Biological weed control in herbaceous crops

In organic farming, weed control is one of the most complex challenges to overcome. Thus, mechanical means are resorted to as an alternative to the use of pesticides…

However, this issue also affects conventional farmers because there are currently not many active ingredients that can be used and in several situations the use of chemicals has become ineffective. Moreover, in this context, especially for short-cycle vegetable crops, it can be difficult to comply with shortage times and this in fact further reduces the possibilities of using chemicals.

Weed control systems on open field crops make use of mechanical means that interact superficially with the soil by undermining or tearing up the weeds, exposing, at least partially, their root system to the air, causing them to dry out more or less rapidly. However, in order to be sufficient to guarantee correct weed management during the entire crop cycle, these interventions must be suitably included in a broader agronomic strategy. These strategies, once normally applied by farmers, have been progressively abandoned due to the wide availability of effective chemical principles. Now that such availability is no longer there and many farmers have embraced the organic world, it is necessary to recover that knowledge and integrate it with the new equipment available today to ensure proper weed control.

Weed control operations

All soil tillage involving loosening, mixing and turning over of the soil surface has a herbicidal action. In some cases, this action is secondary, in others pre-eminent. Before delving into the different types, however, the field must be cleared of the supposed advantages provided by ploughing in controlling weeds. Ploughing, in fact, when carried out regularly, brings back to the surface the seeds buried in previous years which, being on average characterized by a high longevity, largely nullify the weed control action assigned to this operation.

The most effective way of combating weeds is instead to create the best conditions for their germination before the crop is planted. Within the crop rotation it is therefore useful to include periods dedicated to this technique, alternating them between the spring season (to combat macrothermal weeds) and the autumn season (to combat microthermal species). The summer period, after the straw cereal harvest, is also effective in this sense, if the soil water reserve or rain is sufficient to ensure adequate moisture in the first few centimeters of soil, i.e. where the weed seed is able to germinate.

After the main tillage, the soil is prepared for sowing, and depending on the period, it is necessary to wait two to three weeks before sowing the crop. This will be preceded by a very superficial and fast passage with elastic tine or disc equipment, with tools arranged in several rows, with the aim of moving the soil only in the first 3-4 centimeters, devitalizing the germinated or germinating plants. The depth may vary depending on the type and size of the weed, but it is fundamental to avoid bringing new seeds to the surface. Under minimal tillage conditions it may be convenient, for example after tilling with a light cultivator, to intervene with independent disc harrows adjusted to work on the surface.

This technique, known as false sowing, was widely used (in fact, in ancient times it was one of the few weapons to combat weeds) and is now also back in vogue in conventional farming. In fact, alternating between chemical and mechanical weeding reduces the problems generated by the repeated use of chemical herbicides, such as the establishment and spread of resistance phenomena and the possible accumulation of chemical residues in the soil. These, at least for microbial degradation molecules, may also depend on the low biological activity that typically characterizes soils managed for decades with very invasive tillage.

Excellent for these tillage operations in which the objective of weed control prevails, are implements equipped with elastic tines, so-called preparators, if the soil surface is free or with little crop residue, or with flat and dense discs, arranged in several rows, in the presence of crop residue, even abundant. In the first case, harrows can also be used, and in the second case, specialized vertical tillage equipment would be perfect, although not yet available on the Italian market.

Similarly, stubble processing involving very superficial tillage, immediately after harvesting the autumn/winter cereal or rapeseed, has the purpose of interrupting the upwelling of water (by storing the soil’s water reserve) and favouring the degradation of the residue. Moreover, such tillage creates optimal conditions for the emergence of weeds that can then be mechanically removed before they become seed.

Harrowing and weeding

With the planting of the crop, the weed control strategy must change. The organic farm finds in the weeder harrow a fundamental ally, an indispensable piece of equipment on autumn/winter cereals because it is the only means that can be used, so much so that today it is also intended to be used on summer-cycle crops and on many horticultural crops where, however, being in most cases cultivated in spaced rows, weeders can also be used.

The tine harrow is characterized by a series of thin, very elastic, angled tines, with the distal part almost vertical to the ground, equipped with a spring near the attachment point on the frame. This is generally independent of the main frame, allowing a perfect adaptation to the ground. The angle of penetration and the pressure exerted on the ground are generally adjustable. The harrow takes advantage of the difference in development between the crop and the weed: the former must already be well rooted, the latter emerged recently and therefore it can be easily removed. For this reason, harrowing is an operation that must be repeated precisely according to the emergence of the weed in order to prevent it from breaking through the soil.

Used on summer-cycle crops, in addition to providing excellent weed control, it breaks up the surface crust, interrupts capillary rise, safeguarding soil water resources and, possibly, promotes surface burying of fertilizers given in cover.

Hoeing is an operation applied to spaced rows of crops with the aim of eliminating weeds, blocking the capillary rise of water, and burying any fertilizers. To reduce damage to the crop, the weeder is set up to accommodate an operator who intervenes to correct the position of the tool with respect to the row of plants or individual plants.

Only complex optoelectronic systems, i.e. based on electromagnetic waves of the visual field or very close to it such as infrared, made it possible to replace humans and automate this delicate operation.

Two paths are taken. The first limits the analysis to the identification of the cultivated row and therefore, based on this information, governs the element so that it is kept at a predetermined distance from the row itself. The second identifies each individual cultivated plant, fixes its position and on the basis of this governs one or more tools that also perform superficial tillage on the row. When the seedling emerges, they are brought into the inter-row and once it has passed, repositioned inside. These ‘intelligent’ weeders can be used for many crops, including lettuce despite the very dense planting layouts (up to 16 plants per square metre) with which it is cultivated.

Alternative techniques

In crops with high areal profitability, i.e. where the income per unit area is potentially high, weed control can make use of expensive techniques. One of the most effective is undoubtedly mulching, which consists in covering the strip of land subject to transplanting or sowing, with recyclable materials that avoid, compared to plastic ones, the hard task of film recovery at the end of the production cycle. Also effective, but potentially costly and difficult to apply over large areas, are those systems that make use of physical stratagems such as the super-innovative electric current systems or the more tested heat-based systems. The latter aim to ‘boil’ the aerial portion of the plant, bringing it to a rapid death by transferring heat via a variously shaped plate, heated inside, or by direct flame. In the latter case we speak of pyro-diserbos, with a flame fed by butane gas cylinders.

In special contexts, such as the rows of a vineyard or pavements in an urban environment, high-pressure water jets can be used to eliminate pests through mechanical action. Thermal sterilization by, for example, solarization is widely applied under greenhouses in southern Italy. However, it is a technique that is more successful in controlling pests than on weed seeds, which must be in a germinating condition in order to be successfully affected.

Lorenzo Benvenuti

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