Without the means to control weeds,neither the required yield nor
the desired quality of the products deriving from a crop can be guaranteed. 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.
In order to avoid the problems that arise if an active substance is over-used, it is necessary to select a rotation in which herbicides with different modes of action can be applied. Knowledge of the mode of action of herbicides can help towards developing a successful strategy for controlling weeds.
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 herbi-cides) into the plant. Products are classifiedas contact or systemically-active herbicides,depending on the extent and natureof uptake, redistribution and activity within the plant.
1. Foliar herbicides
The contact herbicides belong to this group. They penetrate into the plant exclusively or pre dominantly via the leaf, and are then redistributed only to a limited extent.They therefore cause damage to the weed plant at, or near, the point of penetration.
This means that contact herbicides 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, which interfere with the balance of growth hormones in the plant. Most grass herbicides and bindweed products also work via the leaf.
Foliar herbicides are redistributed in the plant mainly via the transpiration stream that flows through the plant’s vascular bundles.
The assimilates produced by photosynthesis in a particular leaf are only exported if it is producing more than it needs for growth and respiration within its own tissues.The greatest export of assimilates occurs in fully developed,photo synthetically-active leaves when weather conditions are optimal. Young, still-developing leaves do not export sugars – and herbicides applied to them are not re-distributed to any extent. Temperature is important for herbicide efficacy: at temperatures below 10°C, 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.
2. Soil herbicides
The uptake of these active substances occurs via the roots, and is followed by re-distribution within the plant. The herbicides tend to be active in the leaves, or other above-ground parts of the plant, where they disrupt respiration processes and photosynthesis.
The active substances reach the soil through the medium of water, and can remain there for some time. Soil herbicides should therefore only be applied to moist soils: under dry conditions, these products may lose their activity altogether.
Soil herbicides 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.
3. Foliar and soil herbicides
Some herbicides are active via both the leaf and the soil.The relative degree of uptake via leaves and roots of products within this category of herbicides determines the timing of application – whether pre-emergence, early post-emergence or from the 3-leaf stage onwards. Combined foliar and root uptake can be achieved using a mixture of active substances.
4. Safeners
Safeners are herbicide additives that accelerate the breakdown of the active substance within the crop plant. In contrast,they do not interfere with the intended action within target grass weeds, which apparently possess different variants of the relevant target enzymes. The safener activates an enzyme in cereals that accelerates the breakdown of the active substance, thus making the cereal plant insensitive to it.
The secret is that the safener does not activate the corresponding enzyme in weed grasses – they remain sensitive and are killed off.
Mode of action
The mode of action describes the way in which physiological processes within the plant are influenced by the herbicide. In most cases, the active substance binds to a protein, thereby blocking one of the plant’s essential metabolic processes. The protein is usually an enzyme that regulates a particular biochemical reaction within the metabolic chain. However, the inhibition
can also take place at structural and regulatory binding sites. Herbicides tend to possess a single main mode of action, but many also have secondary sites of action at which they can also disturb the plant’s metabolism.
Herbicides should be applied according to the principles of Good Agricultural Practice and Integrated Crop Protection.This means that local conditions, rotation, and possible cultural methods should all be considered when developing a strategy for controlling weeds. Rotation is particularly important, and it has a direct influence on soil cultivation, the incidence of weeds and the ability of a crop to compete with them,and it determines the spectrum of herbicides that is available for weed control.
If herbicide use becomes necessary, the first step towards choosing the most suitable product is to determine which weed species are present – or are likely to appear – and the current and anticipated density of infestation. The prevalent weed flora and the known thresholds of action determine the choice of the best herbicide, based on its spectrum of activity.
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. If all of these factors are considered together, then the product’s full potential can be realised.
But economic factors also play a role in the decision-making process. The decision as to whether to treat pre-sowing, or pre- or post-emergence, influences both the choice of herbicide, and the work-regime.
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. Shifts within the weed population always start with single, resistant individual plants –these are universally present in nature. The repeated use of herbicides with a common mode of action creates a selection pressure that promotes the spread within the population of plants possessing the resistance characteristics. Unless the control strategy is changed, these resistant weeds can spread such that they eventually gain the upper hand, and can no longer be controlled effectively.
Thus to be on the safe side, resistance management should be considered in advance,whilst planning which crop to grow.The key to active substance rotation is to design the rotation so that the same mode of action is not used twice in successive crops,throughout the season.
