How herbicides work？

Date:03-13-2009

Under most circumstances, herbicides are the cheapest and most reliable form of weed control. However, they should be used only after other, cultural options for weed control have been considered and put into practice. To help avoid problems with resistance it is necessary to select a rotation in which herbicides with different modes of action can be applied.

Mode of action
The mode of action describes the way in which physiological processes within the plant are influenced by the herbicide. Herbicides are classified into different groups according to their main site of action within the plant’s metabolism:
1. Photosynthesis inhibitors
2. Pigment synthesis inhibitors
3. Amino acid synthesis inhibitors
4. Fatty acid synthesis inhibitors
5. Cell division inhibitors

How are herbicides taken up by the plant?
In order to be effective, herbicides must be able to move from the spray deposit (foliar herbicides) or the soil solution (soil herbicides) into the plant. Products are classified as contact or systemically-active herbicides, depending on the extent and nature of uptake, redistribution and activity within the plant.

Foliar herbicides
Contact herbicides belong to this group. They penetrate into the plant via the leaf and are redistributed to a limited extent only. They therefore cause damage to the weed plant at or near the point of penetration. This means they tend to be effective mainly against species that lack stored reserves, such as annual weeds.
The uptake of systemic foliar herbicides occurs mainly via the leaf, and is followed by extensive redistribution within the plant. The best-known examples are the growth substances. Young, still-developing leaves do not re-distribute herbicides. At temperatures below 10 C, efficacy of herbicide activity is usually low; whereas at temperatures greater than 25 C, there may be scorching of the crop plant, or reduced activity against the target species.

Soil herbicides
The uptake of these active substances occurs via the roots, and is followed by re-distribution within the plant. Soil herbicides should only be applied to moist soils: under dry conditions, these products may lose their activity altogether. They play an important role in the control of weed grasses and dicotyledons during the pre-seeding and pre-emergence periods, or sometimes in the early post-emergence period. Examples are metazachlor in oilseed rape and flufenacet (in Cadou in cereals. Some herbicides are active via both the leaf and the soil. Examples include the ALS-inhibitors, e.g. Atlantis and Husar or the sugar beet herbicide Betanal Expert.

Safeners
A wide range of commercial herbicides are innately selective in their target crops. However, many have been developed that require a safener for complete selective use.

Safeners mostly work by accelerating the breakdown of the herbicide in the crop but not in the targeted weeds. The product Atlantis, for example, has excellent activity against grasses. It would not be possible to use it in cereals – which botanically are also grasses – if the product didn’t contain a safener thus making the cereal plant insensitive to it.

Decision criteria
Local conditions, rotation, and possible cultural methods should all be considered when developing a strategy for controlling weeds. Rotation is particularly important and it determines the spectrum of herbicides available. If herbicide use becomes necessary, the first step is to determine which weed species are present and their density of infestation.

The prevalent weed flora and the known thresholds of action determine the choice of the best herbicide. The efficacy of a product and the overall success of weed control are influenced by a number of factors, including the growth conditions, the timing and rate of application, the technology used to apply, and other, local circumstances. All of these should be considered. Economic factors also play a role. Depending on the situation, early treatment of cereals can help to avoid work peaks later in the season.

Resistance
Limiting the rotation to one or two crops, and intensive use of herbicides with identical or similar modes of action, favour the development of resistance in weeds. The key is to design the rotation so that the same mode of action is not used twice in successive crops.