Weed control in organic farming

Photo: Tomas Fibiger Nørfelt

Emerging weeds are particularly sensitive to mechanical impact just before they break the soil surface.

Blind cultivation is a cheap and effective way to control weeds when done correctly. Blind harrowing should always be done.

Tips for blind harrowing

• Always blind harrow three to six days after sowing
• Blind harrow at a depth of approximately two cm
• Adjust the speed so that the harrow does not bounce and the tines move quivering but not jumping
• Check regularly that you are achieving the desired effect and that the crop is not damaged
• Be extra careful with blind harrowing of legume seeds during germination - the sprouts can break.

Most weed seeds germinate from the top 3 cm of the soil, and as long as the small sprouts have not developed cotyledons, it only takes a very small mechanical impact for them to die or dry out, see photo 1 above.

In addition to controlling weeds in the crop row, blind harrowing also provides a better opportunity for later weeding or row cleaning at the most effective time to control the weeds.

Photo: Tomas Fibiger Nørfelt

Just as the crop is germinating, you can carefully weed harrow for the first time. Note that when the crop has one fully developed leaf, the kernel is almost depleted of energy and the grain can hardly tolerate a cover crop. Therefore, do not weed harrow around the 1-1½ leaf stage.

Weed harrowing should be performed if there is a high weed pressure. The size of the weeds determines the effect of the control, and the effect is greatest just when the weeds have germinated. If the first weeding is done when the crop is in the 2- to 3-leaf stage, the weeds will often have grown too large for you to achieve the full effect of weeding.

Tips for weed harrowing

• The first weeding is done at a depth of approximately two cm when the crop has just emerged and is up to two cm high. In some cases, some of the crop leaves may be covered, but this is not the goal as it reduces the competitiveness of the crop against the weeds.
• Adjust the speed and pressure of the harrow tines so that the harrow does not bounce and the tines move quivering but not jumping.
• The second weeding is usually carried out 10 days after the first weeding. Here, the crop will typically be in the bushing stage and can tolerate a slightly more vigorous harrowing.
• Check regularly that the desired effect is achieved and that the crop is not covered too much.
• Weeding when weeds have leaves is often not worth the effort.

Weeding in cereals:

• Cereals can be weeded with caution at crop emergence
  Cereals with one fully developed leaf are not very tolerant of being covered.

Weeding in pulses:

• Peas and broad beans that are more than 4 cm tall and have two or more leaves can tolerate a slightly more vigorous harrowing at a depth of 3 cm.

Setting the harrow

For both blind and weed harrowing, the speed should be adjusted so that the harrow does not bounce and the tines move in a quivering, not jumping motion. The inclination of the harrow tines should generally be around 45 degrees and the pressure adjusted so that the tines move at a depth of around 2 cm.

Photo: Lars Egelund Olsen

A good, even seedbed is crucial for the efficiency of row cleaning.

In fields with moderate to high weed pressure, row cleaning can be a good method of weed control.

Advice for row cleaning

• An even seedbed is crucial for effective row cleaning
• Blind harrow always prior to row cleaning. This increases the chance of optimal row cleaning.
• Crops on increased row spacing should be row cleaned. More space and light also makes room for more weeds
• Row clean the 1st time at a depth of approximately 2 cm when the crop rows are visible. Be careful not to cover the crop
• If necessary, clean the row a second time to a depth of around 3 cm and if necessary, add a little soil into the crop row
• Allow plenty of time. Row cleaning can be time-consuming
• Hills, uneven surfaces and many stones will reduce the capacity and efficiency of row cleaning
• Row cleaning does not control root weeds, but can inhibit the propagation of root weeds

The timing of row cleaning is crucial

Weed seedlings are the most vulnerable to row cleaning and have less root hold to avoid the row cleaner. The seedlings will also be sensitive to soil cover inside the crop row. Row cleaning can control slightly larger weeds between crop rows, but the effect is greatest at the cotyledon stage.

Once the weeds have leafed out and the population is high, the camera on the row cleaner may have difficulty finding the crop rows.

Row cleaning in spring crops

• Blind harrowing is performed approximately three to six days after sowing.
• The first row cleaning is done when the grain has two leaves.
• The second row cleaning is carried out as needed. Here it is a good idea to mould some soil into the row to achieve weed control there.

Row cleaning in winter cereals

• The first row cleaning is carried out in October or November when the crop rows are visible.
• The second row cleaning is done in early April when the soil is suitable.

Most winter wheat varieties compete poorly against weeds, so row cleaning is usually profitable and a good idea to avoid weed propagation. Row cleaning should be considered especially for early sowing of winter wheat.

With late sowing in October, weed emergence and the need will be less. If possible, perform a blind harrowing about 7 days after sowing.

If winter rye is sown in late September or early October, row cleaning is not necessary due to its strong spring growth.

Hybrid rye is usually sown around 15 September. The early sowing date results in greater weed germination and therefore row cleaning will usually be economically viable.

Row cleaning of grain legumes

In some growth phases, legumes are quite open and compete poorly with weeds. Therefore, it will often be an economic advantage to sow at 25 cm and carry out one or two blind cultivations and one to three row cleanings.

• The first row cleaning is done when it is possible to push a small amount of soil into the rows with the row cleaner without covering the crop. This is usually when the crop has two to four leaves.
• The second row cleaning is done about 10 days later. Here, you should mould 5 cm of soil into the seed rows to cover or inhibit the weeds that have sprouted inside the seed rows.
• In field peas, the last row cleaning is carried out when the catch wires from one row touch the catch wires of pea plants in another row.
• In broad beans, the last row cleaning is performed when the frame of the row cleaner touches the top of the plants.

Trial results with row cleaning

Organic trials in 2015-2016 have shown that the effect of a well-executed row cleaning is over 60% of the additional yield that can be achieved by removing all weeds from the field.

When there is a high prevalence of aggressive weed species, row cleaning can effectively protect the crop, and when done correctly, row cleaning can provide significant additional yields. At lower weed pressure, there will often be no additional yield from row cleaning.

The crop is usually sown at 25 cm row spacing and row cleaning is carried out with 15-18 cm between rows. If done correctly, row cleaning has an almost 100% effect on weeds between rows, but can also have an effect on weeds in the cotyledon stage in the row if a little soil is added during row cleaning.

Camera equipment to control the cleaner and GPS equipment for the tractor increase capacity and can be a great help on larger areas. In organic trials, row cleaning has been shown to reduce weed coverage of the soil at harvest by 60-70% compared to untreated or blind harrowing.