Mechanical, agrotechnical and biological weed management

Learning Outcomes:

• Explain the difference between direct and indirect weed control practices in organic farming and the different types of methods.
• Choose and recommend the appropriate method for weed control according to the advantages and disadvantage of practices.

Nowadays there are a various number of non-chemical weed control techniques developments. The following are common techniques available to non-chemical weed management strategies for organic farming.

Direct weed control

Direct control needs to be linked with long term preventative measures to maintain the weed population at a manageable level.

Thermal weed control

Thermal weed control includes application of fire, flaming, hot water, steam and freezing. These techniques control weeds without disturbing the soil and do not bring the buried seeds to the soil surface. Several factors (i.e. temperature, exposure time, energy input) can influence the effectiveness of thermal control, however many of these methods only kill the shoots of target plants, thus repeated treatments may be necessary to avoid regeneration. Based on mode of action thermal control methods can be divided into three groups: (i) the direct heating methods (flaming/burning, solarization, infrared weeders, hot water, steaming, hot air), (ii) indirect heating methods (electrocution, microwaves, laser radiation, ultra violet light), and (iii) freezing as opposite plant stress factor.

- Flaming/burning. Plant processes can be damaged by high temperature through protein coagulation and denaturation, increase of membrane permeability and enzyme inactivation. The thermal dead point for most plant tissues is 45 - 55°C after prolonged exposure. Effectiveness of procedure is mostly influenced by plant size at treatment time less than density of weed plant. The most tolerant species cannot be controlled with flaming regardless of the numbers of applications. Flaming is a successful type of weed control, however due to its high cost and higher effectiveness of other methods, it is not used much in crops. Only seeds present in the windrow and on the immediate soil surface below the windrow are affected by burning. For soil stewardship and preservation of organic matter, burning should only be practiced on windrowed straw or on gathered weed materials from patches within the field known as spot burning.

The most commonly applied fuel in the burners is liquefied petroleum gas (LPG), usually propane, however as renewable alternatives such as hydrogen have been evaluated. Flame weeding (Figure 5.14) can be cheaper than hand-weeding but there is a high machine cost. It is concluded that treating an area of 6-20 hectares brings costs down to a reasonable level but treating smaller areas could also be profitable depending upon the crop.

Figure 5.14 Flame weeder (shutterstock)

- Steaming. The application of steam for weed control results in a minor reduction in water quantity and provide better canopy penetration compared to hot water (Figure 5.15). Effectiveness of this method is influenced by temperature of steam, weed species, duration of exposure and plant size. Perennial weed species have the ability to regenerate, thus it is necessary to repeat exposure. Seed coat of annual weed species can offer some protection to steam. Mobile soil steaming is commercially applied to manage weeds in the field and glasshouses for controlling both pathogens and weed and for sterilizing the soil. The interest was renewed for the steam sterilization methods as a result of related concern with the usage of the highly toxic methyl bromide. Steam is applied under pressure beneath metal pans forced down onto freshly formed beds for periods of 3-8 minutes. The steam raises the soil temperature to 70-100°C killing most weed seeds to a depth of at least 10 cm, however weed seeds below the treated layer are unaffected. If there is no further following cultivation treatment, weed control can remain effective for two seasons.

Figure 5.15 Weed killing steamer (shutterstock)

- Solarization.Solarization is a preventive process that exploits the heat of the sun for controlling the weeds. For this, a black or clear plastic cover is laid over the soil surface to trap solar radiation (Figure 5.16). The increased soil temperature kills plants, seeds, plant pathogens and different life stages of pests, thus high soil temperature is declared as a soil disinfection technique. For effective solarization method warm, moist soil and intense radiation is required, that lasts throughout the day. Moisture of soil is required for an effective process. Therefore, irrigation of soil before solarization is necessary. It is also concluded, that the success of soil solarization does not depend on the peak temperature measured in soil but rather on the duration of temperature above a certain threshold (45°C) day by day. For retaining the weed control effect of solarization, the soil must not be cultivated subsequently because otherwise weed seeds present in deeper soil layers (less affected by heating) are brought up to the soil surface and can germinate.

Figure 5.16Solarization as a tool in weed management (shutterstock)

- Infrared radiation.The burner applied in this method uses infrared radiation (IR) to kill the weeds. The burner heats ceramic and metal surfaces that radiate the heat (in the form of IR) towards the weed plants. A ceramic disc heated by gas from a small butane cylinder generates IR when incandescent. Then the so called ‘hot spear’ (projecting metal spike) heated is pressed into the center of the plant to be destroyed and held there for a few seconds (for most weeds. about 1.5 second is enough, however for harder plants increase of time is needed). The intense heat boils the moisture in the plants’ cells that results in them bursting. The leaves will wilt and turn a darker green immediately after treatment. Moreover, the method damages the proteins in the cells, thus in absence of photosynthesis the plant will die. Infrared weeders have the disadvantages of needing time to heat up, the IR panels are sensitive to mechanical damage, and they are more expensive than flame weeders. However, unlike flame weeders, they can be used in situations where an open flame would be extremely dangerous.

- Direct heat.Before application of direct heat for destroying weed seeds in field soil, the soil is cultivated and set in ridges. The worked ridge of soil is lifted, passed through a chamber heated to 68-70 °C by a diesel-fired burner, and then placed back onto the ground, thus providing a band of weed free soil. The depth of treatment ranges from 10 cm for shallow rooted crops to 25 cm for potatoes. The dry heat system comparing to steaming allows faster coverage of a filed.

- Electrocution.There are two types of the systems used for the electrical treatment. The „spark discharge” method  applied high-voltage, short-duration pulses (e.g. 25–60 kV, 1–3 μs) for weed control, plant thinning and the acceleration of ripening. The „continuous contact” method applies a metal applicator connected to a high-voltage source (e.g. 15 kV, 54kW, 30 Ampere). Electric current flows in a closed circuit through the plants into their roots, through communicating roots into neighboring plants, and from there back into a current collector at the soil surface. In such a circuit, the plant forms a resistance. The electric voltage damages the chlorophyll of the touched plants and kills the plant cells. This method is used for pruning and desiccation of root crop foliage, as well as area wide weed control and row crop thinning.

- Freezing (cryogenic weed control).Two different media are applied for the freezing treatments: liquid nitrogen and carbon dioxide snow (dry ice). The cryogenic system applies liquid nitrogen to target weeds through a modified sprayer and then crushes the weeds with a ballasted mechanical roller. Liquid nitrogen is more effective than carbon dioxide, however neither is as effective as flaming. Freezing is only advantageous where there is an obvious fire risk from flaming.

There is some other thermal weed control technics applying infrared radiation, microwave radiation, electrostatic field, irradiation, lasers or ultraviolet light, however these methods are not detailed in this chapter.

Mechanical weed control

A wide range of mechanical weeders from basic hand tools to tractor driven devices are availabe for farmers. These include cultivating tools (i.e. hoes, harrows, tines and brush weeders), cutting tools (i.e. mowers and strimmers) and implements (i.e. thistle-bars) that perform both. Basically, complete burial of seedling weeds to 1 cm depth and cut them at or close to the soil surface is the most effective mechanical method of weed management. Crop and weed population determines essentially the type of implement and the timing/frequency of its application providing effective weed control. For example, fixed harrows are more suitable for arable crops, while others like inter-row brush weeders may be more effective for horticultural use. Disadvantages of mechanical weed control include low work rates, delays due to wet conditions and the subsequent risk of weed control failure as weeds become larger. Weed control is not necessarily better at earlier weed stages because missing late germinating weeds can survive the treatment. The additional cultivations associated with mechanical weeding could harm soil structure and possibly encourage soil erosion. The increased mineralization of soil nitrogen due to cultivation can be a problem or an advantage for farmers.

- Hand tools. Removing weeds by hand is often the most effective way to prevent weed from spreading and therefore becoming a serious problem. Hand tools are more effective for annual rather than perennial weeds due to its capacity of vegetative reproduction. Manually operated weeders are classified as follows:

1. small tools: They are traditional hand-held type hoes applied by the farmers. Although these tools are appropriate for removing weeds between plants and are very effective, operation is only possible in squatting posture and has very low work output. Hand hoes, push hoes and other traditional methods of hand-weeding are still used worldwide in horticultural crops. Hand-weeding is often used after mechanical inter-row weeding to deal with the weeds left in the crop row. Application during the heat of the day in bright sunlight is the best, because under this weather condition weeds desiccate quickly. Recovery or survive of weeds in rainy weather and wet cloddy soils can be happened.
2. spades or chopping hoes: These weeders have straight, curved or pronged blades. Weeds are removed by digging, cutting and uprooting. These are operated in the bending posture. The operation is normally slow and tiring.
3. long handle tools: Long handle tools have a soil working tool fixed at the end of a 1.5 to 2 m long handle. These tools are operated in push, push-pull or pull mode, and in standing posture. These are designed to work under friable soil moisture conditions and give high work output at the early stages of crop growth when weeds are small.

- Harrows. Harrowing is a traditional form of mechanical weed control (Figure 5.19) for dealing with annual weeds but is ineffective against perennial and established deep-rooted weeds. For giving the crop an early advantage, killing the first emerging weeds by spring tine, chain or drag harrows, blind or pre-emergent harrowing can be carried out after drilling but before crop emergence. Early harrowing is successful in case of dry weather but soil moisture is adequate. Disadvantage of blind harrowing is the low efficiency if few weeds emerged and sometimes the slow crop emergence. Harrows also can be applied post-emergence, however, in this way it can cause crop injury. Increasing the working depth from 10 to 30 mm doubles the number of uprooted plants and is further improved by higher soil moisture and faster working speeds. Sorting action of tines increases with wider tines and slower forward speed, while throwing action increases with forward speed, working depth and tin width.

• Chain harrows with round and/or shuttle shaped links bury the weeds but do not pull them up. They are especially effective on light soils and prior to crop emergence, or in short crops. Tine weeders with either rigid or spring-loaded tines, superficially cultivate the whole soil surface and cause less crop damage. They are more effective on lighter soils and less successful on heavy land.


• Weeders fitted with flexible tines (flexi-tines) can be used selectively at the late tillering stage of cereals when the dense crop foliage forces the tines into the inter-row. It is the most effective when weeds are in white thread (weed that have germinated but not emerged) or cotyledon stage. Advantages of flexi-tines are fast speed operation, break of soil crusts, lifting of sections over crop without injury.


• Torsion weeders, with pairs of tines set either side of the crop row offer more precise inter-row. Crops must be extremely well-rooted with sufficient row spacing. Optimal crop stage for application of Torsion weeders is 2+ leaves and very well rooting.


• Rotary-tine weeders, with two ground-driven ‘star’ or ‘spider-tine’ rotors covering each row, also allow inter-row weed control. The angle of the rotors can be set to move soil away from, or towards the row; the latter ridging up the crop to bury small intra-row weeds.

Figure 5.17Harrows in weed management: Chain harrow (left - (shutterstock.com), Tine weeder (middle – I. Tirczka), Finger weeder (right – I. Tirczka)

- Tractor hoes. Tractor hoes cut through the soil at 2-4 cm depth by an ‘A’ or ‘L’ shaped fixed, vibrating or revolving. Increasing the working depth does little to improve weed kill, but higher forward speed increases soil covering of weeds and reduces survival. Soil structure is important: in rough soil weeds may continue to grow in the lumps of soil lifted by the hoe. Desiccation on the soil surface is a critical factor in preventing weed regeneration, and wet conditions after hoeing can decrease the level of control. Hoeing is particularly effective against mature weeds. Hoe weeders control weeds within the inter-row. The shares undercut everything, so it is necessary to steer the hoes very carefully between the crop rows. A good seedbed and precise drilling of the crop are prerequisites for successful hoeing. For avoiding the removal of significant number of crop plants and the covering them by soil, different types of protectors can be fitted. These may take the form of discs, plates or protective hoods.

The powered rotary hoe is PTO (power take-off) driven and fitted with rotating L-shaped blades on a horizontal axle (Figure 5.20). The width of the rotor can be adjusted to different row widths, thus more intensive cultivation of the soil can be performed and can deal with larger weeds. The rotary hoe serves two basic functions: (i) removing small weeds, and (ii) loosening crusted or compacted soil to aid in crop emergence. A further development has been the rotary ground driven weeder or rolling cultivator with usually two ground driven ‘star’ or ‘spider tine’ rotors covering each row. The rotary hoe causes very little disturbance of crop residue, thereby enhancing infiltration and preventing erosion. Its use is generally limited to large-seeded crops such as corn and soybeans, because these crops are planted relatively deep and have root systems that develop fast enough to anchor the young seedlings.

Figure 5.18 Rotary hoe (E. Takács)

- Brush weeders. The brush weeder (Figure 5.19) is primarily intended for inter-row weeding of vegetable crops, however application in cereals also can be performed. Two main types of brush hoe have been developed: (i) with disc brushes operating in the vertical plane on a horizontal axis, and (ii) with circular brushes operating in the horizontal plane on a vertical axis. In general, the brushes are made of fibreglass and are flexible. These weeders working very superficially mainly uproot but do also bury or break weeds. A protective shield panel or tent can be used to protect the crop. When using horizontal-axis brushes, their rotation speed should be only slightly faster than the tractor speed, otherwise too much dust will be generated. For brush hoe on a horizontal axis, working depth is the most important factor in ensuring good weed control. Tractor speed, brush velocity and soil conditions interact to determine the working depth. A higher rotational speed will not improve the effect; however, the bristles will wear out more rapidly. It has the advantage that it can be operated under moister soil conditions than a tractor steerage hoe. When the soil is too hard, the brush weeder will remove only the part of the weeds above the soil, and the weeds will readily regrow. Application on moist soil, the effect will diminish as a result of soil sticking to the bristles. Some models of vertical-axis brushes can have the angle, rpm and rotating direction of the brushes adjusted. Vertical-axis brushes can be adjusted to throw soil towards the crop row or to remove soil and weeds away from the row.

Figure 5.19 Brush weeder (shutterstock.com)

- Mowers, cutters and strimmers. These methods are commonly used in turf, and can be used in vineyards, in orchards, in pastures and in forage crops if used in the appropriate way. Where weeds are much taller than the crop it may be possible to ‘top’ the weed and at least prevent further seeding. Although, cutting and mowing techniques enable us to control the size of weeds and their seed production and to minimize the competition between weeds and crops. Hand held and wheeled strimmers offer the potential to cut down seedling and larger weeds pre-emergence overall, or post-emergence between the crop rows without disturbing the soil surface. These techniques are seldom efficient enough to obtain a total weed control. Cutting and mowing weeds reduces their leaf area, slows their growth and decreases or prevents seed production. Repeated mowing reduces weed competitive ability, depletes carbohydrate reserves in the roots, and prevents seed production. Some weeds, mowed when they are young, are readily consumed by livestock. Mowing (Figure 5.22) can kill or suppress annual, biennial and perennial weeds and help restrict their spread. A single mowing will not satisfactorily control most weeds; however, mowing three or four times per year over several years can greatly reduce and occasionally eliminate certain weeds. Regular mowing helps prevent weeds from establishing, spreading, and competing with desirable forage crops.

Table 5.4 The advantage and disadvantage of main implements applied in integrated weed management in ecological farming

Implement	Positive weed control effect	Negative weed control effect
Plough	Disrupts growth and seed production. Buries seeds produced this year and buries perennial weeds and their below ground root/stem systems.	Weed seeds from the seed bank are moved up to the soil surface.
Cultivator/Disc cultivator	Disrupts weed growth and seed production. Buries seeds produced this year and buries /fragments perennial weeds and their underground root/stem systems.	May stimulate shoot development from below ground root/stem systems of perennial weeds.
Harrow	Destroys/kills small weed plants. Fragmenting root/stem parts of perennial weeds near the soil surface.	Stimulates weed seed germination. May spread viable root/stem parts of perennial weeds.
Roller	Improves germination conditions for the crop.	Improves germination conditions for the weed seeds.
Weed harrow	Covers small weed plants with soil and/or uproots them.	Stimulates weed seed germination. May more or less damage the crop.
Inter-row cultivator	Covers small weed plants with soil, uproots them or cuts them off.	May damage the crop.
Brush weeder	Covers small weed plants with soil or uproots them.	May damage the crop.
Weed mower	Cuts of weeds in growing crops.	If used after stem elongation, the crop will be damaged.

Mulching

Mulch is a layer of various material applied to the soil surface. The mulch provides a physical barrier on the soil surface, blocks nearly all light reaching the surface. It keeps soil surface shaded and cool, reduces daily fluctuations of soil temperature, thus weeds emerging emerge under the mulch do not have sufficient light to survive. For example, when a cover crop is killed by extreme temperature, mowing, or rolling, their residues left on the soil surface as a mulch. Effectiveness is depending on the type of weed. For example, small-seeded broadleaf weeds sprouting is effectively blocked by a 2–3-inch-thick layer of cover crop residues. However, larger-seeded broadleaf seedlings, grass seedlings, and perennial weed shooting from buried rhizomes and tubers get through, but their growth can be delayed by residues of a high biomass cover crop. The mulch effect can be enhanced by the release of allelopathy substances from the decaying residues. Moreover, mulch provides habitat for ground beetles and other predators of weed seeds, as well as microorganisms that can attack and kill weed seedlings. There are different types of mulches according to the nature of the soil covering material: organic (leaves, grass clippings, peat moss, wood chips, bark chips, straw mulch, pine straw, biodegradable mulch, cardboard/newspaper) and synthetic (rubber, plastic, polypropylene and polyethylene, carpet, colored mulch). Mulches can be classified in the following way, as well:

- Sheeted mulches. Black polyethylene mulches are widely used for weed control in organic systems, however they are generally not practical for lower-valued, large-scale field crops. Plastic mulches have dual efficiency, they selectively filter out the photosynthetically active radiation (PAR) and let through infra red light to warm the soil (thermal weed control). Regarding the colour of the mulch it is concluded that white and green coverings had little effect on the weeds, however brown, black, blue, and white on black (double colour) films prevented weeds emerging. The latter has the advantage, that the higher rate of light reflectance is beneficial to the crop. Plastic and other durable mulches have the drawback of not degrade in field. Mulches made from paper (Figure 5.20), non-woven natural fibres and degradable plastics have the advantage of breaking down naturally and can be incorporated into the soil after use. Correct laying of the paper can avoid damage provided by rain or wind. There can be additional environmental benefits if the paper mulch is made from recycled materials such as cardboard cartons. In January 2018, the European Standard EN 17033: “Plastics–Biodegradable mulch films for use in agriculture and horticulture–Requirements and test methods” was released. The standard was developed by the European Committee for Standardization, Technical Committee CEN/TC 249 Plastics and applies to all European Union countries plus Macedonia, Norway, Sweden, Switzerland, Serbia, Turkey, and the United Kingdom. This standard regulates the requirements for biodegradable plastic mulch films (BDMs): their composition, biodegradability in soil, effect on the soil environment (ecotoxicity), mechanical and optical properties, and the test procedures for each of the listed categories. It does not apply to mulch films that are being removed from the fields after use.

Figure 5.20 Mulches made from paper (E. Takács)

- Living mulches (groundcovers). Living mulch consists of a dense stand of low growing species (Figure 5.21) established prior to or after the crop (i.e. undersowing of cereals with clover and grass) to slow the development of weeds and provide other benefits (nitrogen fixation, protecting soil from water and wind erosion, increase enemies of crop pests). Living mulches control weeds in two ways: When they are seeded before weed establishment, they suppress weeds by competition. In some situations, the allopathic properties of living mulches can be used to control weeds. It has been argued that annual weeds would provide a natural ground cover if managed properly. Living mulches are sometimes referred to as cover crops, but they grow at least part of the time simultaneously with the crop. Cover crops are generally killed off prior to crop establishment. Often, the primary purpose of a living mulch is that of improving soil structure, aiding nutrition or avoiding pest attack, and weed suppression may be just an added benefit. Disadvantages of living mulch is, that it competes for nutrients and water with the main crop and this can reduce yields. Although leguminous cover crops have large biomass production and turnover, they are not likely to increase soil organic matter. This is because legumes used as living mulches have greater N contents and a low C to N ratio. So when legume residue decomposes, soil microbes have sufficient N available to enhance their breakdown of organic materials in the soil. Thus, application of legumes is primarily recommended when there is already enough organic matter in the soil.

Figure 5.21 Living mulch (Marigold) and sugarcanes (shutterstock.com)

- Particle mulches. Particle mulches are composed of a mass of material spread on the ground, loose materials like straw, bark and composted municipal green waste (Figure 5.22). The particle mulch may be composed of compost, manure, straw, sawdust, rock, gravel, or any other material that covers the ground. Effectiveness of weed control is directly proportional with the thickness of the mulch layer. Weed seeds in the mulch itself can be a problem if the composting process has not been fully effective or there is contamination by windblown seeds. In straw mulches, volunteer cereal seedlings are a particular problem due to shed cereal grains and even whole ears remaining in the straw after crop harvest. There may be a risk of crop damage from herbicide or growth regulator residues remaining on straw from conventionally grown cereals. With particle mulches like straw that consist of light materials there is the possibility of them being blown around by the wind.

Figure 5.22 Particle mulch (shutterstock.com)

Biological weed control

Biological weed control methods apply living organisms, such as insects, nematodes, bacteria, or fungi, to reduce weed populations. Classical (or inoculative) control describes the introduction of host-specific, exotic natural enemies to control alien weeds. Inundative (or augmentative) control involves the mass production and release of native (usually) natural enemies against native (usually) weeds. The basic criteria for organic products are host specificity and durability. However, as weed populations of mixed species usually occur in the field, this (also) makes their practical applicability difficult. In a broader sense, allelopathy (secondary, inhibitory metabolic products produced by certain plants) is also included. Susceptible weeds will not die, but will suffer significant biological depreciation, so they will not be competitive partners for healthy crops. Preventive cultural practices, together with physical controls such as cultivation, flaming, and mulching, normally include into an organic farm’s weed management strategy, with biological products or agents playing at most a minor role. However, biological processes may contribute to the efficacy of practices such as cover cropping, mulching, crop rotation, and farm diversification in reducing weed pressure. Biological processes that can impact weeds include: (i) herbivory—direct consumption of weed seedlings, or foliage or roots of adult weeds, (ii) disease caused by bacteria, fungi, and other microorganisms, (iii) plant–soil–microorganism interactions that change weed vigor and competitiveness relative to the crop, (iv) allelopathy—suppression of weed growth by substances released by other plants, (v) weed seed consumption and (vi) weed seed decay.

It is essential to test biocontrol agents in detail for host specificity. Much of this is still in the research and discovery phase; however, some biological processes are sufficiently well understood and documented to be utilized as effective methods for enhancement the successful of the overall weed management program. In addition, many diversified farms utilize livestock and poultry as weed consumers, often to significant benefit.

- Allelopathy. It is the effect, when a plant releases natural substance that suppress or hinder weed seed germination and early growth (Figure 5.26). The origin of these substances can be: (i) excretion by living plant roots, (ii) leaching from foliage, and (iii) release during microbial decay of plant residues. These allelochemicals, some of which are potent enough to be considered nature’s herbicides, have the greatest impact on germinating seeds, seedlings, and young plants, retarding their growth, causing visible damage to roots or shoots, or even killing them outright. Many cover crops and a few vegetable varieties have been shown to exert significant allelopathic activity against weeds, especially young annual weeds. Cover crops in the brassica family, including rapeseed, mustards, and radishes, contain a number of compounds called mustard oil glycosides, which break down into powerful volatile allelochemicals called isothiocyanates during residue decomposition, which can affect plant growth as well as microbial activity. Well-documented examples within crops including rye, other cereal grains, sorghum, sorghum—sudangrass hybrids, forage radish and other brassicas, and sweet potatoes. Here is an example, that an allelopathic relationships can be quite specific. For example, sunflower root exudates inhibit seedling growth of wild mustard and other broadleaf weeds but have little effect on grasses. In no-till field trials, rye residues are strongly allelopathic against Amaranthus sp. and Chenopodium album, but not ragweed. There are some cases, when allelopathy is not so effective. Transplants and large seeds are less responsive to allelopathic suppression due to their deep plantation, the allelochemicals produced by a cover crop mulch are concentrated above the soil surface. As specific allelopathic relationships become better understood, crop rotations and cropping systems can be designed to give crops an edge over the major weeds present in a given field. Unlike direct competition, allelopathic weed suppression can persist for a few weeks after a cover crop is terminated. Tilling the top growth in as a green manure causes an intense but relatively brief burst of allelopathic activity throughout the till depth. Leaving the residues on the surface as an in situ mulch creates a shallow (less than 2.5 cm) but more persistent allelopathic zone that can last for three to ten weeks depending on weather conditions.

- Soil microbiota. The ability of the soil’s microbiota to influence the growth and competitiveness of weeds relative to crops has been a subject of much fascinating research. Plant–soil–microbe relationships are highly complex, and research findings have not yet been consistent enough to warrant recommendation of procedures to introduce, encourage, or limit certain soil microbes as weed control tactics.

Figure 5.23 The different release pathways and effects of allelochemicals. The allelopathy plant (left) can release allelochemicals through four pathways (blackarrows): leaching by rain (P1), decomposition of plant residues (P2), exudation from roots (P3) and volatilisation (P4) (Zhang et al., 2021).

Indirect weed control

Management of drainage and irrigation systems

Careful choice and maintenance of drainage and irrigation systems is an important preventive measure to reduce on-field weed infestation. Periodical clearance of weed vegetation established along ditches prevents it from invading the field. Where it is economically feasible, substitution of ditches with subterranean drains eliminates a potential source of weed infestation. Use of localized (e.g. trickle) irrigation systems favour crop development to the detriment of weeds. In contrast, broadcast irrigation systems often favour weeds because most of them have a higher water use efficiency (dry biomass production per unit water used for evapotranspiration) than the crop.

Tillage

One of the most important goals of all tillage processes, among other beneficial effects, has always been to reduce the stock of weed seeds in the soil and to deplete the reserve nutrient reserves of underground vegetative reproductive organs in perennial species. The weed seeds in the soil are placed in more favorable layers close to the soil for germination as a result of the disturbance, and the seedlings can be easily destroyed during a repeated tillage. The use of conventional tillage systems is of great importance in organic farming. It consists primarily of an autumn deep plowing or stubble plowing and then, in the spring of the following year, of the tillage procedures in preparation for sowing (disc, cultivator, harrow, combine, etc.). Later, in the vegetation, several inter-row additional mechanical weed control may become necessary (cultivator, weed comb, weed brush, spoke hoe, etc.). Soil cultivation or tillage, as an effective method has long been involved into control weed management. Various factors, like depth, timing and frequency of cultivation can influence different parameters of the weed population (composition, density and long-term persistence). However, similarly to other weed management methods, tillage also have conflicts. Finer seedbeds produce more weed seedlings but a smooth surface makes the direct weed control easier. Larger clods of soil produce fewer weed seedlings but the rough surface gives emerged weeds protection against direct weeding methods. Soil structure can be damaged by excessive cultivation that leads to erosion in longer term. Although, reduced tillage results in better control of soil erosion, conservation of soil moisture and more efficient use of fossil fuel, but not all soils are suitable for reduced tillage. Tillage is often divided into three forms primary, secondary and tertiary, but there are other cultivations that do not fall into these categories.

- Primary tillage. Primary tillage is the principal method chosen for cultivation prior to crop establishment. It is the first soil-working operation in cropping systems that is performed for preparing the soil for planting. Primary tillage is always aggressive and carried out at a considerable depth in order to control annual and/or perennial weeds by burying a portion of germinable seeds and/or propagules at depths at which weed seeds are not able to emerge. The main tools used to perform primary tillage are mould-board ploughs, disc ploughs, diggers, and chisel ploughs.

- Secondary tillage. Secondary tillage is used to prepare seedbeds and leave a level surface for drilling, thus the soil is not worked aggressively or deeply. The aim is to prepare the soil for planting or transplanting or it is used for carrying out the false seedbed. The equipment for secondary tillage are cultivators, harrows (disc, spring tine, radial blade, and rolling) and power take-off machines applied to a depth of 10 cm. In conservation tillage this equipment could be used as a substitute for ploughs in primary tillage. Conservation tillage is useful for conserving or increasing the organic matter content in the soil and for saving time, fuel and. Although, reduced tillage techniques could cause some problems with weeds, farmers can optimally alternate primary and secondary tillage in order to optimize soil management by changing mechanical actions year after year and thus improving annual and perennial weed species control. The timing of seedbed preparation affects weed populations considerably and is an opportunity to reduce weed numbers that emerge in the growing crop. One traditional method of weed control is the stale or false seedbed technique. Cultivation for seed bed preparation has two contrasting effects on weeds: (i) elimination the emerged vegetation resulting from after primary tillage, and (ii) stimulation of weed seed germination and consequent seedling emergence. Utilize these two effects can be achieved by false (stale) seed bed technique. A stale seedbed is a technique where a seedbed is prepared several days/weeks/moths before planting or transplanting crops in order to stimulate the emergence of weeds prior to sowing. The success of a stale seedbed depends on the length of time before planting and on weed spectrum. Late-emerging weeds will still be a potential problem. Application of the false seed bed technique can reduce weed emergence > 80% compared to standard seed bed preparation. The most important factor beside the temperature is the moisture of soil. In dry years the stale seedbe)d method does not serve as a good method of weed control without the intervention of irrigation. A novel method of reducing seedling emergence is to carry out the seedbed preparations in the dark to avoid stimulating weed seed germination, however this technique does not provide consequent results.

- Cultivation tillage. Cultivating tillage is performed after crop planting in order to achieve a shallow tillage which loosens the soil and controls weeds. For this purpose, cultivators are used which can control weeds in different ways. The complete or partial burial of weeds and their seeds can be an important cause of mortality. Another mode of action is by uprooting and breakage of the weed root contact with the soil. It is preferable to carry out cultivation tillage when the soil is not too wet because it can damage the soil structure and favor the spread of perennial weeds. Cultivators are generally classified according to their application in a crop: broadcast cultivators could be used both on and between the crop rows; inter-row cultivators are used only between crop rows; and intra-row cultivators which are used for removing weeds from the crop rows. For example, the methods against Cirsium arvense: With wire rope method the field is mounded up by using mounding equipment in place of plow. For tillage, the mounds or ridges are dragged down to a greater or lesser extent, depending on the crop, and sown with cereals, for example, or planted with field vegetables. While the seed is now emerging, but the roots of the crops are still short, the tilled ridges are undercut at the boundary between topsoil and subsoil with a wire rope stretched across the hill implement, thus cutting off the thistle shoots. Undercutting with the wire rope can be done both in the fall and in the spring.

Crop rotation

Crop rotation is a basic technique in organic farming to help pest and disease control and to provide optimum soil fertility, moreover weed control is achieved effectively by combining crop rotation with other cultural treatments. Crop rotation involves alternating different crops in a systematic sequence on the same land (Figure 5.27). Monoculture or high proportion of similar crops results in a weed species composition that are adapted to the growing conditions of the crop (for limiting the field thistle, the cereal content should be limited to a maximum of 50%). Rotating crops at different life cycles can disrupt the development of weed-crop associations, through different planting and harvest dates preventing weed establishment and therefore seed production. Since different crops favour different types of weed species, it is important to change between annual and perennial crops in the crop rotation. Autumn- and spring-sown annual crops also favour different types of weed species, which makes it important to rotate between such crops within a crop rotation. Traditionally, potato (Solanum tuberosum) is included in the rotation to reduce weed problems before a less competitive crop is grown. For an organic farmer, consideration of soil fertility level and including fertility building periods in rotation complicate the crop choice. The inclusion of a fallow period in the rotation in known to reduce perennial weeds. It is best to alternate legumes with grasses, spring planted crops with fall planted crops, row crops with close planted crops and heavy feeders with light feeders. Despite the use of rotations, some weeds have been identified as particular problems in organic farming systems. Couch grass (Elymus repens) and other creeping perennial grasses, and creeping thistle (Cirsium arvense) are often declared as the main problem weeds in all organic systems. Blackgrass (Alopecurus myosuroides) and Cirsium arvense can become more frequent when cereals form a significant part of the rotation. Docks (Rumex spp.) are a particular problem in grassland and bracken (Pteridium aquilinum), has become a severe problem in upland areas of pasture.

Figure 5.24 Possibilities for crop rotation (E. Takács)

Cultivar

It is not simply the choice of crop that influences weed development within a rotation, the characteristics of the cultivar such as morphology and growth rate can have a significant effect on both crop and weed development. Cultivar choice and crop seed rate can be effective in suppressing weeds and hence minimising weed control inputs, as well. For example, spring barley cv. Atem has taller development than cv. Triumph and has a major influence in its greater weed suppression. Similarly, number of weed species found on the plots were significantly reduced in the presence of the traditional longer strawed Maris Huntsman winter weed cultivar in contrast with Mercia cultivar. Morphological traits can influence the competitive ability of crops over weeds. For example, earliness of crop ground cover is vital in weed suppression, and research has indicated that larger initial crop seed size can significantly improve early crop establishment and hence increase the competitive ability of winter wheat cultivars. Identifying and quantifying the traits associated with competitive ability against weeds is indeed complicated by the fact that, although different cultivars have unique characteristics, many of these traits can change over development stage. However, differential rooting patterns, early vigour, leaf size and allelochemical properties may influence the ability of a cultivar to suppress weeds and be successfully selected in breeding programmes.

Intercropping

Intercropping process means to grow smother crop between rows of the main crop (Figure 5.25). Increased yield, not improved weed control, is probably the main benefit expected from intercropping. It is declared, that intercrops are able to suppress weeds, however it should be carefully applied. Without any attentiveness, intercrops can greatly reduce the yields of the main crop if competition for water or nutrients occurs. Similar to cover crops, intercrops increase the ecological diversity and use of natural resources by canopying, moreover compete better with weeds for light, water and nutrients. For example, a leek-celery intercrops sown in a row-by-row layout decrease relative soil cover of weeds by 41%, reduce the density and biomass of Senecio vulgaris by 58% and 98% respectively, and increase total crop yield by 10% compared to solo cropping. Increased weed suppression and crop yield has also been demonstrated in many environments for cereal-legume intercrops. As in the case of living mulching, the success of intercropping relies on the best match between the requirements of component species for light, water and nutrients, which increases resource use complementarity and reduces competition between the intercrops. In practice, this means optimizing intercrop spatial arrangement, relative plant densities and crop relative growth over time in any given environment.

Figure 5.25 Sugarcane intercropping with cabbage or cauliflower. (shutterstock.com)

Fertilization

Nutrient level of soil in agro-ecosystem is altered by application of fertilizers, thus they directly affect weed population dynamics and crop-weed competitions. Numerous weeds are high consumers of nitrogen and therefore able to reduce the availability of nitrogen for crop growth. Strong effects in weed control can be detected by timing, dosage and placement of fertilizer. Organic farming uses organic manure and compost to replenish nutrients, which, as a consequence of improper treatment, have a “weed-growing” effect on the viable weed seeds in it. It is known that weeds absorb nutrients earlier and in greater amounts than their associated crops, so they need to be treated very carefully with nutrient replenishment.

Cover crops

Cover crops include a wide range of plants grown for various ecological reasons and cover the soil. Cover crops (Figure 5.26) suppress weeds by competing for resources, moreover their residues laying on the surface of the soil inhibit weeds through physical (barrier to weed emergence and establishment, increase of space for normal development of weeds), biotic (blocking of light, avoidance of temperature fluctuation, alteration of moisture conditions necessary for germination) and allelopathic interactions (compound released from living or decaying plant tissue). In general, the larger the cover crop and greater the biomass or dry matter production, the greater the impact on weeds. Despite these potential benefits, physical and biochemical effects from cover crops may not provide adequate weed control. Weed suppression by cover crop residue can vary from negligible to highly effective for anywhere from two weeks to several months, depending on cover crop biomass and nitrogen (N) content, season, weather, and soil conditions. Warm, moist weather combined with high soil biological activity accelerates decomposition of cover crop residues and their allelochemicals, thus shortening the weed control period. Strawy, low N residues last longer than succulent, high-N residues. Use mechanical control tactics and cultural controls to complement cover crops for weed management. The inclusion of cover crops such as rye, red clover, buckwheat and oilseed radish, over wintering crops (i.e. winter wheat) or forages in the cropping system can suppress weed growth. Highly competitive crops may be grown as short duration 'smother' crops within the rotation. When choosing a cover crop, consideration should always be given to how the cover crop will affect the succeeding crop. Examples of highly weed suppressive cover crops are rye, sorghum, kale, rocket and mustard. In contrast, although direct weed suppression by legumes can be significant, their residual weed control effect is usually lower because the high quantity of N released from their residues after cover crop destruction stimulates weed emergence, especially when legumes are used as a green manure.

Figure 5.26 Benefit of cover crops (E. Takács)

Sanitation

It is possible to prevent many new weeds from being introduced onto the farm and to prevent existing weeds from producing large quantities of seed. The use of clean seed, mowing weeds around the edges of fields or after harvest to prevent weeds from going to seed, and thoroughly composting manure before application can greatly reduce the introduction of weed seeds and difficult weed species. It is even possible to selectively hand-eradicate isolated outbreaks of new weeds, effectively avoiding future infestations. Planting clean, high-quality seed is essential to crop success. Other sanitation factors to consider would include thorough cleaning of any machinery which might have been used in weedy fields or washing stations, and the establishment of hedgerows to limit windblown seeds.