Conventional agriculture has relied heavily on synthetic herbicides over many decades. Today, however, more attention is being brought to their undesired effects.
Herbicides and their soil residues have a high potential for a negative impact on:
As a result of constant exposure to high doses of herbicides, weeds develop resistance to these chemicals (2).
Regenerative agriculture is based on minimal soil disturbance, year-round soil cover, and crop rotation. It is generally considered a sustainable alternative to systems involving conventional tillage. However, the application of regenerative agriculture principles changes the dynamics and populations of weeds, therefore modification of weed management methods is often necessary. Knowing your local weeds, their noxious potential, and their reproductive habits are key to developing a successful weed control strategy.
In regenerative agriculture, crop rotation is fundamental. It is simultaneously the most influential tool in the fight against weeds. Each plant species creates specific and unique conditions (both biotic and abiotic) which set limits for weed growth. In this way, any given crop species can be considered a filter that only allows certain types of weeds to grow while suppressing others (3). If we add different sowing times on top of the rotation of different crops, these filters will alternate and the pressures will intensify, preventing a single weed species to dominate.
Weeds can thrive when they have a synchronised crop cycle (4,5). Conversely, weed growth that is desynchronised with crops provides a unique opportunity for its control. Weeds growing in the absence of crops can be removed with a non-selective herbicide, while weeds in the vegetative stage during harvest can be removed before the seed is sown. Crop rotation allows weeds to be controlled at different emergence times, thus preventing a specific type of weed from repeatedly completing its life cycle.
Various studies (6,7) confirm that regular crop rotation with different sowing times leads to a reduction in the weed seed bank. Another important aspect is related to preventing the formation and accumulation of weed seeds. When drawing up a seeding strategy, rotations for each field should be designed in consideration of the relevant weed species for that particular field.
Crops that grow quickly and can beat weeds (such as corn) are an excellent addition to the rotation.
Incorporating cover crops into the crop rotation offers many benefits, such as reduced soil erosion, improved soil structure and fertility, but also the opportunity to use plants with strong weed suppressing capabilities (8).
Incorporating cover crops into agricultural systems prevents the growth and development of weeds by occupying vacant niches (the cover crop takes up the space that would otherwise be available to weeds).
Cover crop residues can suppress or slow down weed germination and growth (9,10), due to both allelopathic and physical effects (11). Additionally, plant debris on the soil surface can result in less variation in the soil temperature and reduced light penetration, both of which have been shown to inhibit weed germination (12).
As for specific examples, the contribution of allelopathic cover crops such as rye, subspecies of buckwheat, sorghum, or alfalfa are worth mentioning. Cover crops, including rapeseed (Brassica napus) and white mustard, can also prevent weed seeds from germinating and thus prevents their emergence. Legumes are usually not the strongest contenders for weed management due to the nitrogen released by their residues positively stimulating weed growth.
Instead of leaving a field fallow, even for a short time, it is possible to plant annuals or cover crops that grow quickly and take up space before weeds have a chance to emerge and crowd them out. For example, if the plan is to sow cash crops in early spring, it is possible to choose winter-hardy cover crops, such as a mixture of oats and crimson clover, which die back early. For sowing cover crops in late spring and early summer, it is again possible to use winter-hardy crops, for example, hairy vetch (Vicia villosa or Vicia hirsute) and rye. Freeze-killed cover crops can be used, for instance, as mulch or allowed to die naturally.
Empirical studies suggest that tillage does not affect the reduction or increase of weed seed bank density (13, 14, 15, 16). In other words, ploughing your field several times a year does little to reduce, let alone eradicate, weeds on your property. On the contrary! Tilling redistributes seeds throughout the soil profile, which ultimately helps weeds.
In a no-till system, weed seeds penetrate the soil through slow processes (cracks in the soil, fauna...). Therefore, 60-90% of seeds accumulate only in the upper 5cm of the soil (17, 18). This may result in a higher rate of predation (eating by animals), which limits the number of available seeds (19). Ploughing, aka burying seeds, reduces their availability to predators (20). Thanks to no-till, weed seeds accumulate near the surface and rodents, insects, and birds have better access to them. We must not forget to utilise animals as an effective natural tool for reducing the seed bank of weeds(21, 22, 23)!
Ploughing buries seeds below the surface, reducing their availability to predators. On the other hand, no-till encourages the accumulation of seeds near the soil surface where predators have easier access to them, thus reducing the number of weeds.
In contrast with tillage, no-till also promotes biodiversity and predator abundance (24).
Ploughed soil offers better germination conditions for most seeds while providing a stimulus for seeds requiring light for germination or those requiring larger temperature fluctuations (25). Tillage reduces soil resistance to weed root penetration, leading to the establishment, emergence, and growth of germinated weed seeds (26, 27). In other words, more ploughing can mean more weeds.
Ploughing also allows seed seeds to germinate at a greater soil depth (28). If the roots of the germinated weed seeds are located directly on the surface of the uncultivated soil, it is difficult for them to penetrate to the lower layers and they will die more easily.
And what does this look like in practice? It often happens that in the first year after switching to a no-till system, there are more weeds in the field than under the previous system. However, in the following years, things turn around and in no-till, the weed seed bank is depleted within a few seasons (29).
A no-till system and careful management of cover crops can be an opportunity for long-term weed control (30).
The standard approaches to regenerative agriculture (such as more varied crop rotation, cover crops, or the introduction of a no-till system) are not effective in the fight against weeds on their own. Therefore, it is essential to use a combination of practices to the greatest extent possible. The various methods then work together and can cumulatively reduce the number of weeds.
Weed management during the transition to a no-till system is critical, and it may take several years for yield, soil properties, and weed populations to stabilise (31). The implementation of no-till is most successful in agricultural systems that use at least a two-year crop rotation and utilise cover crops or their residues in their seeding procedures.
In the transition to regenerative agriculture and no-till, weeds are often one of the tricky obstacles that farmers must overcome. And, indeed, it is not possible to get rid of weeds overnight if we are not using intensive chemical herbicides, which are not sustainable in the long term (neither ecologically nor economically). Especially in the first seasons, farmers cannot do without patience. Nevertheless, by using several measures at once, the desired results can be achieved relatively quickly (within a few years).
It is important to ask whether it is even necessary (or to what extent) to wage this war on weeds. Even weeds can contribute to desirable ground cover, nutrient stabilisation, habitat creation, or a nutritious food source for pest predators, which do not necessarily reduce yield. As this study shows, yields will not permanently decrease with the transition to regenerative farming. And the soil, landscape, and its inhabitants, both large and small, will rejoice at minimising our interventions in these natural processes.
The list of literature and studies used in this article can be found here.
Do you have questions or comments about the article? Let us know by e-mail at carboneg@carboneg.com! We will be happy to discuss the topic with you individually.
