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The aim of stubble cultivation after rapeseed is to bring as many weed seeds to germination as possible. This is most successful when working very shallow.
Goal of stubble cultivation after rapeseed is to bring as many dormant seeds to germination as possible. This is achieved especially when working very shallow.
Agricultural challenges such as glyphosate reduction in water protection areas, increasingly resistant weeds, conservation of soil water reserves, and the costs of different equipment are bringing tools for ultra-shallow soil cultivation (maximum 2 cm deep) more into focus. Many practitioners wonder about potential results and limitations of such practices. The State Farming Agency Hessen (LLH) was able to obtain interesting results regarding stubble management on a harvested rapeseed field at the Hessian state domain Johannesberg.
For optimal field hygiene, it is important to consider the biological principles of germination. Many weed seeds, grass seeds, and rapeseed seeds have one thing in common: with a traditional cultivation depth for stubble of about 5-7 cm, the buried seeds often go into dormancy.
Weeds like blackgrass may have a primary dormancy based on ripening conditions in summer. This means the fallen seeds do not germinate immediately after harvest. If they are then incorporated, they go into a "sleeping beauty sleep," the so-called secondary dormancy. This can last for years. The weed seed reserve in the soil is significantly enriched.
This should be avoided at all costs from an agricultural perspective. From this secondary dormancy, the blackgrass seed then germinates in subsequent years with sufficient light stimulus and moisture.
Unlike weeds, rapeseed seeds do not have a pronounced primary dormancy. Volunteer rapeseed can germinate immediately. A secondary dormancy, as with grasses, occurs in rapeseed due to too deep initial cultivation.
Significant factors for the development of secondary dormancy of seeds are darkness and dry stress. Studies have shown that volunteer rapeseed, when incorporated into deeper soil layers, can remain viable for more than 15 years through the formation of secondary dormancy (dormancy). This seed is revived by light stimulus.
Volunteer rapeseed then grows in subsequent crops. Often unnoticed, typical rapeseed diseases multiply on volunteer rapeseed plants. The genetics of the varieties also influence the intensity of dormancy. However, there is currently no solid database for modern varieties. The outlined knowledge of rapeseed seed dormancy must be considered in targeted stubble treatment strategies.
In addition to crushing crop residues for phytosanitary reasons, the goal of the first treatment of rapeseed stubble is to create optimal germination conditions for volunteer rapeseed. In this context, the LLH compared various equipment on a practical field in a strip demonstration plot.
The first deployment took place on a loamy soil with a high clay content four days after rapeseed threshing. During harvesting, the research team determined 1.9% of harvesting losses, which means over 2,000 volunteer seeds/m2 with a yield of 43 dt/ha.
The upgrown rapeseed plants in the demonstration strips were counted at defined counting points at several time points. An untreated control strip served as a comparison. In the shallow-cultivated plots, the equipment achieved high germination rates.
The "Knife Roller" and the "Straw Rake" offer high surface performance, crush the brittle straw, and shake out the rapeseed from the crop residues. In these variants, about twice as many rapeseed plants emerged as in the untreated control.
The use of the "Mulcher" offers, in addition to very good crushing of the crop residues, a separation of the mulch material without soil disturbance. Green stems, old weed growth, and pod husks are reliably detected and crushed.
A newly developed tool for ultra-shallow soil cultivation is the "GrindStar" tested in the experiment. It cuts the rapeseed stubble flat at the soil surface and crushes the residues through a cutting and crushing action. Fine soil is produced, ensuring optimal germination conditions. The equipment provides good soil adaptation and works between 0 and 1 cm deep.
The importance of working depth is evident when using the "Short Disc Harrow." The machine, set very shallow at 2 cm, achieves high germination rates. The short disc harrow only scratched the soil surface. At a working depth of 4 cm, fewer plants emerged compared to the untreated control. The open soil surface was coarse-cloddy and offered little fine soil.
The use of the "Wide Tine Harrow" with goosefoot shares also left an open and cloddy soil surface, which did not provide good germination conditions for the rapeseed.
Summary: Shallow cultivation with minimal soil disturbance is advantageous to bring volunteer rapeseed to germination. By separating the volunteer seeds from the straw residues, soil cultivation equipment can create good germination conditions.
The rapeseed seeds remain on the soil surface and are covered by straw. The straw cover binds rising moisture and creates a microclimate in which rapeseed seeds can germinate. At the same time, the straw layer acts as protection against evaporation and serves as a food source for soil organisms. For reasons of field hygiene, a good straw shredding is important in the first operation.
In addition to the effect of the equipment used on the germination behavior of volunteer rapeseed, the effectiveness of equipment for combating it was also examined. Tools with all-over cutting mechanisms generally performed well here. A flat cut at the firm horizon of the soil surface allows the rapeseed plants to be cut at the vegetative cone.
With a deeper intervention into the soil, the roots are pulled out, and the rapeseed plants are laid on the surface. Attention is then focused on de-rooting the rapeseed roots so that they can dry out afterwards. Following harrow tines on the working equipment are helpful. In addition to the good results of the flat cultivator and the GrindStar variants, the Straw Rake scored well in combatting volunteer rapeseed.
The use of the rake is time-consuming. In addition to dry weather at the time of cultivation, the rapeseed must not be too advanced and strongly rooted.
The variants of the short disc harrow showed no effective control of the upgrown volunteer rapeseed. At 2 and 4 cm working depth, complete surface cultivation is not possible with one pass, so some rapeseed plants remain standing and continue to grow.
The predominantly dry weather from the first cultivation to the sowing of the subsequent crop in October limited mineralization on the demonstration field. The Nmin values of the individual cultivation variants did not differ significantly.
However, from older studies, it is known that deeper soil disturbance after rapeseed harvest leads to high Nmin values. The mineralization rate increases with the intensity of soil cultivation. Therefore, shallow cultivation after rapeseed harvest, where the straw remains as much as possible on the soil surface, is recommended.
For professional arable farming, field hygiene plays a crucial role. Deep soil cultivation immediately after harvest should be a thing of the past in many cases.
Even short disc harrows work too deep with full-surface cutting. Therefore, entirely new tools will complement the previous operations in the future. Whether GrindStar, Straw Rake, Mulcher, and others, ultra-shallow soil or straw cultivation on stubble at depths of 0-2 cm will become increasingly important under future agricultural conditions.
