the product, they will undergo its herbicidal activity. Conversely, crop seeds are positioned deeper and escape contact with the product which will have no effect on their germination.

• Application selectivity : Contact of the product with the cultivated plant is avoided during spraying. The herbicide is applied only on the weeds in the inter-row, taking care not to reach the crop row. This technique is mainly used with total herbicides in wide-spacing crops or thanks to precision robotics.

• Anatomical selectivity : Mainly post-emergence products. Penetration through leaves can be hindered by the presence of hairs (trichomes) or by the thickness of the epidermal cuticle. The leaf posture also modifies the adherence of the spray on their surface : grass leaves, upright and narrow, retain droplets less well than those of dicotyledons, often broad and spread out.

• Physiological selectivity : Selectivity can be obtained by differences in physiological behavior between plants. Some herbicidal molecules are less well transported or have enzymes that degrade the herbicide.

Spectrum of efficacy

The efficacy of a herbicide depends on the applied dose. One defines a limit dose of efficacy which may vary depending on the target plant and the application period. The spectrum of efficacy corresponds to all species controlled by a product at a given dose.

Fate of herbicides

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Like other pesticides, herbicides degrade more or less rapidly after their application in the environment :

• They are partly metabolized in the target plant and metabolites of the original herbicide molecule can be found in the environment.
• A small portion is exported by volatilization into the air, by runoff from rain or by leaching into lower soil layers. This portion increases significantly if the product is not applied according to good practices.
• A portion is absorbed by clays and organic matter in the soil before undergoing biochemical and microbiological degradation.

Persistence or residual activity corresponds to the duration during which a herbicide product manifests its activity. It is always necessary to ensure the absence of carry-over effects of a product on the following crop as well as on animal and human health.