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No side effects with mechanical control of root weeds in organic fields

A 4-year trial in Norway has highlighted the effect of various mechanical weeding methods against root weeds on organic fields. The team of researchers from Denmark, Norway and Sweden also investigated whether mechanical weed control, in addition to an effect on weeds, could also increase disease infestation, adversely affect soil structure or affect the final yield.

2020.06.26 | Camilla Brodam

Photo: AU Foto

Organic farmers are in dire need of methods that can effectively combat perennial weeds - also called root weeds. A commonly used method of management is mechanical tillage. Researchers from Denmark, Norway and Sweden have, in a 4-year trial in Norway, investigated which mechanical treatments that works best for different root weed species, as well as how the mechanical tillage affects soil structure, diseases, and yield.

“It is a rather interesting experiment because it is rare in the literature that there are several elements at the same time. Often, we only look at weeds or diseases, but here we have included several elements at once, so that we can see if the mechanical treatments have derived and undesirable effects on soil structure and grain diseases,” explains Associate Professor Bo Melander from the Department of Agroecology at Aarhus university.

Different strategies

The Norwegian experiment was made in organically grown cereals under organic growing conditions and the focus was on common couch and perennial sow thistle, which dominated the experimental area. In total, the researchers investigated six different strategies to manage the root weed:

  1. No specific treatment for root weeds - control field
  2. Root cutting with minimal soil disturbance in the autumn
  3. Row cleaning in the grain sown at 24 cm's row spacing
  4. Row cleaning in the grain combined with plate harrowing in the autumn
  5. Harrowing only in spring
  6. Harrowing with the Danish Kvik-Up harrow both autumn and spring

“Couch grass was one of the worst weed species we encountered in the experiment. Our results show that mechanical treatments are needed in the autumn if a sufficient effect against couch grass is to be achieved in the subsequent growing season. This means that you have to do tillage after the grain has been harvested, which is not so new compared to the Danish experience,” explains Bo Melander.

But tillage has to take place at some depth to have an effect. This way you will be able to destroy the root canals, have them split, starved and dried out.

Delayed seeding

“The other strategies we used did not have the same good effect on couch grass. In addition, we tried a strategy in which we delayed sowing a little in the spring and tried intensive tillage to combat the weeds pre-seeding, but it did not turn out so well. Postponing the sowing is problematic even in Denmark, and in Norway they have an even shorter growing season, so on there it is even more problematic, the yield was very effected by that. Our conclusion was that autumn treatment is needed if you want to have a proper effect on the couch grass the following year. The biggest news, in fact, was that the treatments did not adversely affect the soil structure, nor was any increased disease attacks observed in any of the treatments,” says Bo Melander.

And if the farmer also has the opportunity to clean the rows, he will, according to the researchers, be able to grow the grain at a slightly increased row distance with great benefits. It will only have a positive effect on the general weed control during the cultivation season, but no long-term effect on root weeds should be expected. In Denmark, according to researchers, more and more organic farmers are resorting to row cleaning rather than weed harrowing, or using weed harrowing as a supplement to row cleaning.

Perennial sow thistle is a hard nut to crack

Although the researchers found good methods for couch grass, perennial sow thistle was significantly more difficult to combat.

“We had a lot of perennial sow thistles in the Norwegian trial, but none of the six treatments were particularly successful on this particular weed species. The stock never really came down during the 4 years the trial lasted. This is in line with the Danish experience. It's a troublesome species to deal with. In Denmark we have good experience with double plowing, i.e. plowing after harvest and then again in the spring. In Denmark, it has been shown to be quite effective against thistles, and I also believe that it has an effect on perennial sow thistle. It also does not like a slightly delayed spring plowing, we have seen in Danish conditions,” explains Bo Melander.

The Norwegian experiment shows that autumn treatments are needed if you want to get rid of couch grass, but they also show that some weed types are not very much affected by these kinds of treatments and that they need a different focus. In addition, the experiments show that tillage in the autumn and spring had no adverse effect on the soil structure, it does not cause more disease attacks, just as it has a good effect on the yield in the subsequent crop, because the root weed is controlled. 

“The Norwegian organic farmers can learn from our experiments, among other things, that the control must take place in the autumn after harvest if they want to be sure of an effect. In addition, we also had oats in the trial, and it proved to be a very good competitor to the weeds. And they can perform these treatments without having to worry about disease attacks or destroying the structure of the soil,” Bo Melander says.

Behind the research

Collaborators: Department of Agroecology, University of Aarhus, Denmark; Norwegian University of Life Sciences (NMBU), Department of Plant Sciences, Norway; Norwegian Institute of Bioeconomy Research (NIBIO), Division of Biotechnology and Plant Health, Norway; Department of Crop Production Ecology, Swedish University of Agricultural Sciences, Sweden; Norwegian University of Life Sciences (NMBU), Faculty of Environmental Sciences and Natural Resource Management (MINA), Norway.

Funding: The Research Council of Norway (project no. 207686)

Conflicts of Interest: None

Read more: You can read the publication: “Influence of mechanical weeding and fertilization on perennial weeds, fungal diseases, soil structure and crop yield in organic spring cereals”. It is written by: Lars Olav Brandsæter, Kjell Mangerud, Lars Andersson, Trond Børresen, Guro Brodal and Bo Melander.

Contact: Associate Professor Bo Melander, Department of Agroecology, Aarhus University. Tel: +4522283393. Email: bo.melander@agro.au.dk

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