Mechanical weed control: practical concepts for root crops

The success of mechanical weed control methods is influenced by numerous factors such as the weather, the soil type, the soil condition and the species of weeds as well as their development stage. The machine settings, the working speed, the scheduling of the measures and ultimately also the farmers themselves are equally crucial. Despite modern, functioning technology, it can occur that the desired control success is not always forthcoming and the anticipated work rate is not achieved. 

So that the harrowing and hoeing technology can operate precisely and without disturbances, the following points must be observed even prior to and during sowing: 

• Attention must be paid to careful primary cultivation, seedbed preparation and sowing. A level, well re-compacted seedbed without deep tractor tracks and consistent seed depth placement are ­extremely important for the precise use of mechanical weed control methods.
• ‘Light’ tractors with wide tyres or two tyres as well as tyre pressure control systems should be used in soil tillage wherever possible. This enables deep ruts and harmful soil compaction to be reduced.
• Rolling after sowing may prove sensible if the seedbed is too rough. This enables the establishment of better working conditions for the subsequent mechanical methods. Rolling must be weighed up in locations that are at risk from erosion.
• In the case of primary cultivation without the use of a plough and mulching, excessively coarse and long plant residues can impede the work of harrows and coulter hoes. Plant residues can build up or the hoeing tools can become clogged. The use of mulchers, cutter rollers or cutting tools to shred the organic substance improves the performance of the mechanical control implements.
• It is crucial to coordinate the sowing and hoeing technology. To do this, the number of sowing and hoeing units must match. To ensure disturbance-free operation of the hoeing tools, the sowing units must be set to the precise row spacing and the track markers must be adjusted correctly for precise row closure. This is where the most frequent errors occur by far in practice.
• Sowing with parallel guide systems is extremely advantageous for the subsequent work with a coulter hoe. Sowing in absolutely straight rows can extensively relieve the driver, particularly in the case of coulter hoes without a steering system.
• At the edge of fields, the first row of seeds must be planted a sufficient distance away from the edge so that the outer coulter hoe unit can operate undisturbed. 

A high quality of work with a high work rate at the same time is important to many farmers. However, this can quickly result in conflicting aims in mechanical weed control methods. The basic principle is that quality of work takes precedence over the work rate.

• The time required to correctly set the implements and to regularly check the success of control and crop losses during the work is regularly underestimated. The conditions can vary extensively from field to field. Constant checks are therefore required during harrowing and hoeing and the machine settings have to be adjusted time and again. There are no generally applicable implement settings or working speed.
• When using tine harrows and coulter hoes in sensitive crops or during early stages of development, a reduced working speed of around 3 and 5 km/h often has to be implemented to prevent crop damage. This reduces the work rate.
• Only limited time slots might be available due to weather conditions. Efficient technology that is adapted to the area to be worked must therefore be provided. Using hoes or harrows early in the morning in dew-covered crops, for instance, is only recommended to a limited extent or not at all, and may only be possible as of the late afternoon at the earliest.
• Phases of unsettled weather with rainfall usually prohibit the use of mechanical methods. As a result of this, scheduled or due work cannot be completed on time, leading to the risk that the weeds ­develop too extensively and can then no longer be controlled in a satisfactory manner.
• Planning and scheduling the required measures necessitate continuously monitoring the weather and checking the fields as regards soil condition and weed and crop development.
• Settings for and the implementation of mechanical methods should only be undertaken by suitable, qualified staff who have good knowledge of crop production and also come equipped with calmness, experience, motivation and the ‘finesse’ needed to handle this technology. 

• It must essentially be remembered that the controlling effect of tine harrows is based largely on uprooting and covering the still small weeds during the filament and cotyledon stages. High efficiencies of over 80 percent per pass can be achieved with the harrows in these stages. The removing and covering effect quickly decreases as the size of the weeds increases.
• Regular field inspections and precise monitoring of weather forecasts are needed to schedule the use of harrows specifically in the filament to cotyledon stages. Black grass, wind bent grass, common lamb’s quarter or knotweed species, for instance, can only be controlled most effectively up to the cotyledon stage.
• A sheet of glass or Perspex placed on the ground can help when inspecting the field. The weeds emerge earlier under the sheet and therefore make it easier to schedule harrowing measures, particularly for blind harrowing prior to emergence.
• It must also be taken into account that each soil movement encourages further weed seeds to germinate. Harrowing should therefore be carried out as shallow as possible. The following harrowing operations must be aimed at re-germinating weeds.
• The covering effect of the harrow declines significantly with larger weed plants approximately as of the second foliage leaf pair. Whilst it can be increased by increasing the working speed, this comes at the expense of crop compatibility. The removal of weed plants in advanced stages declines considerably due to their more extensive roots.
• The desired intensity of the harrow is varied via the working speed, the working depth and, ­depending on make, by means of either the spring pre-load or the setting angle of the tines. The harrow settings and working speed must be regularly checked on the field and adapted if necessary.
• However, crop losses of between 1 and 3 percent per harrow pass are often unavoidable depending on the crop type and size. In the event of significantly higher losses, the settings and working speed must be corrected. The possible harrowing losses should be taken into consideration during sowing by increasing the seeding rate by approximately 10 – 15 percent.
• There are no generally valid specifications for the settings or driving speed. Numerous aspects such as the make of harrow, soil type, soil condition, weed size, crop type and size, for instance, are ­determining factors.
• Driving very slowly in speed ranges from approximately 2 to 5 km/h is necessary during early, sensitive crop stages. Working speeds of around 5 to 10 km/h are possible in larger crop stages or when blind harrowing crops that are sown deeper.
• In order to minimise crop losses due to breaking off, care measures should only be undertaken as of approximately the late morning, when the cell pressure in the plants decreases and the plants are more elastic.
• Wherever possible, the work should also be carried out in sunny and windy weather in order to ­ensure that the weeds are dried quickly.
• The soil surface should be dry and free-flowing at the time of harrowing. Harrowing should be ­forgone in the case of excessively moist soil conditions, unsettled weather and low evaporation. Harrowing must only be undertaken when navigability is ensured, the soil is free-flowing and the weather is frost-free, dry and sunny.
• Newcomers, above all, often use harrows too often because they are concerned about excessive residual weed development. Depending on the type of crop and the development stage, excessive harrowing intensities can lead to growth impairments that impact on yields. It is also ecologically and economically nonsensical to harrow the very last dead nettle or the very last pansy from the crop. Consideration must also be given to the protection of ground-nesting birds and small game. 

• Hoeing should generally be carried out as shallow as possible. However, the objective of cutting the weeds off over the entire surface must always be ensured. Excessively deep cultivation must be avoided, as seeds are increasingly encouraged to germinate on the one hand and water loss is ­increased on the other hand. The erosion risk factor must also be taken into account.
• A variety of different hoeing tools such as double-disc coulters, flat hoeing blades or angled blades are available for the area between the crop rows. They must be coordinated to the soil types that occur in the region. If several coulters are used between the rows, sufficient coulter overlap must be ensured to guarantee full-area hoeing.
• The objective is to guide coulters as close to the crop as possible without covering or damaging it. Protective discs or protective plates on the coulter hoe are therefore required in small crop stages. This particularly applies in the case of camera-guided coulter hoes. Higher working speeds can be achieved with these, as a result of which the risk of covering increases.
• As for harrowing, the time selected for hoeing should be the late morning or the early afternoon. Sunny weather ensures that the weeds are dried quickly.
• When using coulter hoes with camera control, it must be ensured that the tractor’s lower link side struts are locked to avoid undesired lateral movements. The lifting struts must be set so that the hoe is guided parallel to the surface of the soil and can plunge evenly into the soil. The upper links can be used to adjust the hoe horizontally or set it gently against the tip of the hoeing shares in more difficult soil conditions.
• Numerous tools are also available for controlling weeds within the crop rows. Depending on soil type, crop and development stage, these include finger hoes, roller harrows, torsion spring tines, trailing harrows and flat ridgers as well as various other ridging bodies.

Pseudo tillage’ can prove suitable if the pressure caused by weeds is expected to be high. Primary cultivation, followed directly by seedbed preparation (pseudo tillage), is carried out three to four weeks before the crop is scheduled to be sown. The subsequently emerging waves of weeds can then be controlled with harrows or other soil tillage implements that operate at shallow levels before the actual sowing. The tillage encourages other weed seeds to germinate. Depending on weed pressure, these are controlled either with a further pass or by tillage during crop sowing. The selected tillage depth should be as shallow as possible in order to not impair the moisture conditions for seed germination. Prior to tillage, however, the depth from which the weeds are germinating must be checked in order to control as many of the seedlings as possible. Knotweed species, in particular, can germinate from deeper layers of soil. Deeper tillage may then possibly be required after all. Flame weeding technology can also be used as an alternative, particularly for weeds that emerge from deeper layers of soil. This would offer the advantage that further weed seeds are not encouraged to germinate by excessively deep soil tillage and that the soil does not dry out too much. This method is comparatively expensive, however.

The necessity of a pseudo seedbed must be weighed up, because disadvantages such as an ­increased risk of erosion due to wind and water as well as the drying out of the seedbed have to be taken into consideration.

As maize is very weak in the face of competition from weeds during early growth, mechanical control has a significant influence of the success of cultivation. Additional preventative weed control measures must also be taken into consideration. 

Give consideration to preventative measures

• The objective is to achieve rapid emergence and early growth after sowing so that the maize has and maintains a growth advantage over the weeds.
• For rapid emergence, it is crucial that the soil has reached a temperature of at least 10 to 12 °C at a depth of 5 to 6 cm by the time of sowing. Sowing should additionally be carried out during a prolonged warm weather phase. Later sowing dates should therefore be scheduled and maize varieties that mature early on should be selected accordingly.
• Preference is to be given to quick-growing maize varieties that are able to tolerate low temperatures. Species with a planophile leaf arrangement lead to earlier ground coverage and are therefore able to suppress the weeds better. The state organic grain and silage maize trials conducted by the Landwirtschaftskammer Niedersachsen also demonstrate noticeable differences between varieties.
• A pseudo seedbed (see section 2.1.5) can prove suitable if the pressure caused by weeds is expected to be high. 

Torsion spring tines

Torsion spring tines are another additional tool for the coulter hoes. Sprung tines whose ends are slightly angled work on both sides of the crop row. Both tines operate by vibrating at a flat setting angle. The inclination and the distance from the crop are adjustable. The weeds are covered or also exposed so that they can dry out. Good control success is to be anticipated, particularly on looser, more sandy soils. 

Roller harrows

On roller harrows, steel tines are cast into a plastic disc in a star formation. The star-shaped tools are set at an angle of 30 ° diagonal to the direction of travel. The setting angle can also be adjusted. During operation, they are caused to rotate and comb through the soil over the entire surface. The weeds are torn out, but are primarily covered.

Thermal weed control in maize 

In locations with very high weed infestation, thermal control can be implemented following the emergence of the maize. The Organic Farming department of Landwirtschaftskammer Niedersachsen has tested this method in three-year trials.

The trials showed that flame weeding with a flame weeder that operates over the entire surface is possible without problems in the maize plants’ 1st foliage leaf stage (BBCH 11). Only the leaf that has formed at the side dies off due to the heat. The shoot of the young maize plant remains vital. The maize plants regenerate comparatively quickly after flame weeding by forming new leaves. Plant losses do not occur in this stage, and no retarded growth was ascertained. As the weeds are also usually small during this early maize stage, very high control success of 80 to 90 percent has been achieved.

Later flame weeding dates were also investigated. It was shown here that the maize can still be flame weeded over the entire surface in the 2nd foliage leaf stage (BBCH 12). Only very slight maize losses occurred. However, the regeneration of new leaves took longer and the maize needed more time to make up for the growth gap again compared to the control. Control success was around 80 percent. The advantage of flame weeding is that neither harrowing nor hoeing are carried out until it is undertaken, meaning that no further weed seeds are encouraged to germinate beforehand or due to the actual flame weeding. In the trials, one to two subsequent passes with coulter hoes were usually sufficient to control late weed development.

This method becomes more interesting if row flame weeding technology is available. The space between the maize rows could then be hoed mechanically and only the maize rows themselves flame weeded. This would significantly reduce gas costs and control success within the maize rows would be ensured.
 

Only mechanical methods and preventative measures have to be applied in organic sugar beet growing. At present, weed control can only be undertaken satisfactorily in organic sugar beet growing with the additional use of a manual hoe. In addition to the high costs, which significantly impact on the growers’ profitability, it is above all the availability of seasonal workers that poses a major problem. High control success therefore has to be achieved through preventative and mechanical measures, a demanding goal.

During their early growth, sugar beets are comparatively weak in the face of competition. They only achieve a high level of shading and therefore high weed suppression capacity when the rows close. The basic principle for ­mechanical methods is to always mechanically control the emerging weeds in the early stages, from the cotyledon stage to around the 1st foliage leaf pair at the latest, so that the beets always have a growth advantage.

Pseudo seedbed prior to sowing

A pseudo seedbed is suitable for controlling weeds prior to actual sowing. This method has already been described in section 2.1.5.

Thermal control prior to sugar beet emergence

The thermal control of weeds or potato halum using gas flame weeding technology is one of the standard measures used in organic field vegetable and potato growing. The use of flame weeding technology can also be used in beet growing prior to the emergence of the beets. The advantage of thermal control is that no further weed seeds are encouraged to germinate. Particularly in humic, more sandy locations in which the soil heats up faster and high weed pressure occurs early on, flame weeding prior to the emergence of the beets can lead to high control success and is to be given preference over blind harrowing. Using Farmflex rollers as coverers is recommended when sowing beets, as they leave closed sowing grooves behind. Conversely, finger pressure rollers leave behind a relatively open sowing groove, as a result of which the beet pills, which have already begun to sprout, can be significantly damaged by flame weeding.

Unlike in conventional potato growing, in which the finished ridges are often shaped during planting, the ridges are usually only built up over the course of the growth phase in organic farming. The basic principle involves repeated cultivation of the potato ridges by alternately harrowing and earthing up. The objective is to control the emerging waves of weeds at an early stage by exposing and covering them.

Various implements are available for cultivating the ridges. Besides tine harrows and chain-link harrows, various ridging implements such as share and disc ridgers as well as roller hoes with different ridge-forming plates can also be used. Rotary ridgers are preferably used on loamy soils.

Start with the first weeds

Weed control should be started after planting as soon as the first weeds are in the filament stage, i.e. just prior to emergence, or in the cotyledon stage. The control measures must then be regularly repeated with each newly emerging wave of weeds. Depending on the weather, the time span between the individual measures can be between five and 14 days. If the control measures are delayed for too long, the weeds increasingly develop leaf and root mass and can then no longer be controlled reliably. 

Avoid injuries

Following the emergence of the potatoes, care must be taken during the control measures not to damage the lateral roots and stolons, and later on the developing potato nest. The tools must be set to operate at a shallow depth.

Excessive coverage of the potato leaves must be avoided on earthing up. However, differences between varieties must also be observed in this case. Varieties with leaves lying low down on the ridge are more sensitive than varieties with stems that are positioned steeper.

A staged ridge structure is usually easy to implement in lighter soils. In more loamy soils, the ridge is usually built up with a rotary ridger as the potatoes emerge. Covering the young plants in the rosette stage is not a problem. They usually grow out of the ridge again quickly. Further waves of weeds can then be controlled by harrowing and earthing up over the continued course of growth.

• DLG Organic Farming Committee
• Markus Mücke, Consultant Organic Crop Production, Landwirtschaftskammer Niedersachsen
• Christian Kreikenbohm, Consultant Organic Crop Production, Landwirtschaftskammer Niedersachsen

Editorial office 

• Dr. Achim Schaffner, Project Manager Organic Farming, DLG e.V.

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Title image: Markus Mücke, Landwirtschaftskammer Niedersachsen